Development of a Novel Concept of Efficient Superconducting ...

Development of a Novel Concept of

Efficient Superconducting Magnet for

Radioisotope Production Cyclotron

Desarrollo de un nuevo concepto de imán superconductor

eficiente para un ciclotrón de producción de radioisótopos

Author:

Javier Munilla López

Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas – CIEMAT

Universidad Pontificia Comillas – UPCO

Supervisors:

Dr. Fernando Toral Fernández

Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas – CIEMAT

Dr. Mario Castro Ponce

Universidad Pontificia Comillas – UPCO

Madrid, September 2019

Development of a novel concept of efficient superconducting magnet for a radioisotope production cyclotron

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iii

Summary

Radioisotopes are nowadays extensively used in several applications, including

material science, industry or nuclear medicine. In the specific case of nuclear medicine,

radioisotopes play an essential role which can be classified in terms of two main categories:

Diagnosis and Therapeutics. A radiopharmaceutical for diagnosis is customized to be located

into specific organ or tumor by taking advantage of its metabolic process before the

radioactive decay occurs. After that, when radioisotope decays, the emitted radiation is

detected and analyzed by imaging monitors and techniques. On the other hand,

radiopharmaceuticals for therapeutics emit controlled damaging radiation to targeted cells

(e.g. cancerous tumors) while minimizing the radioactive dose to the surrounding healthy

tissues.

Radioisotopes found in nature are typically not suitable for the human body. They

belong to elements with a clear lack of biocompatibility or their half-lives are so long that the

radiation suffered by the human body could be even more toxic than the disease to be cured.

Those of them with a proper combination of properties are not so common because they

simply decay too fast to accumulate. Therefore, radioisotopes of medical interest must be

artificially produced.

Nowadays 80% of all diagnostic medical scans use 99Mo, which is needed to produce

99mTc. At this moment it can be only produced at reactors. There are just 17 reactors

worldwide producing radioisotopes in 2019, so reactor-produced medical isotopes rely

exclusively on the availability of these nuclear reactors. Some of them were constructed in

the 1960s and they are approaching the end of their life. Future reactors are projected but

they need time to be operational at nominal capabilities. This is even more critical when

taking into account that the demand has been increasing above the expectations during the

last years. This trend could be extrapolated to the future. There is a real possibility of a near

future in which the global production cannot satisfy the total demand of radioisotopes.

Accelerator-based radioisotopes can be considered as an emerging alternative, providing a

complementary production scheme to take care of a wider range of different radioisotopes.


iv

The main objective of this thesis is the development of a procedure for compact and

efficient superconducting magnets suitable for the production of radiopharmaceuticals. The

AMIT project, promoted by CDTI (Centro de Desarrollo Tecnológico Industrial) was an

excellent framework to carry out this process while a prototype can be manufactured. One of

its main objectives was to improve the accessibility of the radiopharmaceuticals production

technology (18F and 11C). In addition, general and exhaustive arguments based on all the

feasible alternatives can be detailed for an actual tailored system based on the specifications

of a national project.

This Thesis describes each of the steps to consider in the design of a compact

superconducting magnet, analyzing the existing alternatives in the state of the art and their

potential improvements. Later on, the final decision based on the particular case of the AMIT

project specifications is explained. The proposed accelerator is a classical cyclotron

(Chapter 1). This Thesis follows the classical route for the development of these devices:

conceptual (Chapter 2), electromagnetic (Chapter 3), mechanical (Chapter 4) and engineering

(Chapter 5) designs. Procedures well-established in the literature will be of general

application, using analytical or numerical tools according to each case. On the other hand,

there are some particular cases where innovative or custom-made procedures and /or tools

will be proposed. Some examples are the internal cooling of the coils and the hybrid

cryogenic circuit, which are critical to minimize the size, or the support and alignment system

which is critical for optimized efficiency.

Once that the general procedure and the particular AMIT design is finished, the

manufacturing procedure for the prototype is shown in Chapter 6. The results for this

prototype are exposed in Chapter 7, while focus is applied to the electromagnetic and

thermal results. This Thesis includes an assessment of the possible steps to continue

improving this prototype, based on the previous analysis and the results obtained. Additional

dissertation is included about the possible improvements that could be incorporated in a

hypothetical second version of this prototype or similar equipment. Finally, some applications

are outlined: The procedure proposed in this Thesis could be used to potentially improve the

size or the efficiency of the devices currently used for them.

LuisGarciaTabares

Resaltado


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Resumen

El uso de radioisótopos en la actualidad está ampliamente extendido en diferentes

campos, abarcando desde la ciencia de materiales o la industria hasta la medicina nuclear. En

éste último, los radioisótopos son componente fundamental de los radiofármacos empleados

dentro de dos categorías principales: Radiodiagnóstico y Radioterapia. En el primer caso, los

radiofármacos son especialmente formulados para, al ser administrados al paciente, poder

estudiar los procesos metabólicos de algún órgano o tumor en particular, combinando la

actividad metabólica propia de la formulación del radiofármaco con la actividad radioactiva

del radioisótopo, que puede ser analizada. Por otro lado, las técnicas de radioterapia emplean

la propia radioactividad del radiofármaco como componente activo en el tratamiento de la

enfermedad, como puede ser la eliminación de células cancerosas.

Los radioisótopos que se pueden encontrar en la naturaleza no pueden ser

empleados de forma práctica en ninguna de estas técnicas de medicina nuclear, bien porque

pertenecen a elementos tóxicos para el cuerpo humano, o porque sus vidas medias son

demasiado largas (produciendo efectos secundarios dañinos que no compensan el efecto

positivo) o demasiado cortas (y por tanto no es viable su extracción, procesado y utilización

en dosis suficientes). Por tanto, los radioisótopos de interés médico deben ser producidos

artificialmente. Las dos principales vías de producción existentes actualmente son los

reactores nucleares y los aceleradores de partículas.

Actualmente el 80% de los procedimientos diagnósticos con radioisótopos a escala

mundial emplean 99mTc producido a partir de 99Mo que a su vez ha sido producido en un

reactor. Debido a que el número de reactores operativos a nivel mundial está decreciendo y

no se espera que, al menos a medio plazo, vaya a aumentar su número o capacidad

productiva, se plantea la necesidad de un cambio para afrontar las necesidades de

radiofármacos que, por otro lado, se espera que sigan aumentando su demanda en las

próximas décadas. Los aceleradores de partículas como productores de radioisótopos se

plantean como una alternativa complementaria emergente.

El objeto de esta tesis es el de desarrollar un procedimiento para imanes

superconductores compactos adecuados para la producción de radiofármacos. Por otro lado,


vi

el proyecto AMIT, impulsado por CDTI (Centro de Desarrollo Tecnológico Industrial) supuso un

marco excepcional para llevarlo a cabo con la fabricación de un prototipo por lo que además

de desarrollar una argumentación general y exhaustiva basado en todas alternativas

tecnológicamente viables, ésta se concreta en decisiones e innovaciones a medida para las

especificaciones reales de un proyecto nacional. Entre sus principales objetivos se encontraba

el impulso de la tecnología de producción de 18F y 11C haciéndola más accesible.

El autor de esta tesis plantea cada uno de los pasos a considerar en el diseño de un

imán superconductor compacto, analizando las alternativas existentes en el estado de la

tecnología actual junto a su potencial de innovación, justificando y detallando las soluciones o

innovaciones concretas para el caso particular de las especificaciones del proyecto AMIT. En

esta tesis se justifica un ciclotrón clásico como el acelerador más adecuado (Capítulo 1). A

continuación avanza en cada uno de los apartados tradicionalmente empleados en estos

dispositivos: diseño conceptual (Capítulo 2), electromagnético (Capítulo 3), mecánico

(Capítulo 4) e ingeniería de diseño (Capítulo 5). Procedimientos bien establecidos en la

literatura serán de aplicación general, empleando herramientas analíticas, numéricas o

cálculos con software comercial según cada caso. Por otro lado, en algunos casos particulares

donde se justifique, serán propuestos procedimientos y/o métodos innovadores o

desarrollados a medida. Algunos ejemplos son el concepto de refrigeración interna y el

circuito criogénico híbrido, claves para optimizar el tamaño del sistema, y el sistema de

soportes y alineamiento, claves para su eficiencia optimizada.

A continuación se exponen los principales pasos y detalles constructivos (Capítulo 6)

para finalmente exponer y analizar los resultados (Capítulo 7) obtenidos de forma objetiva,

clasificados de nuevo en resultados electromagnéticos y mecánicos (fundamentalmente

térmicos). La tesis incluye una valoración de los posibles pasos a seguir desarrollando o

mejorando de este prototipo y una valoración, fundada tanto en el análisis previo como en

los resultados obtenidos, de las posibles mejoras que podrían incorporarse en una hipotética

segunda versión del imán o en otros equipos similares. Finalmente, se esbozan algunas

posibles aplicaciones en las que un procedimiento como el detallado en esta tesis podría ser

fácilmente adaptado y potencialmente interesante para optimizar el tamaño o la eficiencia de

los dispositivos actualmente empleados en dichas aplicaciones.

Acknowledgements

I would like to give my sincere gratitude to those who made possible this Thesis. The

first one is my supervisor Fernando Toral who is responsible of the Particles Accelerators Unit

at CIEMAT. His tenacity, deep knowledge and wide engineering view were the core of this

work. The financial support was supplied mainly by the AMIT consortium and CIEMAT, while

the academic support was received from Mario Castro and Universidad Pontificia Comillas.

It has been a pleasure to be involved in the development of a complete particle

accelerator from the very beginning. There is still a lot of work to develop and the CIEMAT

team is looking forward on it. Along these years, this team has been changing so some

colleagues collaborated just punctually or temporarily, I would like to appreciate their efforts

and participation. For those involved from the initial stage (all of us sharing their time with

other projects), Luis and Jose Manuel were responsible for making it possible from the very

first concept and creating the adequate working frame. The beam dynamics were perfectly

carried out by Conchi, the control system by Cristina and Antonio, the RF system by Dani, the

diagnosis by Ivan, shielding by Jose Ignacio, the ion source by Diego, etc. The manufacturing

and assembly processes could not success without a great team of CIEMAT technicians: Jesus,

Jose Luis, Pablo, Dani and Luismi widely helped to overcome any difficulty. This includes all

the unexpected issues which arise when the theoretical design becomes real in the workshop.

I am very glad that I could be part of this team, hoping that the expectations were fulfilled.

Several other companies and external people were also directly involved in this work,

being Antec (Antecsa) and TVP (The Vacuum Projects) the main companies. From Antec, Rafa

and Borja developed a great work manufacturing the magnet and I am very grateful with

them and their know-how, especially with the superconducting coils. And of course Leire,

whose diligence was critical for bringing order into the engineering project. The

manufacturing of the cryostat was possible thanks to the expertise of Pepe, and the

technicians from TVP, in this field including complex assemblies and joints.


viii

The collaborations from other institutes were also needed and I really appreciate the

help and knowledge coming from experts at each field. ALBA-CELLS provided an excellent

magnetic measurement bench for even longer time than initially expected. From the

Cryogenic Group at CERN, I would like to thank Matthias and Friedrich for their valuable input

in terms of the first version of the CSS, and also some suggestions and interesting debate for

the modifications.

Last but not least, I am very thankful to my parents and family, as they provided the

foundations for my scientific career while encouraging and giving me unconditional support. I

am very glad to count on my friends and other partners at the different aspects of my life, as I

have received tons of confidence from them. And of course thanks to Sandra, because her

invaluable help and understanding along the last steps of this Thesis.


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Table of Contents

Summary .................................................................................................................................... iii

Resumen ...................................................................................................................................... v

Acknowledgements .................................................................................................................. vii

Table of Contents ...................................................................................................................... ix

List of figures ............................................................................................................................. xi

List of tables .............................................................................................................................. xv

Chapter 1. Introduction ........................................................................................................... 1

1.1 Radioisotope production .......................................................................................................................... 1

1.2 State of the art ....................................................................................................................................... 10

1.3 The AMIT Project ................................................................................................................................... 15

1.4 Thesis objectives ................................................................................................................................... 15

Chapter 2. Cyclotron Conceptual Design ........................................................................... 21

2.1 Main specifications ................................................................................................................................. 21

2.2 A Compact Superconducting Cyclotron .................................................................................................. 23

2.3 Cooling concept ..................................................................................................................................... 29

2.4 Definition of magnet interactions with other cyclotron subsystems ......................................................... 34

2.5 Summary of Concept Design ................................................................................................................. 37

Chapter 3. Electromagnetic Design ..................................................................................... 39

3.1 2D concept and refinement (Pseudo 2D) .............................................................................................. 39

3.2 3D design ............................................................................................................................................. 49

3.3 Quench simulation .................................................................................................................................. 52

3.4 Summary and Contribution about the Concept Design ........................................................................... 55

Chapter 4. Mechanical Design .............................................................................................. 57

4.1 Conceptual design of the cryostat .......................................................................................................... 57

4.2 Mechanical calculations ......................................................................................................................... 60

4.3 Evaluation of thermal losses .................................................................................................................. 65

4.4 Refrigeration system .............................................................................................................................. 78

4.5 Alignment and support system ................................................................................................................ 86


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4.6 Current leads .......................................................................................................................................... 90

4.7 Vacuum system ....................................................................................................................................... 93

4.8 Cryogenic Supply System ....................................................................................................................... 94

4.9 Summary and Contributions about the Mechanical Design ................................................................... 100

Chapter 5. Engineering Design .......................................................................................... 101

5.1 Connection box .................................................................................................................................... 101

5.2 Material choices ................................................................................................................................... 103

5.3 Fabrication techniques ......................................................................................................................... 106

5.4 Validation of the refrigeration scheme. ................................................................................................. 113

5.5 Summary and Contributions about the Engineering Design .................................................................. 121

Chapter 6. Fabrication and Assembly ............................................................................... 123

6.1 Superconducting coils ........................................................................................................................... 123

6.2 Helium vessel ....................................................................................................................................... 131

6.3 Cryostat ................................................................................................................................................ 134

6.4 Iron assembly ....................................................................................................................................... 137

6.5 Connection box assembly .................................................................................................................... 138

6.6 Summary of Fabrication and Assembly Chapter ................................................................................... 141

Chapter 7. Magnet Testing.................................................................................................. 143

7.1 Pre-cooling tests .................................................................................................................................. 143

7.2 Liquid Helium tests ............................................................................................................................... 146

7.3 Modified CSS tests ............................................................................................................................... 151

7.4 Autonomous operation of the cyclotron ................................................................................................ 153

7.5 Transfer line tests ................................................................................................................................. 156

7.6 Summary of Magnet Testing Chapter ................................................................................................... 158

Chapter 8. Conclusions. Future developments ................................................................ 159

List of Publications ................................................................................................................ 163

References .............................................................................................................................. 165


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List of figures

Fig. 1.1. Beta decay and scanning concept ................................................................................................ 3

Fig. 1.2. Basics of a cyclotron ..................................................................................................................... 8

Fig. 1.3. Cyclotron poles according to the AVF concept: Rutgers Cyclotron [24] ..................................... 10

Fig. 2.1. Radiation energy (horizontal axis) needed for providing a 11

C single dose in terms of time and

beam current (vertical axis). .................................................................................................................... 22

Fig. 2.2. Superconducting transition of a resistive material ..................................................................... 27

Fig. 2.3. Sumitomo cryocooler RDK415 load map .................................................................................... 33

Fig. 2.4. RF system overview (already installed on cyclotron magnet) .................................................... 36

Fig. 2.5. Cyclotron mid-plane cross section showing extracted particle ideal trajectory. ........................ 37

Fig. 3.1. Load line of the magnet for a given design ................................................................................ 42

Fig. 3.2. Magnetic field map with left edge as symmetry axis (left). Load Line of the magnet from 2D

design. Nominal operation is highlighted by the circle (right) ................................................................. 46

Fig. 3.3. Cyclotron magnetic model (3D and its pseudo-2D) .................................................................... 47

Fig. 3.4. Magnetic field map in the final Pseudo 3D magnetic model(left) and magnetic field vs radial

position graph (right). .............................................................................................................................. 49

Fig. 3.5. 3D magnetic model. ................................................................................................................... 50

Fig. 3.6. Overview of vertical magnetic forces on each coil (z axis). The force is positive towards the iron,

that is, coil forces are repulsive ................................................................................................................ 51

Fig. 3.7. Schematic of the magnet protection circuit. .............................................................................. 55

Fig. 3.8. Simulation of the current decay and resistive voltage evolution for a quench at nominal current

with a dump resistor of 3 ohm ................................................................................................................. 55

Fig. 4.1. Magnetic forces in coils and first schematic of a casing with openings for the RF vacuum

chamber. .................................................................................................................................................. 58

Fig. 4.2. Coil alignment concept inside the casing. .................................................................................. 59

Fig. 4.3. Coil model for averaged mechanical properties evaluation ....................................................... 61


xii

Fig. 4.4. Magnetic force density and radial stress distributions inside the coil without shrinking cylinder.

.................................................................................................................................................................. 62

Fig. 4.5. Radial and hoop stresses distribution in the coil and shrinking cylinder at the three main load

cases ......................................................................................................................................................... 63

Fig. 4.6. Von Mises stress in Casing at worst load scenario (Amplified strain for easier view) ................ 64

Fig. 4.7. Vapour pressure of different gases at cryogenic temperatures.................................................. 67

Fig. 4.8. Thermal conductivity of selected materials ................................................................................ 69

Fig. 4.9. Thermal FEM Model of cold mass support................................................................................. 74

Fig. 4.10. Heat load to casing ................................................................................................................... 76

Fig. 4.11. Cryogenic concept for AMIT cyclotron including the CSS system for autonomous operation. . 80

Fig. 4.12. Schematic flow diagram of the custom code to compute the cooling fluid behavior ............... 84

Fig. 4.13. Schematic of dedicated FEM model for transient calculations (ForcedFlowN.m)..................... 84

Fig. 4.14. He Pressure evolution in case of a quench. ............................................................................... 85

Fig. 4.15. Supporting concept: detailed view of one support (left) and overall view (right)..................... 87

Fig. 4.16. Directional deformation (axial displacement) of casing after cool down. ................................ 88

Fig. 4.17. Stresses at supporting structure while casing is cold ................................................................ 89

Fig. 4.18. Supporting structure under horizontal magnetic misalignment (1 mm) .................................. 90

Fig. 4.19. Current leads concept design .................................................................................................... 91

Fig. 4.20. Temperature distribution at the conduction heat exchanger for the warm end of HTS current

leads. ........................................................................................................................................................ 92

Fig. 4.21. HTS Current leads shunts for quench protection: temperature distribution (left) and time

evolution (right) for the most pessimistic scenario .................................................................................. 93

Fig. 4.22. Vaccum system scheme ............................................................................................................ 94

Fig. 4.23. a) The Cryogenic Supply System (artistic view). b) 3D model of the low loss Transfer Line. ..... 95

Fig. 4.24. Cryogenic Supply System schematics as supplied by CERN (left) and modified at CIEMAT ready

for cyclotron magnet cooling (right)......................................................................................................... 98

Fig. 4.25. Helium flow circuit .................................................................................................................... 99


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Fig. 5.1. Connection box: front (top) and rear view without main flange (bottom) ............................... 102

Fig. 5.2. Magnetization curve of the procured ASTM 1010 material ..................................................... 104

Fig. 5.3. Welding preparation (casing) .................................................................................................. 107

Fig. 5.4. Cross section of the iron yoke. Each individual part is drawn with a different pattern. .......... 111

Fig. 5.5. Cross sectional view of coil and casing mock-up ..................................................................... 115

Fig. 5.6. Test setup for cooling scheme validation ................................................................................ 116

Fig. 5.7. Coil inside the casing for test. Insert ready to be installed in the test cryostat ........................ 117

Fig. 5.8. Temperature distribution computed by FEM simulations on Coil(a) and cryostat (b).............. 118

Fig. 5.9. Coil temperature during cool down on Single Coil Test: comparison of measurements (dots) and

calculations (dashed line)....................................................................................................................... 120

Fig. 5.10. Equilibrium temperature of coil as a function of LHe mass flow: comparison of measurements

(dots) and calculations (solid line) assuming 5 W of heat loss in the transfer line for any mass flow. .. 120

Fig. 6.1. Coil manufacturing: Winding (Left) and thermoretractable tape wrapping (Right) ................ 124

Fig. 6.2. Insulation defect at the inner diameter of SC2 coil .................................................................. 125

Fig. 6.3. Inductance spectrum of SC2 and SC3 ....................................................................................... 129

Fig. 6.4. Superconducting coil including temperature sensor ................................................................ 131

Fig. 6.5. (left) Coil assembly inside the casing. (right) Detail of the welds ............................................. 132

Fig. 6.6. Setup for leak test at 77K of the finished casing ...................................................................... 133

Fig. 6.7. Casing covered by low emissivity aluminum foil ...................................................................... 133

Fig. 6.8. Preliminary fitting of thermal shield around casing(left). Detail view of rods thermal anchoring

manufacturing: slots improve flexibility (right)...................................................................................... 135

Fig. 6.9. Electrical measurements on cryostat after rods assembly ....................................................... 136

Fig. 6.10. Thermal testing of the cryostat with liquid Nitrogen ............................................................. 136

Fig. 6.11. Iron assembly (left) and pole assembly (right) ....................................................................... 137

Fig. 6.12. Dedicated supporting structure during connection box manufacturing ................................ 138

Fig. 6.13. Input pipe assembly. (left) Welding setup. (right). ................................................................. 139


xiv

Fig. 6.14. Connection box ready to be closed ......................................................................................... 140

Fig. 6.15. Intermediate assembly step of the connection box cryogenic connection to rigid transfer line

................................................................................................................................................................ 140

Fig. 7.1. Setup for first magnet cooling test ........................................................................................... 143

Fig. 7.2. Cooling down curves using liquid Nitrogen ............................................................................... 144

Fig. 7.3. Setup for magnetic measurements ........................................................................................... 145

Fig. 7.4. Magnetic field measured at low current (0.65 A) ..................................................................... 146

Fig. 7.5. Cooling down curves using liquid Helium from a Dewar .......................................................... 147

Fig. 7.6. Magnetic field measured at cyclotron center ........................................................................... 149

Fig. 7.7. Magnetic field measured at low current (30 A) ........................................................................ 149

Fig. 7.8. Broken strain gauge after powering tests ................................................................................ 150

Fig. 7.9. Autonomous cool down of AMIT cyclotron with a dummy load ............................................... 152

Fig. 7.10. (left) Modified CSS tests and dummies. (right) He pump circuit ............................................ 153

Fig. 7.11. Autonomous operation of AMIT cyclotron .............................................................................. 154

Fig. 7.12. Autonomous cool down of AMIT cyclotron ............................................................................. 154

Fig. 7.13. CSS and transfer line with dummy load test ........................................................................... 158

Fig. 8.1. Thesis objectives: completed (green) and partial/in progress (orange) ................................... 160


xv

List of tables

TABLE I SELECTION OF CYCLOTRONS FOR RADIOISOTOPE PRODUCTION ............................................... 14

TABLE II THESIS OBJECTIVES .................................................................................................................... 19

TABLE III NOMINAL THERMAL POWER AVAILABLE FROM CSS ................................................................ 34

TABLE IV MAIN MAGNET SPECIFICATIONS .............................................................................................. 39

TABLE V EFFECT OF CU/NCU RATIO ON THE QUENCH PROTECTION RESULTS ....................................... 45

TABLE VI MAGNET PARAMETERS DEFINED FROM 2D MODEL ................................................................ 46

TABLE VII MAGNETIC FORCES ................................................................................................................. 52

TABLE VIII QUENCH SIMULATIONS SUMMARY ....................................................................................... 54

TABLE IX MECHANICAL PROPERTIES OF MAGNET MATERIALS AT WORKING TEMPERATURE ............... 60

TABLE X AVERAGED MECHANICAL PROPERTIES OF COIL (4.2 K) (COMPUTED) ...................................... 62

TABLE XI INTEGRAL THERMAL CONDUCTIVITY FOR SELECTED MATERIALS [W/m] ................................ 70

TABLE XII CYCLOTRON MAGNET: THERMAL BUDGET OF COLD MASS .................................................... 77

TABLE XIII RESIDUAL GAS CONDUCTION ................................................................................................. 77

TABLE XIV CYCLOTRON MAGNET: THERMAL BUDGET .......................................................................... 114

TABLE XV STEADY STATE HEAT LOSSES CALCULATIONS FOR SINGLE COIL TEST ................................... 119

TABLE XVI COILS METROLOGICAL MEASUREMENTS ............................................................................. 125

TABLE XVII COILS ELECTRICAL MEASUREMENTS ................................................................................... 127

TABLE XVIII COIL TRAINING ................................................................................................................... 130

TABLE XIX MODIFIED CSS PERFORMANCE ............................................................................................ 152

TABLE XX THERMAL PERFORMANCE OF THE CYCLOTRON REFRIGERATED BY THE CSS ........................ 155

TABLE XXI TRANSFER LINE THERMAL BALANCE AT SECOND STAGE ..................................................... 158

Chapter 1. Introduction

1.1 Radioisotope production

Cancer, cardiovascular and brain diseases are found among the top leading causes of

death in 2016, according to the World Health Organization[1]. Thus, the development and

implementation of techniques to provide a better understanding of the causes and allowing

earlier diagnosis and/or treatment should be promoted.

Radioisotopes are nowadays extensively used in several applications, ranging from

material science to industry, transportation or national security. In the specific case of nuclear

medicine, radionuclides play an essential role which can be classified in terms of two main

categories: Diagnosis and Therapeutics. Both of them use radiopharmaceuticals based on

specific molecules which include radioisotopes in their formulation [2].

When using radiopharmaceuticals for diagnosis, a radioactive dose is given to the

patient and then dynamic processes can be studied in combination with imaging devices. The

interesting point is that these radiopharmaceuticals can be customized to be localized into

specific organs, tumors or cellular activities. Therapeutic techniques are typically more

innovative but already important nowadays. They are based on radiopharmaceuticals which

emit controlled damaging radiation to targeted cells (e.g. cancerous tumors) while minimizing

the radioactive dose to the surrounding healthy tissues [3].

Radioisotopes for Diagnostic Radiopharmaceuticals

For diagnostic procedures, radiopharmaceuticals must interact with the patient body

before the radioactive decay occurs. After that, when radionuclide decays, the emitted

radiation has to escape the body to be detected by a specific device. Such a simple

explanation is enough to extract important conclusions about the preferred radioisotopes to

be used.

First of all, lifetime should be long enough to keep radiological activity from the

production stage to the moment at which radiopharmaceutical molecule finishes the


2

expected biological processes. This includes the transportation/storage, administration to the

patient and any biological process involved (e.g. delivery from the blood to the cells inside the

organ to be diagnosed).

On the other hand, radiation emitted by radioisotopes should ideally vanish quickly

once the diagnostic procedure finishes. This would reduce the radiation suffered by the

patient, which may be dangerous, so the dose should be limited to the minimum needed for

the diagnostic.

Based on the same safety criterion, radiation emitted by the radioisotope should

have low energy, but high enough to be efficiently detected by the imaging devices taking

into account the attenuation inside the body patient.

These points are taken into consideration by the two main techniques for nuclear

medicine diagnostics, which are SPECT (Single-Photon Emission Computed Tomography) and

PET (Positron-Emission Tomography). Their differences rely on the radiation mechanism.

SPECT uses gamma-emitting radioisotopes. These gamma particles (electromagnetic

radiation or photons) can be detected by cameras surrounding the patient (or a single camera

moving around the patient, for example). When the information in the photons detected by

all the cameras is processed (reconstruction), reports with very valuable data can be

generated for the medical doctors, including 3D images or 2D slices, both of them static or as

a function of time [4]. The most widely used radionuclides for SPECT are 99mTc and 123I. Their

emission energies are 140 keV and 159 keV, respectively, while half-lives are 6 and 13 hours.

PET is a similar technique but involves a different radioactive source. PET

radiopharmaceuticals include a positron-emitting isotope which decays generating a positron.

The positron is annihilated by a nearby electron and this process results in the simultaneous

emission of two 511 keV gamma rays in opposite directions [5]. These two reactions are

presented in equations (1) and (2). The main advantage of PET compared to SPECT is that in

the former there are two photons generated and detected, each one with its own time to

reach a detector (Fig. 1.1, extracted from [6] and [7]). This additional information can be used

for a more precise image compared to SPECT, where just one photon is emitted from each

nucleoid.


3

(1)

(2)

Fig. 1.1. Beta decay and scanning concept

Some technological limitations, such as the efficiency and time resolution of

detectors are important for PET performance. Common PET scanners use “coincident”

detections on opposite detectors, so a LOR (Line Of Response) is evaluated for each

coincidence event. Advanced PET scanners have been improved by increasing detector

specifications (and computing capabilities). One of the improvements, for example, is the

time resolution for each detection, which could be fine enough to take into account the

timing between both detected gamma particles, providing a more precise location (in the

order of centimeters) rather than the LOR. This technique is called TOF-PET (Time of Flight

PET)

The main PET isotope is 18F, since it has proven to be the most accurate non-invasive

method of detecting and evaluating most cancers. In this specific case, 18F is combined in a

radiotracer molecule called fluorodeoxyglucose (FDG), commonly known as 18F-FDG or FDG-

PET [8]. Once it is inside the patient, as it is similar to glucose, it is taken up by high-glucose-

using cells so real FDG distribution in the body is a good indication of “standard” glucose

distribution. The distribution of glucose absorption by cells is the information that medical

doctors need for diagnosis and follow-up the patient’s response to a certain treatment [7],

[8]. The best example is cancer diagnosis, because as it is well known by experts in the field,


4

cancer cells develop an increased glucose uptake and depict metabolic abnormalities even

before morphological alterations are visible [10].

In addition, new procedures in diagnostics using radioisotopes are being developed.

For example, PET/CT, which basically combine PET with Computed X-ray Tomography at the

same time [8]. Processed information extracted by both methods simultaneously allows

better diagnosis than both techniques separately. This strategy can be used also for other

diagnostic procedures, for example MRI (Magnetic Resonance Imaging) resulting in PET-MRI

[11].

Radioisotopes for Therapeutic Radiopharmaceuticals

Radioisotopes for therapy are quite different from diagnosis ones even when the

radiation can be localized in the required organ in the same way as diagnosis (through a

radiopharmaceutical which is able to follow certain biological path). In this case radionuclides

providing high ionizing and short range radiation are needed, as the main objective is to

deliver a large dose to the targeted cells [12]. There are two main benefits, the first one is to

eliminate cancerous tumors (or at least control their growth) because they are sensitive to

damage by radiation. The second one is to use it as palliative, for example to relieve pain in

case of cancer-induced bone pain.

Radiopharmaceuticals for therapy use α, β- and Auger emitters. The best selection is

based on each specific situation. Half-lives for these radioisotopes are typically longer than

for diagnosis, so that the dose is applied for longer time and keeps causing damage to the

cancerous cells, even if they are still replicating. It is likewise for the case of long-lasting pain-

relief palliative treatment. In these cases lifetimes in the order of hours are typical, while a

few days can be also found [3].

Auger electrons are very well suited for small tumors and disseminated cancer cells

because their typical range is in the order of nanometers and their energies are very low

(from eV to keV). Auger electron cascades coming from 99mTc are also used as a therapeutic

agent (not as the most widely use of being an imaging agent as explained before). Their

therapeutic effectiveness occurs mainly due to the extensive DNA fragmentation of the

carcinogenic cells [13].


5

Targeted Alpha-particle therapy (TAT) uses α emitters as radioisotopes. These α-

particles (two protons and two neutrons, identical to a 4He nucleus) are more energetic than

Auger electrons and their effective range for energy relief is in the micrometric scale. 223Ra

and 225Ac are examples of α emitters for therapeutic uses against prostate cancer and

leukemia, respectively.

Finally, a β- emitter is a radioisotope which undergoes β- decay, so that one neutron

from its nucleus is converted into a proton, an electron and an electron antineutrino. As this

radiation can travel higher distances before the deposition of its energy (millimeters), they

are better suited for treating bigger lesions or macro-metastatic. The most popular β- emitter

is 131I which for example can be used for hyperthyroidism therapy.

Radioisotopes Production

Radioisotopes found in nature are typically not suitable for the human body. They

belong to elements with a clear lack of biocompatibility or their half-lives are so long that the

radiation suffered by the human body could be even more toxic than the disease to be cured.

Those of them with proper combination of properties are not so common because they

simply decay too fast to accumulate. Therefore, radioisotopes of medical interest must be

artificially produced.

There are two main sources to produce these radioisotopes (for both imaging and

therapeutic purposes): nuclear reactors and particle accelerators. The choice between them

relies on several considerations [14].

First of all, not every radioisotope can be produced (or not efficiently) in both

schemes. Those radioisotopes which are neutron-rich, like 99mTc, are generally produced in

research nuclear reactors while neutron-deficient ones (18F for example) are typically

produced via charged particle reactions in accelerators.

In terms of radioactive waste and nuclear weapon proliferation risk, which are the

two main global concerns on this matter, accelerators are in a better position. Moreover,

operational and decommissioning costs are lower for accelerators, as it is also the initial

capital investment. On the other hand, production rate of reactors is usually higher, so that

the cost per dose could be lower than accelerators.


6

Nowadays 80% of all diagnostic medical scans use 99Mo, which is needed to produce

99mTc. At this moment it can be only produced at reactors [15].

During the last decade, this scenario has started to change due to the reactors

shutdowns, for example OSIRIS in 2015 and NRU in 2016, the growth of radioisotope

therapies and the expanding demand of different radionuclides, including short-life ones.

According to IAEA (International Atomic Energy Agency), there are just 17 reactors worldwide

producing radioisotopes in 2019 [16], so reactor-produced medical isotopes rely exclusively

on the availability of these nuclear reactors. Some of them were constructed in the 1960s and

they are approaching the end of their life. Future reactors are projected or being constructed

currently, but they need time to be operational at nominal capabilities. This is even more

critical when taking into account that the demand has been increasing above the

expectations during the last years. This trend could be extrapolated to the future. Moreover,

emerging markets bring additional demand. There is a real possibility of a near future in

which the global production cannot satisfy the total demand of radioisotopes.

Accelerator-based radioisotopes can be considered as an emerging alternative,

providing a complementary production scheme to take care of a wider range of different

radioisotopes. It could be used for both imaging and therapy techniques compared to a

globally centralized scheme of (mainly) 99mTc supplied by a limited number of producers.

There are a number of possible accelerators to produce radioisotopes, as well of

production routes [17]. Hadron accelerators can produce radioisotopes “directly” (cyclotrons,

linear accelerators, electrostatic-machines, etc…). Photo-induced reactions can be used with

electron machines. Neutron-induced reactions (based on high energy spallation sources) or

particle-induced fission reactions can be obtained in accelerator-driven reactors.

Each production route could show advantages and drawbacks depending on the

nature of the radioisotope to be produced. The best route should provide the maximum yield

of radioisotope and the minimum level of impurities. In addition, some other parameters like

costs, facility versatility for efficiently producing several radioisotopes, or the overall footprint

could be critical for a final decision on the selected accelerator.

Finally, it is important to mention that the accelerator itself is not the only

component to be developed for a future mature technology. Targetry, raw radioisotopes


7

processing and recycling should be developed at the same time to result in effective

radiopharmaceutical facilities.

At this moment, when different accelerators are compared, cyclotrons are becoming

the most popular option because they are relatively simple and cheap while they are a quite

mature technology. Different cyclotrons have been developed, including some that are

already commercially available.

Cyclotrons for Radioisotope Production

A cyclotron is a quite simple accelerator machine which accelerates (and guides)

charged particles through a designed path using magnetic and electric fields. The basic

concept of a cyclotron is shown in Fig. 1.2 (extracted from [18]). Particles are emitted in the

ion source inside a uniform magnetic field. Due to the Lorentz force, they bend their

trajectory according to their physical parameters (energy, mass and electric charge). Along

with this circular movement they are crossing electrical fields which accelerate them by

electrostatic gradients. These particles could be as simple as H+ or H- or highly charged heavy

ions.

By using such a machine, many particles can be accelerated up to certain energy.

There are two critical parameters to consider from the point of view of radiopharmaceutical

production: energy and current. Both are referred to the accelerated particles extracted from

the machine. Energy value should be high enough to use these particles for

radiopharmaceutical production. Higher current or energy leads to quicker production rate.

From the design point of view, the main cyclotron parameters are the electric and

magnetic fields. The magnetic field is responsible for bending particle trajectories, so stronger

magnetic fields provide the possibility to reach higher energies at a given radius. The electric

field is responsible for the acceleration: the stronger the electrical field, the higher the energy

supplied to the particles at each turn. Then, a strong electric field means that a certain value

of final energy can be achieved after a reduced number of turns, and therefore lower particle

losses.

In fact, a cyclotron is usually partitioned in three different systems to be solved quite

independently: Ion Source (Production of particles), RF system (Electrical field) and Magnet


8

(Magnetic field). An additional system, Targetry, is needed in case of a cyclotron for

radioisotope production.

The Ion Source is responsible for providing the needed amount of particles to be

accelerated at a specific place, timing and energy.

The RF system is responsible for providing the electrical field. As from the basic

equations of a charged particle moving inside a uniform magnetic field, orbit period is

constant even if the energy changes, (as long as relativistic effects are negligible) a simple

alternating field (constant frequency) can be used for accelerating the particles twice per

turn.

Fig. 1.2. Basics of a cyclotron

For the specific case of a cyclotron magnet and to keep particles turning around, a

“uniform” magnetic field must be supplied to the trajectories region. Then, a cyclotron

magnet is basically a dipole for the most simple configuration [19]. When the magnetic field is

studied in detail, some possibilities can be found depending on the kind of cyclotron:

Classical, Synchrocyclotron or Isochronous [20].

Classical cyclotrons (also called Lawrence cyclotrons) are axially symmetric magnets,

in which the magnetic field is basically uniform but a small decreasing gradient in the

magnetic field strength along radius is designed to provide some focusing to the particles.

This is called weak focusing. Focusing is the action of bending particle trajectories in such a

way that small deviations of a particle in position or velocity from the ideal trajectory are


9

compensated [21]. Two possibilities can be found in terms of focusing strategy: weak and

strong.

Weak focusing is provided by the field itself as explained in the previous paragraph

and classical cyclotrons do not include any strong focusing. If a number of particles move

inside a magnetic field starting at slightly different initial positions, relative distances between

them will change around the movement and eventually these distances could be reduced.

Mathematical equations can be developed to show that the condition for an axially

symmetric cyclotron to perform using weak focusing (for both radial and axial stabilities) is

eq. (3), being eq. (4) the definition of the n-index. Also, axial and azimuthal components of

the magnetic field should be constant to satisfy the stability conditions.

These equations are only valid for non-relativistic dynamics. In case of enough energy

to reach relativistic effects, particle mass is not constant anymore (it becomes a function of

speed) and vertical focusing is not available anymore. The main consequence is that there is a

limitation on the maximum energy that a classical cyclotron can achieve.

(3)

(4)

Strong focusing could be included by design in the magnet to improve focusing

effects on the particles. One magnet can be designed, for example, as the azimuthally varying

field (AVF) which includes vertical strong focusing. This AVF concept is shown in Fig. 1.3 and it

is an example of an isochronous cyclotron.

Alternating magnetic fields along circular paths produce an alternating focusing-

defocusing effect that can be tuned for optimized beam dynamics. A collateral effect coming

with AVF cyclotrons is that particles trajectories are not circles, because bending force is not

equal at valleys (larger air gaps at the magnetic circuit yield low field) and hills (smaller gaps

with higher field). The strategy of isochronous cyclotrons to keep accelerating relativistic


10

particles is to use a constant frequency RF signal, but a radially increasing magnetic field to

provide a constant period for particles trajectory even if they change their mass.

Another variation to handle relativistic effects is the synchrocyclotron. In this case, RF

frequency is not constant. Frequency must change at the same rate than the period of the

particles along the acceleration (mass variations result in period shifts).

Focusing, stability and some other physical parameters of these machines and the

particles are intensively evaluated and computed according to beam dynamics techniques

and procedures which are out of the scope of this thesis. More information can be found, for

example, in references [22] and [23].

Fig. 1.3. Cyclotron poles according to the AVF concept: Rutgers Cyclotron [24]

In any of these cyclotrons, when used for radioisotopes production, once that the

particles reach certain energy, they are sent to the so-called “target system” in which, by

means of physical and/or chemical reactions, the radioisotopes will be produced [25]. A very

important consequence of the extraction of particles, which is made at mean-plane (because

it is the plane in which the particles are actually moving), is that the coil arrangement must be

compatible with the free path needed by the particles to reach the target.

1.2 State of the art

Starting with the first cyclotron, developed in 1932 at Berkeley, the number of active

cyclotrons has raised rapidly. More than 1000 cyclotrons are used worldwide nowadays for

nuclear medicine or research. There are already some review and state-of-the-art papers


11

which summarize the evolution of cyclotrons including the most typical configurations and

approaches, for example [26],[27] or [28]. They cover the whole range of applications and

possible parameters. According to many authors [29] [30], cyclotrons for radioisotopes

production are better classified in terms of their energy, which also defines the major

nuclides that they can produce.

Cyclotrons used for imaging techniques like PET need low energies. A minimum

energy value of 3.7 MeV is needed for producing 15O, while a machine accelerating single

negative particles up to energy in the order of 10 MeV could be used for producing positron-

emitting radionuclides like 11C, 13N, 15O and 18F. Cyclotrons reaching medium energies,

between 15 MeV and 30 MeV, are used for producing radioisotopes for SPECT and several

other PET isotopes. Finally, cyclotrons reaching higher energies, in the order of 100 MeV,

have the capability of producing many more radionuclides for radiotherapy.

Intense R&D is being developed to produce new cyclotrons for nuclides production at

both low (≤ 10 MeV) and high energy (≥ 30 MeV). Low energy cyclotrons are enough to

produce the most demanded nuclides, while they are susceptible of a greater improvement in

terms of volume, weight and operational costs to make them affordable and more efficient.

New concepts on high energy cyclotrons could potentially result in a new universe of

nucleoids or medical procedures to be discovered taking advantage of this available energy

spectrum. Besides, the same improvements in terms of volume, efficiency and reliability are

interesting for existing but innovative techniques like hadron therapy.

About the state of the art in the low energy range, one of the last designs, ION-12SC,

was developed by Ionetix [31]. It is a compact superconducting cyclotron able to reach

12 MeV and 10 µA which are used to provide 13N-ammonia PET radiopharmaceutical. The

extremely short life of 13N isotope (t1/2 = 10 min) yields to a need of a cyclotron producing it

just beside the patient. Compact and cost-effective design is critical in this situation in which

scale economy on doses production cannot be applied. For this reason, the cyclotron is

expected to be portable in a truck, since cyclotron weight is as low as 2 tons [32].

Also, the company ABT Molecular Imaging has recently developed a compact low

energy cyclotron for nucleoid production, which is called BG-75 [33]. Its specifications state a

final energy of 7.5 MeV for positive ions and less than 5 µA beam current. This cyclotron is the


12

core of an integrated solution optimized for 18F production on demand. Simple operation, low

power consumption and reliability are the key points which ABT decided to work on for this

solution. Moreover, a self-shielding device is provided for being able to fit a complete PET

production facility in a small room, which is an advantage for its installation at the PET

demanding sites.

An interesting proposal by VECC (Kolkata, India) is based on the concept of a

superconducting ironless AVF cyclotron. The magnetic field shape is completely generated by

the main coils plus 4 sector coils. Expected performance would reach a beam of 25 MeV

protons weighting just 2 tons [34].

There are some other commercial designs on low energy cyclotrons available

nowadays:

1. ISOTRACE is based on the Oxford Instruments cyclotron OSCAR-12, modified

to provide a beam current of up to 50 µA at a fixed energy of 12 MeV. Its

weight is 4.5 tons.[35]

2. GE Healthcare has developed MINItrace, a cyclotron to accelerate 50 µA of

9.6 MeV H-. The weight for the total system, able to provide 18F, is about

50 tons[36].

3. Siemens Healthcare design, Eclypse RD Cyclotron, is focused on flexibility,

being able to produce 13N, 15O, 11C and 18F. It accelerates H- up to 11 MeV,

with a magnet weight of 10 tons [37].

4. Cyclone 11 is the main cyclotron in this category from IBA, which accelerates

H- up to 11 MeV and it is based on the previous version Cyclone 10 [38].

Moving from the low to the high energy range, R&D is mainly focused on reducing the

footprint and operational costs for the whole facility. But in this case, magnet design is not so

challenging because of the restrictions coming from the physics laws underlying the

operational concept, as explained in the previous section. As the energy is raised, the radius is

increasing due to the difficulty of achieving strong magnetic field. Because of this reason,

“compact” cyclotrons for high energy levels are, anyway, much bigger than low energy ones

and the impact of an effort on reducing the magnet it is not so critical for the facility. The

LuisGarciaTabares

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13

main R&D projects for high energy cyclotrons are not focused on the compactness of the

cyclotron, but on the optimization of some other important components, like gantries and

beam lines for delivering the particles to the proper destination.

In fact, at these energy levels, up to 300 MeV, cyclotrons are competing with

synchrotrons [39] and LINACS (LINear ACcelerators) [40], as they are also candidates for

providing proton or carbon ions for hadrontherapy. Moreover, the possibility of using a low

energy cyclotron followed by a LINAC to increase energy before the beam reaches the

patient, which is called cyclinac, is being explored by the TULIP project at CERN [41]. An

interesting review on the comparison of every solution can be found at [42]. The energy of a

cyclotron is typically a fixed value, so absorbers are needed to fine tuning the energy

deposition on the patient, but the time needed for changing the energy is in the order of

100 ms and some additional and dangerous radiation can arise from them. Conventional

synchrotrons can vary the energy by themselves, but in a time scale in the order of seconds.

This is not quick enough for moving organs, like heart or lungs. Linacs and cyclinacs are the

best options according to their energy variation flexibility, as they can supply proton beams of

varying energy in just few milliseconds. The disadvantage of linacs is the volume needed for

the facility. Cyclinacs are more compact but they are more complex and expensive as they are

in fact two accelerator systems coupled together, both needing their own design and

maintenance.

Table I summarizes this brief explanation on some existing cyclotrons for comparison.

Some others are also included with their references for further information.


14

TABLE I SELECTION OF CYCLOTRONS FOR RADIOISOTOPE PRODUCTION

Cyclotron

name Company

Energy

(MeV)

Current

(µA)

Magnetic Field

(T)

Weight

(tons) Ref.

BG75 ABT 7.5 5 1.8 3.2 [43]

GENtrace GE 7.8 ≈25 2.2 6.7 [36]

MINItrace GE 9.6 >50 2.2 9.1 [36]

Eclypse Siemens HC 11 >120 1.9 11 [37]

Cyclone10 IBA 10 >150 1.9 12 [44]

ISOtrace Oxford Ins. 12 ≈50 2.4 4 [35]

Cyclone11 IBA 11 120 1.9 13 [44]

ION12 Ionetix 12.5 ≈2 4 2.3 [31]

PETtrace GE 16.5 >100 1.9 22 [36]

Cyclone18 IBA 18 150 1.9 25 [44]

TR24 ACSI 24 >300 2.1 84 [45]

Cyclone30 IBA 30 <1500 1.7 50 [44]

Cyclone70 IBA 70 <750 1.6 145 [44]

BEST70 BEST 70 700 1.6 195 [46]


15

1.3 The AMIT Project

The AMIT (Advanced Molecular Imaging Technologies) project was a Spanish research

project focused on developing the core technology needed for molecular imaging in Medicine

and Biomedicine, in special for human brain and mental diseases [47]. It worked in two main

challenges: Increasing the capability of medical centers to access to radiopharmaceuticals and

improving the tools for a better diagnostic coming from the information provided by PET

scans.

The first scope of this project fits perfectly as an application for this thesis, in which

several designs of a machine able to produce radiotracers for PET (18F and 11C) will be

explored and a prototype will be manufactured according to the guidelines developed here. A

cyclotron fulfilling the specifications of compact, autonomous, efficient and based on-

demand production of radioisotopes will be a great achievement for cheaper and wider

access to radiodiagnosis.

1.4 Thesis objectives

The main objectives of this Thesis include the goals related to the development of a

really innovative solution or the exploration of the frontiers of technology regarding compact

and efficient superconducting magnets for radioisotope production cyclotrons. In order to

accomplish the challenge of developing such a cyclotron under AMIT project premises, partial

objectives will be defined. Secondary objectives are not so critical for a successful machine

but they are desirable for an optimal result. These are summarized in TABLE II.

Main objectives

The first objective is the development of a procedure for the design, manufacturing

and testing of compact magnets suitable for efficient cyclotrons aimed to low energy

radioisotope production. This is a multidisciplinary work, and the state of technology will be

checked in several fields at the same time. The main challenge of this thesis is to provide a

procedure for the design of such magnets while considering the whole picture and the

associated technologies.


16

The efficiency of this magnet design will include decisions related to manufacturing,

operational and maintenance costs besides the radioisotopes production ratio.

Compactness will be defined in comparison with similar cyclotrons but an additional

feature will be required: it should be compact enough for installation in radioisotopes user’s

facilities (e.g., hospitals).

These previous objectives (efficiency and compactness) can only be achieved if one

additional requirement is satisfied: This cyclotron must be able to operate in a non-dedicated

facility. This means that specifications in terms of power supply, refrigeration demands,

operational risks and environmental effects should be limited enough for this kind of facilities.

Finally, the last main objective is to provide a magnet design able to be used as a

prototype. Every component should be able to be easily dismantled, minimizing production

dead times due to maintenance or any other technical manipulation of the machine. This is

extremely important for example for the targetry system and the ion source, as they could be

changed for experimental reasons or for producing a different radioisotope. This first

prototype magnet should be versatile enough for technology development and testing,

providing interesting data for further designs in the future.

Secondary objectives

Some additional objectives are desirable. They could be also considered as a first

attempt to check the feasibility of further improvements in a second version of the prototype.

The first secondary objective is the magnetic field optimization according to beam

dynamics for reaching the best results in terms of current and radioisotope production ratio.

Measuring accurately the magnetic field will require the use of a MMB (Magnetic

Measurement Bench). In the framework of a Collaboration Agreement, ALBA-CELLS

(Cerdanyola, Spain) will provide the MMB for these measurements. This device will be

installed at CIEMAT-CEDEX facilities. Mechanical relative positioning between field sensors

and magnet is also important, so that metrological equipment will be needed during the

measurements. Finally, a custom program will be coded for processing the data, so that this

objective will be considered completed once that the magnetic field is optimized and


17

measured in the whole volume where particles will be accelerated and guided, from the ion

source to the target.

The second additional objective is to provide a system as autonomous as possible.

This requirement means that additional supplies should be reduced as much as possible so

that operational costs are minimized. Even when the main objectives are met (i.e., the system

is efficient), autonomy is a further step on the overall specifications. Nevertheless, it could be

a critical point to take into account when installation in a non-dedicated facility is expected.

One example of this matter is the possibility of using common power lines for feeding the

whole system with no additional electrical installation or supply needed. Another one is to

provide the whole refrigeration power needed from conventional systems. Some cyclotrons

require a dedicated facility for refrigeration or periodic refilling of expensive cryogens. These

issues should be avoided if possible. Collaboration with the Technology Department at CERN

is expected for this objective, because they have a lot of experience on particle accelerators,

including of course expertise on refrigeration schemes.

Both secondary objectives are important in a medium-term future, but this work will

focus on the main ones. The secondary ones will not be considered mandatory because of

two main reasons: Firstly, there are significant technological risks associated with them as

complexity increases noticeably, and secondly, these two objectives are not completely

within the scope of this work. Both of them are being done in collaboration with different

institutes, so that schedule is not overlapping exactly with the timeframe of this Thesis.

Alternative objectives

Even when secondary objectives are not considered mandatory for this work, some

alternative objectives will be defined just in case the secondary ones cannot be achieved.

Some possible problems when measuring the magnetic field in the whole volume

inside the cyclotron could happen. For example, the availability of the MMB is not guaranteed

as it is property of ALBA-CELLS. Moreover, this cyclotron will be the first system to be

measured with it, so additional difficulties could arise. In these cases, or any other in which

the magnetic field cannot be accurately measured with the MMB, the alternative objective

will be to measure the magnetic field by means of conventional measuring methods. These

measurements have been done in the past at CIEMAT using own tools. The accuracy of the


18

measurements will be diminished in the 3D volume compared with the MMB expected

specifications. In this situation, the magnetic field quality could be limited if this secondary

objective is not met. Anyway, this alternative objective of adjusting the magnet by measuring

its magnetic field by conventional tools should be enough for the particle acceleration, but

only for a limited beam current. A beam current lower than expected will reduce the

radioisotope production ratio but the beam energy will allow to produce them.

If thermal losses are higher than expected and autonomous operation cannot be

achieved, an alternative procedure will be foreseen for a proper refrigeration scheme. This

alternative method could be based on conventional techniques like cryogen refilling or some

other hybrid solutions to reach the main objective of a working and reliable cyclotron.

Finally, a situation in which the magnet cannot completely provide the expected

magnitude of magnetic field is also a possibility. In this case, a weaker magnetic field would

reduce the energy values of the particles able to reach the target. For a given radioisotope,

there is a hard physical constraint in the energy value needed. If the expected energy is not

achieved, at least this minimum value for the desired radioisotopes production should be

reached. Obviously, in this case the production ratio and, therefore, the system efficiency

would be reduced.


19

TABLE II THESIS OBJECTIVES

Main Objectives Secondary Objectives Alternative Objectives

Development of a procedure for designing accelerator magnets for radioisotope production

Optimized magnetic field according to beam dynamics for optimum production ratio

Tuned magnetic field good enough for extracting radioisotopes

These magnets must be compact and efficient

Autonomous system, no special requirements or refills, including cryogenics

Hybrid solution for cryogenics, including conventional cooling schemes if needed

One magnet will be manufactured for testing the technology and design method

LuisGarciaTabares

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LuisGarciaTabares

Resaltado

Chapter 2. Cyclotron Conceptual Design

As explained in Chapter 1, this Thesis will study and develop a method for designing

magnets for low energy cyclotrons for radioisotope production while optimizing compactness

and efficiency. In this Chapter, the main concepts behind the design of such machines will be

analyzed, with special attention to those features and choices which are significant for the

magnet layout.

2.1 Main specifications

A variety of parameters must be fixed since a cyclotron is a complex machine which

includes many subsystems, so different studies were done to decide their values. The first of

them is the particle energy needed for producing the radiopharmaceuticals. As the AMIT

cyclotron will be the application of this method, it must be able to produce at least 18F and

11C.

The time needed for producing one dose of 11C is shown in Fig. 2.1. It is extracted

from [48] and it was calculated from data obtained from the IAEA Nuclear Medicine Database

[49]. The production time depends on the energy and particle current, once the energy is high

enough. Basically, 6.5 MeV is the minimum energy to produce it, but as far as half-life time is

so short, a higher rate is needed to produce single doses in a reasonable time. A typical

current value for a small cyclotron is in the order of 10 µA. This is limited by the ion source

and the magnetic field map accuracy. Ion sources technology is a quite mature field in which

no great advantages can be foreseen in the near future, while the magnetic field map

accuracy is supposed to be of standard quality for an accelerator machine at this stage of the

design (field non-uniformities measured in few units per 10000).

From these assumptions, producing 11C (half-life of 22 min) with a 10 µA beam

current in a reasonable time (in the order of 7-8 min each dose, one third of the

disintegration rate) will require at least 8.5 MeV.


22

On the other hand, the specific type of cyclotron must be selected. As explained in

the state of the art Section, each cyclotron concept provides certain advantages and

drawbacks. For a low energy and high beam current, a classical cyclotron is much cheaper

than any other possibility. Furthermore, increasing the magnetic field will result in a

comparatively much complicated ion source and focusing scheme. Weak focusing is enough

given the beam parameters (mainly energy) so there is no need for a strong focusing

machine.

Fig. 2.1. Radiation energy (horizontal axis) needed for providing a 11C single dose in terms of

time and beam current (vertical axis).

Once the energy is fixed, eq. (5) provides a mathematical correlation between the

minimum beam trajectory radius r and the magnetic field B in terms of particle mass m and

electrical charge q for a classical cyclotron.

It is quite straightforward that the higher the magnetic field is, the lower the radius,

so that the compactness is maximized. Some features could be used to achieve that objective:

(5)


23

1. Ferromagnetic materials as iron can be used to enhance the magnetic field

intensity, but they are limited by saturation. Even more, particles need a high

vacuum environment to be accelerated without collisions, so at least one iron

gap is needed in the magnetic circuit to allocate the vacuum chamber. Also,

the ion source needs some room for the particles to be injected into the

cyclotron. This air gap could be optimized to mitigate its effects on the

magnetic field as much as possible, but saturation at high fields is

unavoidable (around 2 T).

2. Bigger coils or higher nominal current can of course increase the magnetic

field intensity, but the overall cyclotron size would be increased, likewise the

cooling power.

3. Another possibility is to use superconducting technology, which allows a good

compromise to increase the number of ampere-turns while compactness is

preserved. In fact, this compactness has consequences for the major

concerns of the near-future challenges of radioisotope production worldwide.

2.2 A Compact Superconducting Cyclotron

In the past, a layout of large dedicated facilities for radioisotope production was

found to be a cost-effective method for supplying the serialized production of

radiopharmaceuticals demanded by high-density population areas (cities). At this moment,

the medical sector is claiming for new features to radiopharmaceuticals supply that overcome

the next main drawbacks [50]:

- Limited availability of radiopharmaceuticals on demand: a large facility designed for

a serialized production of just one or few species of radioisotopes is not flexible

enough to accommodate an “on-demand” production strategy.

- Limited lifetime of radioisotopes (inevitable physical constraint) results in an

inefficient consumption of radiopharmaceuticals. Delivery of a radioactive material

from the production center to the hospital and then to the patient can be expensive

and time consuming. Half-life time for 18F is about 110 minutes. This means that the


24

amount of active radioisotope is halved after two hours and therefore the

production must be enlarged accordingly to the time gap from the production to the

final use.

- The scheme itself is inherently biased to a very limited market, dominated by a small

number of competitors, each one typically controlling one large area or region. The

main reason to explain this situation is the great capital investment and,

consequently, the need of a long-time demand of a large amount of serialized

production of radioisotopes.

Thus, a new production method is desired to make possible the following

improvements:

- Flexibility for providing single doses of non-standard radioisotopes which could be

useful for improving diagnosis or treatments of specific diseases or patients (custom-

made radioisotopes)

- Radiopharmaceuticals production at the hospital or institute themselves, reducing

delivery time and radioactive decay of the active molecule. It will enhance the ratio

of useful to manufactured amount of the compound.

- Possibility of producing its own non-standard radioisotopes will enhance the market

competitiveness. This could reduce the costs for these custom-made

radiopharmaceuticals and improve their availability to a larger number of potential

patients.

- Finally, on-site production in non-dedicated facilities will make radiopharmaceuticals

accessible to small cities or not so densely populated areas.

On the other hand, by using large and dedicated cyclotrons it is relatively simple to

reach enough energy and current on the accelerated particles. Because of that, cyclotrons

have been typically chosen as the ideal accelerator for isotope production [51]. However,

such a simple configuration yields some drawbacks: radiation protection of the whole facility,

size, weight and costs (manufacturing and operating).

One of the main points to be dramatically changed from the “traditional” production

technique is the accelerator machine layout. Typical accelerators used for


25

radiopharmaceuticals production are large dedicated cyclotrons, while new compact

cyclotrons can offer real advantages for an installation close to the final users. Shielding size

depends on the cyclotron dimension. The whole facility footprint could be drastically reduced

by using a compact cyclotron while the radiation levels are kept at minimum values. In fact,

these are the most important concerns when installing radioisotope production facilities close

to the final users (mainly in cities). Some other ancillary subsystems should be included in this

analysis. Power supply and electrical consumption of the facility could be an issue for existing

hospitals in terms of installed power capacity and/or operational costs. Also, additional

specialized manpower could be needed in the hospital for maintenance or, at least,

operation.

Superconducting compact cyclotrons are expected to be the most suitable machines

for solving or minimizing most of the aforementioned problems [52]:

- Superconducting magnets can reach stronger magnetic fields by using much higher

current density. This means a smaller machine for a given energy. The cyclotron

weight is approximately proportional to the third power of the extraction radius.

- Taking advantage of iron properties to enhance magnetic flux felt by particles and to

reduce magnetic field levels outside the machine (reduce fringe field) means that a

superconducting magnet decreases particle trajectory radius for a given energy. The

total iron weight scales as the total volume (power 2 to 3 of the particle radius

trajectory), and so the iron weight (which is the main component of total weight) is

dramatically reduced.

- Investment costs could be reduced. Some parts are of course more expensive than in

a non-compact or non-superconducting cyclotron (e.g., the coils) but some of the

most relevant initial costs can be reduced, being the shield the main one.

- Operational costs can be also reduced by superconducting technology. Even when

superconductors require cryogenics, the electrical supply is dramatically reduced in

comparison to the equivalent resistive magnet (normal conducting).

Superconductive state is a special matter phase in which the material shows zero

electrical resistivity. This superconducting behavior has been found in some materials, and, as


26

any other phase transition, superconducting-resistive transition depends on several intrinsic

parameters. For a given material the main ones are temperature, current density and

magnetic field. These three parameters for type I superconductors are linked to a transition

surface named critical surface. Any combination of these parameters above the critical

surface will produce resistive behavior of the material and vice versa. Critical surface of NbTi

is shown at Fig. 2.2 as found in [53].

The greatest advantage of a superconducting magnet for being accelerator magnet,

as explained in the previous chapter, is the current density. Then, accelerator magnets are

typically designed for the highest feasible current density. Nevertheless, cost-effective criteria

should be considered. For example, a slight improvement in current density could be not

profitable if the manufacturing costs are dramatically increased. In fact, the suitable

possibilities are not so many because of some important reasons [54]:

- Most of the superconducting materials handled by current technology are ceramics.

Ceramics are typically fragile, and they cannot be extruded to wire shape so easily as

for example ductile metals (common copper wire). This implies a lot of problems in

terms of manufacturing reliable long cables. The developing costs of a new (or a

custom-made) wire are huge. On the other hand, using a well-known and available

wire can be much more efficient in terms of economic terms, even if it is not the

optimum from the magnetic point of view.

- Operating temperatures for superconducting wires require cryogenics. Low

temperature superconducting wires need Helium for refrigeration, since there is no

other cryogen capable to operate at such a low temperature. High temperature

superconductors could be operated being refrigerated by Helium, nitrogen or a few

other possibilities (argon, neon, oxygen and hydrogen). In any case, the operational

temperature of the magnet is quite related to the liquefaction temperature of the

cryogen and its latent heat (vaporization energy). This temperature can be slightly

modified by changing cryogen pressure, but there is no doubt that the working

temperature of the wire is not a completely free parameter.

- Finally, the magnetic field in the superconducting wire is coupled to the magnetic

field map and size requirements for the accelerator magnet.


27

Being constrained or limited by the possibilities about material, manufacturing

process, temperature and magnetic field, the only free parameter to optimize is the current

density for the chosen wire for the given environment (cryogen, temperature, etc.). There is a

maximum value of the current density, so-called critical, but the designer is free to choose the

operating current density below the critical one.

Fig. 2.2. Superconducting transition of a resistive material

Another important aspect of accelerator magnets is the so-called field quality. This is

about how perfect is the magnetic field in the volume in which particles are travelling. This is

critical for circular accelerators, because any imperfection of the magnetic field at a certain

point will be applied to the particles at each turn. Moreover, as particles are not moving in

the ideally centered trajectory, small imperfections in the magnetic field could produce

misalignment amplification, spreading out the particles from the desired trajectory. For this

reason, accelerator magnets should be able to bend particle trajectories without beam losses.

Field quality is measured in units per 10000 compared to the main harmonic, which means

much better fitting of the ideal magnetic field profile to the real one than any other

conventional magnet. Because of this, accelerator magnets and, in particular, compact

superconducting cyclotrons, typically need some fine tuning once manufactured to reach field

quality requirements. It is more complex for superconducting magnets than resistive ones

because the fields are higher and the dimensions smaller, so the field is more sensitive to

misalignments and dimension errors.


28

Compact cyclotrons are also much more complex in terms of the ion source, targetry,

cryogenics… All these topics should be studied in detail. This Thesis is focused on the magnet,

thus magnetic field, structural and thermal design will be the addressed challenges. All the

others are out of the scope (as far as their interactions do not affect the magnet), so

reference [55] is recommended for more information about them.

Once that a superconducting magnet has been set as the preferred option and the

energy is fixed (8.5 MeV minimum), coming back to eq. (5) and discarding the effects of iron

saturation (because the coils can provide higher magnetic fields than the 2 T limitation of the

iron), the new limit on the magnetic field is linked to the choice of the superconductor

material (critical magnetic field).

Considering the materials available in the market, NbTi is well established as a mature

technology. For a superconducting magnet up to 4 T, NbTi technology is the common

solution, so it is quite easy and cheap to procure NbTi wire long enough for the coil

fabrication. On the other hand, NbTi material is not so good for values of magnetic field

above 4 T because its critical current density decays noticeably. In order to produce a magnet

working at higher magnetic fields, MgB2 or Nb3Sn could be selected, but these materials are

quite exotic choices as their technology is not well established and nowadays the material

itself is too much expensive within the scope of a cheap cyclotron. A conceptual design of a

HTS-based magnet cyclotron has been described in [56] as a possible path to be explored in

the future.

This limitation on technical and commercial availability of the superconducting wire is

then the reason to set the magnetic field at 4 T.

As explained in the introduction, there are a number of cyclotron concepts. For that

magnetic field (4 T), the classical cyclotron or AVF could be used. A classical cyclotron needs a

stronger electrical field for acceleration but an AVF is much more complicated for high

magnetic fields as 4 T. A classical cyclotron was selected as the best choice for a compact and

efficient machine taking into account the desired electromagnetic fields and energy [57]

because the more complicated strong focusing provided by the AVF cyclotron is not necessary

at this energy level.

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2.3 Cooling concept

As explained in the previous Section, superconducting state is reached just under

certain conditions. Typical parameters involved, for “classical” superconductor materials, are

temperature, magnetic field and current density. Because of the first requirement, cryogenic

design and supply become critical tasks to face for a superconducting machine development.

Cyclotron cold mass configuration

The superconducting coils will require cryogenic temperatures to develop

superconductivity, but there is also a possibility of cooling just the coils or the whole magnet

(including, for example, the iron yoke if it exists):

- “Cold iron” layout: Both the coils and the iron are cooled down. In this situation, the

iron itself could be part of the support structure for the coils. The magnet would be

very compact, because there is no need for thermal insulation inside the volume in

which the magnetic field is high. The main drawback for this solution arises when

analyzing thermal transients. As low cooling power is desirable for an efficient

magnet, cooling down the whole iron yoke (in the order of tons for low energy

cyclotrons) and the coils (hundreds of kilograms) would take much more time to be

cooled down than just refrigerating the coils.

- “Warm iron” layout: For this option, coils are cooled down while the iron yoke is at

ambient temperature. Cold mass is small, but additional free space between iron

and coils is needed for support structure and thermal insulation.

Regarding the total volume for the magnet, both are similar because for the first one

the active parts of the magnet are optimized in terms of volume, but the supports and

thermal insulation for the whole iron will be much bigger.

As a conclusion, transient operation for both configurations should be considered. For

magnets including small mass of iron yoke, or when cooling power is high enough for a quick-

enough cooling down for the application, the “cold iron” solution could be a good candidate

and it should be studied. In fact, a recent patent was settled in United States for a cold iron

cyclotron [58].


30

In the specific frame of this work, the “warm iron” solution is preferred for AMIT

cyclotron given the constraint on the cooling power. As a first rough estimation, one

commercial cryocooler can cool 200 kg of copper down to around 4 K in about one week.

Cooling down 300 kg of copper plus 3 tons of iron would spend about two months:

Given the specific heat of iron from ambient temperature to 4.2 K [59], around

77000 J/kg are needed to cool down it. Assuming a mean value for cooling power of the

second stage of the cryocooler around 50 W [60], it would take 53 days to cool down 3 tons

of iron. This simplified calculation is not taking into account the coil and supporting structure

which could be estimated as an additional 10% more time. Of course, this is just a rule of

thumb but enough to discard cold-iron concept when the whole cryogenic supply relies on

just one cryocooler and the availability of the system is critical for production rate.

Cryogen supply

Up to four different possibilities can be found for the cryogen supply to

superconducting devices:

- Bath-type refrigeration: In this design, certain amount of liquid cryogen is directly

injected into the system. Enthalpy change in the cryogen is used to refrigerate the

material, which is kept at the vaporization temperature of the cryogen. Evaporated

Helium can be used for refrigeration of parts at intermediate temperatures. The best

and most important example is the current lead refrigeration. They provide a path for

the current to flow from ambient temperature (power supply) to the coil (liquid

Helium temperature), so it is an important input heat source. It can be reduced

following several strategies to refrigerate the current leads with the Helium vapor

before letting it go out. This is the simplest way to cool down the material, but it is not

typically efficient for a long-lasting and continuous operation machine. It requires a

periodic refill of cryogen, which can be expensive.

- Conductive refrigeration: One or more cryocoolers are in contact with the material to

be cooled down. A cryocooler becomes cold by means of a thermodynamic cycle

inside, while the refrigeration cryogen stays inside the cryocooler. Thermal contact is

critical in this strategy and limited power is available for each cryocooler. Moreover,


31

both magnetic and cryogenic systems are completely related to each other: some

space close to the coils needs to be available for the cryocooler cold head. Also, as

cryocoolers efficiency is reduced by magnetic fields, some shielding could be needed.

Finally, accessibility for maintenance operation of cryocoolers should be taken into

account during design. Because of these reasons, magnetic circuit design is linked to

the thermal one in this scheme. Conductive refrigeration is also the only configuration

which cannot be easily scaled in terms of cooling power. Once that the design is fixed

for a given cryocooler, it cannot be updated by changing it or adding higher amount of

cryogen like in the other options. This should not be a drawback for a well-known

product, but this work is focused on a prototype machine which could be used as the

baseline for a number of updates or improvements in the future.

- Hybrid solution: One or more cryocoolers are used in this scheme, but they are not in

direct contact with superconducting material, which is actually refrigerated by certain

amount of liquid Helium like in the bath-type configuration. The difference is that the

evaporated Helium is liquefied again by the cryocooler inside the cyclotron cryostat.

Liquefied Helium returns to the bath, so in this case the objective is to keep a constant

liquid level without any refill.

- Flow refrigeration: Certain amount of cryogen is cooled down and then it is pumped

into the system to be cooled down. It could be an open or closed cycle; it depends if

the exhaust cryogen is recovered or not. Cryogenic and superconducting systems can

be designed independently. Large systems are usually based on this configuration as it

allows developing a scalable cryogen liquefaction facility based on the thermodynamic

cycle preferred for that specific application.

As this work is intended to provide a design procedure for single cyclotrons to

produce on-demand radiopharmaceuticals close to the patient, there is no option for a large

liquefaction cryogenic facility. This type of cyclotrons should be autonomous and cheap, so

that bath type refrigeration is discarded as it would require periodic refill of cryogen by skilled

technicians, which means also higher operational costs. The conductive and Hybrid solutions

are quite interesting possibilities, which in fact were explored. They were finally discarded

because of the compactness objective. They would require accommodating the cryocooler


32

close to the coils, so that the iron size (and so the cyclotron size) would increase. Field quality

could be affected also because the cryostat and iron designs are cumbersome.

Flow refrigeration was then the preferred option for this work. For autonomous,

cheap and compact radioisotope production facility, a closed cycle cryogenic system is

selected, in order to eliminate the need of periodic refills or the construction of a large

cryogenic system. The design and commissioning of a custom cryogenic system able to

provide and pump the needed liquid Helium is a difficult task. Therefore, autonomous

liquefactor system for producing this liquid Helium was developed in collaboration with CERN.

It was called Cryogenic Supply System (CSS). It includes one commercial cryocooler inside.

Using just one cryocooler for liquefaction was found to be feasible (and challenging) by CERN

cryogenic experts. This decision perfectly fits into the main objective of a compact and low

cost facility (for both investing and operational costs, like maintenance or electrical

consumption).

The core of CSS is one SRDK-415D Sumitomo cryocooler [61]. It is based on a Gifford-

McMahon cycle able to provide up to 1.5 W of cooling power at its second stage (at 4.2 K)

and 35 W at its first stage (at 50 K). Typical load map from supplier is presented in Fig. 2.3.

The actual working point will depend strongly on the thermal power demanded by the system

at each stage. The temperature of the second stage must be low enough for Helium

liquefaction (i.e., 4.2 K at ambient pressure), thus the thermal power at the first stage could

be 30 to 60 W while its temperature will be around 40 to 70 K for reaching 1.5 W of cooling

power at the second stage. These values are valid just for the cryocooler, but additional losses

are expected when assembled inside the CSS system.

The CSS is an assembly of heat exchangers, heaters and temperature sensors that

receives two inlets of gas Helium (ambient temperature and cold gases). It cools them down

to first and second stage temperatures, respectively. Once the steady state is reached, the

latter one will become liquid Helium.


33

Fig. 2.3. Sumitomo cryocooler RDK415 load map

This CSS system will be used along with this work as the baseline for the thermal

design of the magnet so available cooling power will be considered constrained below 1,5 W

which is one of the most challenging specifications for this work. Nevertheless, at the present

state of the art of cyclotrons, they are typically designed for bath-type (higher amount of

cooling power demand compared to 1.5 W) or conductive refrigeration by means of several

cryocoolers.

Theoretical values for the CSS, according to its concept design [62], are detailed in

TABLE III. Nominal powers for the first and second stages depend mainly on the selected

cryocooler, which is the most powerful among the commercially available nowadays.

Cryocoolers with less power have not been considered because the design is already very

challenging for this available cooling power. Nominal mass flow takes into account Helium

properties, refrigeration power and overall dimensions of the system. Nominal cooling power

supplied by the available cryocooler is 1.5 W at 4.2 K. Latent heat of liquid Helium (at 1.3 bar

which is the expected value for nominal pressure, close to ambient, to be justified later) is

about 19 J/g, so a mass flow of 0.1 g/s is considered for first rough design of the system. This

preliminary number will be important for the thermal design. It is a very low number, and

special care should be taken to evacuate 1.5 W at constant temperature (vaporization

temperature) in a limited space (coils) far away from the cryocooler (at least outside the iron

as exposed before).


34

TABLE III NOMINAL THERMAL POWER AVAILABLE FROM CSS

Parameter Value

First stage nominal power and temperature 1.4 W @ 4,5 K

Second stage nominal power and temperature 25 W @ 40 K

Nominal mass flow (Helium) 0.1 g/s

2.4 Definition of magnet interactions with other cyclotron subsystems

Basic subsystems included at the cyclotron are:

- Superconducting magnet

- Accelerating subsystem

- Ion source

- Target and particle extraction

- Control system

All of these systems must be compatible between each other, in terms of

specifications, availability, robustness, and, of course, geometrical constraints.

Even when this Thesis is about the development of the superconducting magnet,

some details of the other subsystems will be explained. Interfaces between subsystems are,

at several points, important or critical design constraints for the magnet.

2.4.1 RF subsystem

The accelerating subsystem will be based on a single 180º dee. Given the expected

magnetic field and radius, an electric field up to 60 kV will be needed [57]. These numbers are

considered as input parameters for this work as their definition and the work done on the

conceptual RF design to get them is out of the scope. So just as a brief explanation, the higher

the electrical field, the lower the number of turns needed to reach certain energy (energy

supplied at each turn is proportional to the field intensity) and it could lead to lower beam

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35

losses ( lower losses coming from collisions in the non-perfect vacuum and lower chances for

instabilities to grow). The selected electric field of 60 kV is quite a big number for a compact

cyclotron. It will need a considerable volume to avoid breakdowns, even if the design is

optimal. Besides, too much high voltages could produce stripping losses and would increase

RF losses (proportional to V2). In any case, it will certainly affect the magnetic yoke geometry

and magnetic field quality, so it is important to take it into account from the very beginning.

RF resonator was designed to be out of the iron yoke for magnet compactness, but a

connection between the resonator and the dee (which is of course inside the magnet) should

be provided to allow the current flow between them. Also, refrigeration water will be used at

the dee for keeping temperature during operation at safe values. Because of these reasons,

and the maximum vertical amplitude of the beam according to beam dynamics analysis (+/-

6 mm from midplane), the following free volume through the magnet should be provided:

Length 280 mm, height 74 mm. The height directly determines the air gap in the magnetic

circuit. This choice is a tradeoff between the necessary number of ampere-turns and the RF

power.

Moreover, a second cylindrical hole of 210 mm radius should be drilled to allow the

resonator to be as close as possible to the iron, minimizing the total dimensions of the

cyclotron and, the most important reason, to reduce power losses in the RF cavity [57]. A

draft image of this concept is shown in Fig. 2.4.

In fact, these holes in the iron could produce perturbing resonances in the beam

dynamics, so it is needed to replicate them at the opposite side in the sake of symmetry. As

these additional holes must be drilled, ion source and targetry will take advantage from them

to provide accessibility.

Finally, the magnet should be able to withstand not only its own weight and magnetic

forces, but also the weight coming from the RF system, which will be partially hanging from

the iron yoke for alignment reasons.


36

Fig. 2.4. RF system overview (already installed on cyclotron magnet)

2.4.2 Ion Source

The Ion source is responsible for providing the particles to be accelerated (or

energized to be more precise in general).

For a compact and low cost cyclotron, it is preferable to include an internal ion

source. The beam transport from an external ion source through the iron and inside the high

field region is cumbersome. However the interfaces between the internal ion source and the

magnet are also challenging.

First of all, the internal ion source should be small in order to be accommodated in a

quite restrictive volume inside the cyclotron. Due to the strong magnetic field and the limited

electric field for beam extraction, the first orbit radius is small. The ion source has to be

placed inside the volume defined by the first orbit.

Moreover, the ion source is a system that needs significant maintenance and tuning.

Because of this reason, there should be an easy and quick procedure to tune and even

replace the ion source system or any of its components.

This internal ion source is inside the RF cavity for optimizing the inner volume of the

cyclotron. Therefore, the RF cavity must be accessible to be extracted from the iron yoke and

easily opened for accessing this subsystem [63].


37

2.4.3 Target and extraction

The AMIT extraction system is based on the stripping foil method. This is basically a

thin carbon foil (pyrolytic graphite) which strips off the electrons of the H- particles. As H-

particles become protons (H+) when the nuclei lose both electrons, their trajectory bends into

the opposite direction of the incoming beam. Stripping foil nominal position was established

at 108 mm radius by beam dynamics study (Fig. 2.5), so the magnet was designed to provide

a good quality focusing field up to 110 mm [64].

The available lateral hole opposite to RF resonator was used for the extraction path

of the protons. This is for allowing the targetry system to be as close as possible to the carbon

foil, reducing the beam losses and radiation. The targetry system will use the extracted

protons to produce the radioisotopes needed for pharmaceuticals. This is also out of the

scope of this thesis, more information can be found in [65].

Fig. 2.5. Cyclotron mid-plane cross section showing extracted particle ideal trajectory.

2.5 Summary of Concept Design

In this Chapter, the Author describes a general procedure for the conceptual of

superconducting magnets when the compactness and the efficiency are the most important

objectives. Each decision is analyzed and finally a solution is defined based on the

specifications coming from the AMIT project. A brief description of other additional

subsystems in the cyclotron is provided when they affect the magnet design.


38

The main contribution from the Author in this Chapter is the compilation of the state

of the art and the related literature to provide a reasonable methodology and guidelines for

superconducting magnets development. The contents of this Chapter were widely discussed

between most of the members of the Accelerators Technology Unit (CIEMAT), providing a

multidisciplinary overview and agreement. In addition, the innovative cooling concept was

developed in closed collaboration with an external institute and its experts.

Some of the contents of this Chapter were shown in AIME16, Madrid (Academia-

Industry Matching Event) [66] by the Author, and in the IPAC17, Copenhagen (International

Particle Accelerator Conference) [64].

Chapter 3. Electromagnetic Design

As it was already explained, on one hand, classical cyclotrons are relatively simple in

terms of the required magnetic field map. On the other hand, very tight precision should be

reached in order to provide weak focusing to the particles and finally extract enough number

of particles (current).

In the particular case of AMIT cyclotron, magnet specifications (TABLE IV) are coming

from the beam dynamics and the combination of efficiency and compactness, as explained in

previous chapter.

TABLE IV MAIN MAGNET SPECIFICATIONS

Parameter Value

Cold mass distribution Warm iron

Magnetic field 4 T

n index 1.5 %

Extraction radius 108 mm

Air Gap Height >74 mm

3.1 2D concept and refinement (Pseudo 2D)

The three usual electromagnet designs able to create the uniform field required by

the cyclotron are evaluated below:

1. Solenoid: This electromagnet is based on a single coil and it is the simplest

possibility. If this coil is long enough compared to its diameter, the field inside

the coil at the mean plane is uniform along the cross-section. Solenoid coil is

discarded for this cyclotron because there is no option for mean plane access.

As explained in the previous chapter, a cyclotron requires a path at mean


40

plane for particles extraction, access for ion source maintenance and space

for the electrical connection of resonator and dee.

2. Helmholtz coils: This configuration is similar to the previous one, but it

consists of two short coils instead of a single, long one. The uniformity of the

mean plane field is not as good as the solenoid one, but it can be designed to

meet the required uniformity. Mean plane access is intrinsic to this

configuration and its value (related to the distance between coils) can be

adjusted during design stage to meet specifications.

3. Maxwell coils: This configuration is similar to the Helmholtz one, but

additional coil/s are included having higher diameter than the Helmholtz

ones. This or these coils can be aimed from design to accomplish different

purposes. For example, this configuration can enhance the uniformity of the

magnetic field or reduce the fringe field. This configuration is clearly the most

complex one in terms of both design and manufacturing. Also, additional coils

result in higher manufacturing costs and sometimes in bigger magnets. Mean

plane access can be obtained as far as outer coils are also designed to be far

enough from it.

The Helmholtz arrangement of coils is selected for this specific cyclotron as it can

provide the required uniformity of the field while simplicity and low cost are preserved.

Once that the concept design of the cyclotron coils is fixed, the proposed design

method starts by a simple geometry definition in a 2D axisymmetric model. Analytical models

cannot be used in this case because of the saturation of the iron yoke starting at fields close

to 2 T. A user-friendly FE (Finite Element) software (QuickField [67]) is used for this first

approach. This software provides a simple and quick environment for 2D FE models including

magnetostatic ones. All the elements use linear shape functions. Non-linear properties of

ferromagnetic materials can be modeled by means of curves defined by points. It also

includes some other features to provide quick solutions, such as completely automatic

meshing algorithms. It is not as powerful in terms of flexibility or accuracy as some other

commercial packages (for example Opera [68] or Ansys Maxwell [69]) but it is proposed as a


41

quick starter software for a first rough design. Afterwards, it will be refined by using more

complex (3D) and accurate models.

At this stage, the most important parameters to define are the nominal current, the

loading factor on the load line and the magnet protection method. The first decision must be

taken on the superconducting wire to be used. In the following paragraphs, reasonable

starting parameters for the numerical calculation will be chosen.

A low nominal current will allow the use of a monolithic wire insulated with varnish.

This choice is very convenient for a cheap coil: there are many wires commercially available,

the winding tooling is not complicate and the winding techniques are well-known.

Furthermore, the thermal losses through the current leads are somehow proportional to the

current, which is very important taking into account the limited cooling capacity. A starting

guess for the design would be a current interval between 100 and 300 A.

Some of the wire properties are fixed once the material to be used is selected; while

some others (e.g., engineering current density) will depend on the actual wire. NbTi

technology is quite mature, but just a few number of wires are available in the market. Due to

the small demanded quantity and the necessity to reduce costs, a commercial wire is

preferred, so the design will select wire parameters from a bunch of possibilities.

The maximum current that a superconducting wire can carry (without changing to

normal state) is called critical current. As any other intrinsic critical variable (temperature and

magnetic field), it is a material property, in this case of NbTi. However, manufacturing

techniques or wire layout (superconducting filament diameter mainly) can slightly improve or

reduce actual critical current, so real performance should be analyzed by using the supplier

information.

A safety margin should be defined for the electromagnetic design. For a

superconducting magnet, it is defined as the maximum ratio between actual current in the

wire at nominal operation and the real critical current given the temperature and magnetic

field for any point of the coil. This is called load line margin (or loading factor) as explained in

Fig. 3.1 (extracted from [70]). Given the material properties of the actual wire and the

temperature, a curve is plotted in a J-B graph as the superconductive-resistive transition (pink

line). If actual magnet operation is below that curve, it is superconducting. By evaluating


42

operational point of the wire for several current values, a load line on the same graph can be

plotted (blue line). The green line is the actual magnetic field seen by the particles in the

aperture. The value of current at which both blue and pink curves intersect is the critical

current of the magnet. The ratio between the lengths of the line from the nominal point to

the intersection divided by the total length from the intersection to the origin is called the

load line margin. Note that critical current of the wire and critical current of the magnet are

different values as the latter depends on the magnet geometry.

Fig. 3.1. Load line of the magnet for a given design

If the load line margin is small, there is a high risk that quenches trigger often. It

would sharply reduce the efficiency of a radioisotope production cyclotron with limited

cooling power. On the other hand, superconducting Helmholtz coils usually show a good

mechanical performance, because the electromagnetic force distribution is quite symmetric,

which eases the design of the support structure. Based on these remarks and practical

experience, one can assume 70% as a good starting value for the working point on the load

line [71].

Another important parameter is the copper to non-copper ratio (Cu/nCu) of the wire,

which is defined as the amount of copper in the wire divided by the amount of non-copper

material (NbTi). A high value means that the wire includes a large amount of copper and just

a little superconducting material and vice versa. A high ratio yields three main advantages:

- The higher the value, the better the thermal stability: In case of a quench (sudden

loss of superconductivity state), the superconducting filaments become resistive and Joule

losses could increase coil temperature or voltage up to dangerous levels. By adding copper to

the wire, during normal operation all the current flows through the superconducting


43

filaments and in case of a quench there is a parallel path for the current to flow: the copper. It

is not as good as the superconductor, but Joule losses will be much lower than for the

resistive NbTi.

- Electrical stability. In case of a quench in one coil, the current will decay due to the

developed resistance and a voltage proportional to the product of coil inductance by di/dt

will rise on the other coil. If there is just a small amount of Cu, the current will decay very fast

(because of resistance) and the voltage can reach dangerous values. Higher Cu/nCu ratios are

important to keep electrical resistance at reasonable values in case of quench so that the

current decays slowly and the peak voltage is safe.

- Thermal stability. Superconducting material resistance is zero at nominal conditions

(constant current). Nevertheless, there are electrical losses during current transients (for

example the ramp-up when powering), even at superconducting state. This copper will help

on evacuating the generated power to the outer surface of the coils and/or spreading it to

decrease the peak temperature. Filament size is controlled as this parameter is the most

important one for this kind of losses.

On the other hand, a high Cu/nCu ratio shows the following drawbacks:

- The higher the Cu/nCu ratio, the higher the weight of the wire and the higher the

cost as the amount of material to be extruded in the billet is increased.

- The higher the Cu/nCu ratio, the lower the engineering current density. For a given

number of ampere-turns, coil size will be increased.

Moreover, NbTi is a brittle material while long filaments are needed to produce high

quality superconducting wire (high critical current). Extruding a brittle material is not easy as

it will break in the process, so it is extruded in a billet beside some amount of a ductile

material: copper. Billet geometry is carefully designed by supplier for providing certain

specifications on the produced wire, including Cu/nCu ratio, filament size, arrangement of

filaments, etc. These details are out of the scope of this work, so the attention will be paid at

the performance parameters stated by the suppliers for the available wires on the market.

In short, there are a number of criteria with sensitivity of opposite signs, so it is

necessary to find a trade-off to choose a reasonable starting value for the Cu/nCu ratio.


44

Taking into account the hard constraint on the cooling power, which leads to a low nominal

current, one can deduce that the magnet self-inductance will be relatively high and, as a

consequence, peak voltage and temperatures are prone to reach dangerous values during the

quench. Based on this reasons and know-how, one can expect that a Cu/nCu ratio between 3

and 5 will be adequate for this application and available on the market.

Magnetic design would require as boundary condition a value for the minimum

separation between the iron and the coils, which will be defined as the minimum distance

technologically feasible for the thermal insulation and mechanical support of the coils

surrounded by the warm iron. For the concept design, a typical cryogenic solution for a

magnet cooled by liquid Helium will be used, so it will include a thermal shield and a high

vacuum chamber (a good reference on the state-of-the-art about cryostat design can be for

example [72]). A number of parts will be accommodated in this free space between iron and

coil: casing as supporting structure of the magnetic forces of the coils, thermal shield for

insulation of the cold mass, and cryostat wall as vacuum vessel. A rough number for this first

design is usually chosen by know-how and previous experience, and a review of existing

cryostats results in typical values from 30 to 60 mm [73], depending on the geometry and the

complexity.

For a compact design this dimension is critical: first of all, higher distance to the iron

means higher distance to the magnetic axis, and so the efficiency of the coils to produce the

magnetic field is reduced. Moreover, coil cross-section needs to be increased and so the

outer cryostat radius is increased even more. Iron total volume would increase as well in a

positive feedback loop. Because of its critical effect on the compactness, 30 mm will be

selected as the baseline for starting the magnetic design of the magnet. This value for such a

complex geometry is very challenging but seems to be feasible. Magnetic design of the

magnet will be done for 32 mm in order to accommodate +/- 1 mm movement of the cryostat

for tuning and assembly tolerances.

The total ampere-turns to provide 4 T at the center with the requested air-gap and

beam extraction radius can be easily checked by means of the FE software. A number of

iterations are needed to find optimal values or, at least, a well-balanced proposal:


45

1. First, select a wire from suppliers accomplishing the given ranges for nominal

current, load line margin and Cu/nCu ratio.

2. Evaluate the needed number of turns. Define the coil cross-section geometry and

position.

3. Analyze the results: ratio between peak field at coils and center field, real load

margin, magnet weight, field homogeneity and magnet protection.

4. Perform next new design iteration.

All of these parameters should be taken into account for a cyclotron design, but there

is no analytical cost function to mathematically search the best option. The main advantages

and disadvantages for minimizing or maximizing any of these parameters have been already

slightly explained. For every superconducting magnet to be designed all the boundary

conditions should be considered and design decisions should be taken consequently.

As an example, the results regarding the magnet protection are presented in TABLE V

when some Cu/nCu ratios are compared. Peak voltage and temperature are not very sensitive

to the copper content. It can be explained as a result of the huge self-inductance, between 40

and 100 H, depending on the iron saturation. The current decay is very slow, in the order of

10 seconds, and thermal conductivity is very good in this type of coils, so generated heat is

well distributed all over the coil. However, a high copper content is chosen to enhance the

thermal stability of the wire and avoid premature quenches in case of cooling instabilities.

TABLE V EFFECT OF CU/NCU RATIO ON THE QUENCH PROTECTION RESULTS

Cu/nCu Maximum

temperature (K)

Maximum Resistive

Voltage (V)

2 101 1532

3 99 1387

4.5 98 1315

5 97 1270

6 96 1240


46

Eventually, a 2D electromagnetic design of the magnet will be reached by this

procedure, including iron yoke and coil geometry, current and number of turns. The results

are shown in Fig. 3.2 and TABLE VI.

Fig. 3.2. Magnetic field map with left edge as symmetry axis (left). Load Line of the magnet

from 2D design. Nominal operation is highlighted by the circle (right)

TABLE VI MAGNET PARAMETERS DEFINED FROM 2D MODEL

Parameter Value

Wire Diameter (bare/insulated) 0.85/0.90 mm

Coil cross section (width/height) 70.55/76.53 mm

Nominal Current 108.6 A

Number of turns 4235

Self-Inductance (nominal current, both coils) 38.35 H

Cu/nCu ratio 4.5:1

Distance from coil to iron 32 mm

The RF cavity will spoil the axisymmetric geometry of the iron. In order to speed up

simulations during the concept design, a pseudo 3D model is proposed. The calculation is

made with a 2D model, but the results are very close to those obtained with an actual 3D

model. It is based on the distribution of the iron reluctance in the magnetic circuit at different

angular positions. The 2D iron model is split in two different parts. The first one will represent


47

the axisymmetric iron fraction (shown with scarce line density at Fig. 3.3). For the second one,

modeling the parts with holes at certain angular positions, its material magnetic properties

(permeability) are considered to be an averaged value of iron and air, weighted by their

volume fractions along the revolution. As the air relative permeability is 1, the relative

permeability µ* of this custom material is the iron permeability reduced by the ratio between

the actual iron volume divided by the total volume.

Magnetic field tuning should be the next step to refine. Boundary conditions from

beam dynamics analysis are considered at this stage.

The proposal from this work is to automatize an iterative method for reaching a

magnetic field profile as close as possible to the optimum one. It is proposed to apply

modifications just on the iron pole, since the field quality at the gap is mainly depending on

its geometry.

Fig. 3.3. Cyclotron magnetic model (3D and its pseudo-2D)

The iron pole was defined as the shape resulting by connecting a number of points

along the radius. Radial position for each point is fixed and axial position can vary according

boundary conditions (for example, the minimum value of the gap to insert the RF cavity).

Axial positions are displaced and the magnetic field profile is evaluated for each trial.

Magnetic field shape is compared to the ideal one (given by beam dynamics analysis), by

means of a nonlinear cost function , defined as eq. (6). This function includes the optimum


48

shape of the magnetic field from the particles dynamics point of view (G) and the actual

magnetic field evaluated for the iron shape under analysis (B). Numerical derivatives on the B

field are evaluated at each iteration (dB), according to eq. (7).

For a given discretization of both profiles, four terms are evaluated and weighted

according to A0…A3 parameters. The first one is related to the actual magnetic field in the

center of the magnet compared to the optimum value at that point. In the second one, a sum

of the absolute values of their differences along the radius is evaluated. Also the sum of the

absolute value of the differences of their numerical derivatives is included, which is actually

related to the n index value. Finally, the last term is included as a hard constraint: actual

magnetic field profile should never show a positive derivate over the radial direction at any

position because it will produce instability and defocusing on the beam dynamics. The A3

parameter should be set large enough for the algorithm to discard any field shape in which

this fourth term is not zero.

This simple proposal, which is a quite typical procedure for multivariable decision

analysis, can be easily adapted to any other accelerator magnet once that the desired shape

of the magnetic field is known.

Values for the A parameters, the initial iron design and the procedure on how the

iron yoke shape should be changing from previous iteration to the next one should be defined

according to previous expertise on similar magnets and operational resources available for

the optimization.

( ) ∑{ (‖ ‖) (‖

‖)

(‖ ‖ )}

(6)

(7)


49

In the framework of this work, an initial guess was introduced manually and

parameters were changing following linear steps. The pole shape was generated in Matlab

[74] (v. R2016a) for each evaluation. Then the geometry is sent to QuickField [75] (v. 5.8)to be

evaluated and the result is read and processed by Matlab.

From the point of view of the beam dynamics, the best solution is an almost constant

magnetic field but including a small negative gradient along the radius. Final design is shown

in Fig. 3.4. It includes the magnetic field profile. It is important to note that due to the

axisymmetric condition, the derivative of the magnetic field at the center is always zero. Thus,

it is physically impossible to reach a perfectly constant n index. In addition, the larger the pole

diameter, the easier is to design a smooth profile for a given extraction radius, but increasing

the pole dimension will have a huge impact on the overall cyclotron dimensions.

A convincing result from beam dynamics analysis ([64]) was found quite quickly, so no

further efforts were paid on programming a custom code for finding the real absolute

optimum design.

Fig. 3.4. Magnetic field map in the final Pseudo 3D magnetic model(left) and magnetic field vs

radial position graph (right).

3.2 3D design

Once that a 2D design is completed, 3D calculations should be performed to check

important features for any superconducting accelerator magnet:


50

- Real magnetic field quality in the whole volume. A 3D model allows considering the

detailed geometry and evaluating the effects of all the asymmetries of the iron on the beam

trajectory.

- Real magnetic forces distribution. Any misalignment of the coils will produce a

magnetic force which cannot be modeled, generally speaking, under the assumptions of an

axisymmetric model. Obviously, some of the possible misalignments would break

axisymmetric conditions.

Fig. 3.5. 3D magnetic model.

For the 3D (Fig. 3.5) evaluation of magnetic field and forces, Opera software [68]

(v. 16) was used. A 3D model including the main features of the iron was modeled (holes for

RF cavity for example) and the magnetic field was mapped. The magnetic field map was

transferred to the beam analysis group, which checked the complete orbit of particles and

validated the magnet design.

As a qualitative comparison, a desktop computer (Intel Core i7, 16 GB RAM) can

compute a 2D or a pseudo-2D model in QuickField in about 2 minutes while a complete 3D

model in Opera requires about 3 hours. This is the main reason for optimizing the design

using a pseudo-2D model. Unfortunately, a complete 3D model is needed to evaluate the

effect of any misalignment of the coils.

Tight tolerances for magnet manufacturing and high positioning accuracy for the

supporting system would reduce the magnetic forces to be handled, because the

misalignment between the magnetic axis of the iron and the coils would be small. In that

case, the supporting system could be slimmer and perform lower thermal losses. On the

other hand, stress measurement and control become also critical if a high precision

positioning is required.


51

The definition of the maximum allowed tolerances for manufacturing and assembly

should be done according to all these details. The supporting system design will be detailed in

a specific chapter and it should be designed in parallel to this 3D model. A positioning

accuracy of +/- 1 mm for the coils in all three directions is considered to be a reasonable

number from the experience of alignment of some other precision mechanical systems [73].

Finally, the maximum lumped forces on the coils can be evaluated under a

combination of misalignments at simultaneous directions and they will be used as inputs for

supporting structure design (TABLE VII and Fig. 3.6). The magnetic field calculated under the

assumption of misalignment combinations will be used for checking feasibility via beam

dynamics analysis.

Fig. 3.6. Overview of vertical magnetic forces on each coil (z axis). The force is positive towards

the iron, that is, coil forces are repulsive


52

TABLE VII MAGNETIC FORCES

Parameter Value

Coil Force (Axial Force @ Nominal Current) 71 kN

X-Axis Force @ 1 mm X misalignment 4000 kN

Y-Axis Force @ 1 mm Y misalignment 3400 kN

Z-Axis Force @ 1 mm Z misalignment 6700 kN

3.3 Quench simulation

A quench event is, of course, a potentially dangerous situation for a superconducting

magnet. Generally speaking, quenches should be avoided but they are always present and the

following considerations must be taken into account for magnet design:

- Robustness. Under unexpected operational conditions, any of the parameters

involved in the phase transition to resistive state could provoke the quench. Examples of this

could be an increase in local temperature at any point or the injection of higher current from

the power source.

- Magnets usually need some training before reaching nominal operation conditions.

Due to manufacturing tolerances, material inhomogeneity, poor design of the support

structure or any other reason, superconducting coils do not usually reach nominal current at

their first run. Some (or several) runs are needed to reach the ultimate current, which is

above nominal current by a given safety margin to guarantee the stable operation of the

magnet at nominal conditions. Training is a well-established procedure for superconducting

magnet manufacturing. A superconducting magnet is expected to reach nominal conditions

after a reasonable amount of quenches, ideally reaching a higher current after each run [76].

The design should focus at least on two main topics. Both of them can be treated as

independent features.

- Electromechanical effects: Once that the first small volume of the coil transits to the

resistive state (for any reason), a high amount of heat will be generated in it (Joule effect).

Eventually, the whole coil will be warmed up and completely transited to resistive mode. A


53

critical parameter to be checked at this point is the maximum temperature inside the coil

(this is called “hot-spot temperature”) and it should be designed to be low enough to avoid

damaging the coil or its components (the resin and insulation materials are typically the less

temperature resistant). For highly inductive magnets it is important to check the maximum

voltages induced in the coils due to the quick transition of the current in the coil (Lenz law),

otherwise insulation breakage could occur.

- Fluid-Mechanical effects: This is not usually considered. Most of superconducting

devices are refrigerated by a bath-cooling concept so ambient pressure (or slightly above) is

the nominal condition. Moreover, many of them include also a Helium reservoir in the cold

mass cryostat for thermal insulation. It can provide a large effective area for Helium flow at

the event of a massive liquid evaporation (which is the case in a quench). Under these

conditions, no problems are expected due to the high gas pressure from the fluid vaporization

in the case of a quench. In conclusion, fluid-mechanical behavior of quench is in fact more

related to the mechanical and refrigeration design than to the electromagnetic design.

Therefore, it will be analyzed in the next chapter.

There are several models, procedures and even custom codes for quench protection

design and to check coil integrity after a quench. This is out of the scope of this work so for

more information check reference [77]. For the specific case under development in this work,

custom made code from CIEMAT, so-called SQUID, was used [78]. TABLE VIII collects the

results of the simulations. Firstly, the quench was simulated for the coils and the power

supply, without any protection. It is assumed that the power supply is switched off by its

overvoltage protection, providing a path for the current to keep circulating. Results are

slightly pessimistic, because no AC losses are included, although they will take place in some

other parts: the copper thermal shield, the aluminum shrinking cylinder, the iron yoke, etc.

The hot-spot temperature is quite low, but the peak voltage to ground is high, about 650 V,

which takes place at the splice between both coils.


54

TABLE VIII QUENCH SIMULATIONS SUMMARY

Dump Resistor

Hot Spot

Temperature

Developed Resistive

Peak Voltage

Peak Voltage to

Ground

Self-Protection No 98 K 1315 V 657 V

Dump Resistor 3 Ω 88 K 964 V 600 V

There is a classical resource to reduce the hot-spot temperature and peak voltage,

which is the use of a dump resistor. The voltage reduction is somehow proportional to the

value of the resistor. Therefore, its upper value is limited by the maximum accepted voltage

to ground in the circuit. For this kind of superconducting wire, insulated with varnish, a

maximum voltage from coil to ground of 500 V is commonly fixed. It means that a dump

resistor around 3 Ohm is adequate for this application. The dump resistor is in parallel with

the magnet, with a diode in series to avoid dissipating power in the resistor during normal

operation. An active switch is in series with the power supply, to disconnect it in case of

quench (Fig. 3.7). The delay time of that switch is not critical for the magnet protection

because the current decay is very slow. The stored magnetic energy at nominal current is

241.3 kJ. In case of a quench at nominal current, 55 kJ are dissipated at the resistor. The

current decay and the evolution of the resistive voltage are shown in Fig. 3.8. The peak

voltage due to the developed resistance is reduced about 30%. In the most pessimistic case,

when the electrical circuit is grounded at the current lead of the non-quenched coil, the peak

voltage to ground takes place at the inter-coil splice. It is about 600 V, which is not a large

improvement compared with the self-protected case study. However, the dump resistor was

kept in the design because it is necessary for the fast discharge of the magnet in case of an

interlock (for instance, power failure or thermal runaway at current leads). In fact, the actual

values will be more pessimistic, due to the contribution of the induced losses in the near

conducting elements. Therefore, it was decided to keep 500 V as the standard test voltage for

the magnet assembly. Only in the case that the leakage current is high, higher voltage or

longer test time will be used, to be decided according to the circumstances.


55

Fig. 3.7. Schematic of the magnet protection circuit.

Fig. 3.8. Simulation of the current decay and resistive voltage evolution for a quench at

nominal current with a dump resistor of 3 ohm

3.4 Summary and Contribution about the Concept Design

In this Chapter, the Author describes a detailed procedure for the electromagnetic

design of superconducting magnets. Each possibility is briefly discussed and then a decision is

taken based on the particular specifications of the AMIT project and the state of the art.

The main contribution from the Author in this Chapter is the description of a

comprehensible discussion on how to proceed to design a superconducting magnet when

compactness and efficiency are critical. Referenced books and papers can be found for

designing accelerator magnets, but none of them are focused on the efficiency or

compactness as the most important parameters.

Chapter 4. Mechanical Design

As a result of the design choices explained in the previous chapters, the

superconducting coils will be cooled by a Helium flow and the iron will be at room

temperature. Besides, the thermal losses must be small and the existing space for the

cryostat walls is only 30 mm (available distance from the coils to the iron). Finally, the

cryostat must be able to hold the magnetic forces when the coils are powered. In the next

paragraphs, the cryostat design will be carefully analyzed, with special attention to the

thermal losses, compactness and mechanical analysis.

4.1 Conceptual design of the cryostat

The expected electromagnetic forces in the coils can be studied in two components:

axial and radial ones. In this coil configuration, axial forces tend to move the coil closer to the

iron yoke. Depending on the value of the air gap and the distance between coils, axial forces

could be attractive. Radial forces tend to expand the coil to a larger diameter. This effect is

typically known as magnetic pressure.

In radial direction, as far as the coil is axisymmetric, it results in zero net in-plane

force. On the other hand, each coil will suffer a net vertical force which is of opposite

direction and the same magnitude.

Moreover, as the coils are thick according to their cross-section dimensions [52],

positive radial stresses will arise inside the coil. These radial stresses should be avoided

because of the brittle nature of the wound coil at cold

Finally, as a relatively large clearance area is needed at the mean plane of the magnet

(to accommodate the RF cavity and the targetry components), the supporting structure of the

coils will suffer a bending moment coming from the axial forces at each coil.

Because of these reasons, a special structure, so-called casing (Fig. 4.1), should be

designed to:


58

Support axial forces of both coils, keeping them at their positions. As they are

supported inside a unique structure, no net axial forces are transferred to the

cryostat, whose wall can be kept thin, able to take care of the atmospheric

pressure only

Provide radial stiffness to hold the radial electromagnetic forces and apply

some compressive pressure avoiding positive radial stresses inside the coil.

This effect cannot be fulfilled with a casing surrounding the coils, because of

the different thermal contraction coefficients of the materials and the

manufacturing tolerances. However, if a shrinking cylinder surrounds each

coil, the casing design and assembly eases noticeably.

Provide space for the cryogen flow to be in intimate contact with the coils.

Alternative solutions based on a Helium flow inside a pipe glued to the coils

or weld to the casing have been analyzed, but they are less compact and

effective.

Provide a procedure for fine tuning of coil position and alignment once the

cryostat is finished and the iron yoke is assembled around.

Fig. 4.1. Magnetic forces in coils and first schematic of a casing with openings for the RF

vacuum chamber.

The casing will be made in a non-magnetic grade of stainless steel, because it does

not affect the magnetic field quality. It is stiff, easy to weld and its thermal contraction


59

coefficient is close to that of the coils. The best choice for the shrinking cylinder is aluminum,

since it can be precisely machined and it contracts more than the coil.

The alignment concept between coils and casing is based on four dowel pins inserted

in the shrinking cylinder, parallel to its axis. They are able to guarantee the concentricity

between coils and casing. In case of some clearance, small vertical movements of the coil

inside the casing are also guided through these pins. The corresponding holes in the casing

are slotted to cope with the different thermal contractions of coil and casing, as shown in Fig.

4.2. The supports of the cold mass will be described in detail later on.

A thermal shield is necessary to decrease the losses by radiation. It should be cooled

at an intermediate temperature. Based on the cooling choice of using Helium flow and a two-

stage cryocooler, it is straightforward to conclude that the thermal shield must be linked to

the first stage cold head. Since Helium flow is available, the heat exchange of the thermal

shield with a cooling pipe is more efficient than using conduction cooling.

A vacuum chamber must surround the previous layers to allow insulating vacuum

between the cold mass and the room temperature.

Finally, there will be a connection box, including the inlet and outlet of the cryogens,

the current leads, the instrumentation feedthroughs, the vacuum ports and other ancillary

elements which will be described in detail in Chapter 5.

Fig. 4.2. Coil alignment concept inside the casing.

SS pin (x4) through SS Support

Coil

Al. Cylinder

SS Structure

Coil Supports


60

4.2 Mechanical calculations

First of all, the electromagnetic forces taken by the coils will be analyzed. Typical first

rough estimation for these forces and the induced stresses can be made with analytical

expression or numerical methods [70]. For the specific geometry of the AMIT cyclotron coils,

with no shrinking cylinder, radial stresses from -6 MPa to 2 MPa and hoop stresses up to 110

MPa can be found in the coil. Usually, the first rough criteria allow up to 150 MPa of

compression in NbTi coil and 150 MPa of tension in aluminum, while positive radial stresses

should be completely avoided inside the coil. TABLE IX shows the mechanical properties of

the materials used in this Thesis.

TABLE IX MECHANICAL PROPERTIES OF MAGNET MATERIALS AT WORKING TEMPERATURE

Material

(@4.2 K)

Young Modulus

(GPa)

Yield Strength

(MPa)

Poisson

Ratio

Shear Modulus

(GPa)

Contraction

(@296-4,2K)

NbTi 77 150 0.30 20 1.87e-3

Copper 138 450 0.34 52 2.92e-3

Insulation 2.5 - 0.35 0,93 10.3e-3

Epoxy 7 - 0.28 2,75 6.4e-3

AISI 1010

(@300 K)

205 - 0.29 80 -

316L 208 1000 0.30 82 2.97e-3

7075 79 690 0.30 30 4.2e-3

The coil is made with NbTi round monolithic wires of 0.85 mm bare diameter,

insulated with a 25 µm thick layer of polyimide varnish. Due to the high number of turns, the

best solution to ease fabrication is to produce a fully impregnated coil with epoxy resin.

TABLE IX shows the mechanical properties of the materials used for the coil and casing [79].

Therefore, the coil is a matrix composite material. The mechanical properties of such a

complex composite will be modeled in Ansys to obtain smeared out properties which can be

used as a simple input parameter for the magnet model. That way, the computing time of the

magnet mechanical model is strongly reduced. Fig. 4.3 shows the FEM model used to


61

evaluate the 2D and 3D mechanical properties of the wound coil. The idea is to model the

composite as an assembly of each constituent, taking into account the exact distribution of

materials. Boundary conditions will be modeled as bonded body, as the wires will be

impregnated and cured together.

Bonding capabilities will be checked to assure that no cracks appear, and no inner

voids will be assumed. Once the model is ready, a set of simulations are conducted to

evaluate mechanical properties on each direction. The orthotropic properties can be assumed

due to the geometric constraints (once that the helicoidal behavior of the wire is neglected

because of the large radius of the coil compared to the helix pitch). Therefore, for each

direction, a fixed support will be applied to one face and a fixed traction pressure will be

applied to the opposite side. Additional constraints could be needed for identical strain on

the loaded side. This procedure can be used also for the calculation of average coefficients of

thermal contraction.

Fig. 4.3. Coil model for averaged mechanical properties evaluation

TABLE X summarizes the results of the simulations on the mechanical properties of

the composite. The coil was modeled as behaving in the same way in both transverse

directions, which is close to reality, because the coil cross section is nearly a square. This

means that an orthotropic behavior is expected and the table includes just the minimum

number of independent elastic parameters for this kind of material [80].

rz


62

TABLE X AVERAGED MECHANICAL PROPERTIES OF COIL (4.2 K) (COMPUTED)

Young Modulus

(GPa)

Poisson

Ratio

Contraction

(@296-4,2K)

94 2.99e-3

r 35 3.90e-3

z 35 3.93e-3

r 0.08

z 0.08

rz 0.35

Once that the material properties are defined in detail, the complete 3D FEM analysis

can be done. Fig. 4.4 depicts the 3D coil model evaluated in Ansys including the iron yoke and,

therefore, the real magnetic field inside the coil.

Fig. 4.4. Magnetic force density and radial stress distributions inside the coil without shrinking

cylinder.

Positive tensile stress can be found in the coil. Its value is not high, but stability of

magnet operation is a must, so that a shrinking cylinder will be included to avoid quenches

triggered because of the brittle behavior of the epoxy resin of the coil. Moreover, some

interference is needed even at room temperature between the cylinder and the coil to

eliminate the tensile stresses at cold when the coil is energized. The choice of the

interference is made to have some margin (compressive stress) while not applying high

stresses to the coil or cylinder.


63

Three load cases have been analyzed: assembly at room temperature, cool-down to

operating temperature and powering at nominal current. Fig. 4.5Fig. shows the radial and

hoop stresses along a radial path at each load step. In all cases, the maximum stresses are

safe and there are not radial tensile stresses in the coil.

Fig. 4.5. Radial and hoop stresses distribution in the coil and shrinking cylinder at the three

main load cases

The casing around the coil should be designed to hold the coils and the Helium inside,

so that both magnetic forces on the coils and Helium pressure should be taken into account.

This casing will be the inner core of the magnet so this design must take into account the

most pessimistic scenario and additional safety factor. It is the most expensive and the most

difficult part to reach once the cyclotron is manufactured, so that it should be one of the

most reliable components of the magnet. Fig. 4.6 shows FEM results on this casing when

maximum axial forces are applied to the upper inner surface of the casing and there is the

maximum Helium pressure of 7 bar inside (outside the casing it is vacuum). Yield strength for

Path definition for stress evaluations:FROM INTERNAL Face of COIL TO EXTERNAL FACE of Shrinkage

Stresses at Warm Stresses at 4,2K

X Axis(mm)

Y Axis(MPa)

Stresses at 4,2K and Magnet ON

Radial (Left Axis)

Hoop (Right Axis)

Radial (Left Axis)

Hoop (Right Axis)

Radial (Left Axis)

Hoop (Right Axis)

COIL AL. COIL AL. COIL AL.


64

316LN steel is around 350 MPa at ambient temperature and around 1000 MPa at 4 K. The

wall thickness of the casing is not homogeneous, because the outer side of the coil is

surrounded by the shrinking cylinder. Deformation of the casing is also important because of

the small distances between components of the cryostat. From all of these points, the final

design of the casing includes wall thickness values of 8 mm (inner wall) and 4 mm (outer

wall). The inner wall is thicker to allow space for the helium flow channel. Maximum

equivalent stress of 265 MPa and maximum displacement of 0.27 mm are found as shown at

Fig. 4.6. The number for the pressure level inside for the simulation is proposed from the

maximum pressure of Helium during cooling down (3 bar, which will be justified later) and the

maximum overpressure allowed in case of quench. In the case this casing, as liquid helium is

flowing inside a small channel and pipes from outside the magnet, this overpressure could be

high and thus, a 7 bar was selected for calculations. Later on, the pressure relief system will

be defined accordingly.

Fig. 4.6. Von Mises stress in Casing at worst load scenario (Amplified strain for easier view)


65

4.3 Evaluation of thermal losses

Thermal losses are probably the most important calculation for a compact and

efficient superconducting cyclotron. As it is well known, there are three mechanisms for the

heat to flow between two bodies at different temperatures: conduction, convection and

radiation. All of them must be considered for a competitive cryostat design.

In this first stage, only thermal losses from the magnet itself will be studied.

Contributions from cryogenic supply system will be included later on.

There are quite good references on cryostat design for liquid Helium temperature

(e.g., see [72] and [81]). Some recommendations are quite well established, and some

specific materials are usually selected for the construction as there is a lot of experience on

their behavior at such a low temperature.

These common procedures are of course included in this cryostat design as it will be

explained along this chapter, but the Author proposes a quite innovative approach in how the

design will try to push the limits to reach a really low-thermal-loss cryostat given the

compactness and geometry constraints.

4.3.1. Vacuum

Generally speaking, it is quite easy to overcome convection losses. Just by keeping

some distance from the cold mass to the cryostat wall at vacuum is enough to drastically

reduce this type of losses. Nevertheless, real vacuum, even ultra-high vacuum, is not

completely free of losses. Convection effects can be negligible but residual gas conduction

should be considered. Also, proper design for optimizing the vacuum level will require to

include some details in manufacturing drawings, welding procedures, materials and surface

treatments as they will be critical to reach good vacuum levels and, finally, minimum residual

gas heat conduction. These details will be explained in Chapter 5.

In order to evaluate which vacuum level is needed, its effects on the thermal losses

should be defined. According to kinetic theory of gases, mean free path of molecules is


66

√2 (8)

Where R is the gas constant, T is the temperature, d is the mean diameter of

molecules, NA is the Avogadro number and P is the pressure. In order to be in residual gas

conduction regime, instead of the fluid one (for which thermal losses are lower), mean free

path must be greater than the distance between surfaces. At ambient temperature and given

a distance for the cryostat in the order of millimeters, maximum pressure at residual gas

regime at warm is in the order of 10-3 mbar. This is indeed the typical vacuum level which can

be easily reached by means of a turbomolecular pump at warm, even if the conductance is

not very good because there are trapped volumes difficult to evacuate.

Then, one can assume that a vacuum pump has already reduced the pressure inside

the cryostat and cool-down has started. Any residual gas inside the cryostat will be

condensed on the cold surfaces. This cryocondensation depends on the gas species and

surface temperatures. It has two main effects:

1. It will reduce even more the residual pressure inside the cryostat.

2. It will lead to a segregation of gas species. As cryocondensation depends on

the vapor pressure of the gases inside the cryostat, the actual composition of

residual gas inside the cryostat (not condensed) will change.

Given the final temperature of cold mass (refrigerated by liquid Helium, so around

4.5 K), basically all the non-condensed gas molecules will be Helium.

Residual gas regime according to gas kinetic theory (for example [82]) leads to this

heat transfer function between two surfaces (number 1 is cold, 2 is the warm one):

( ) (9)

Being S1 the cold surface, p the pressure, a coefficient depending on the gas species

and α a coefficient that depends on the gas species, the temperatures and the geometry.

There are some analytical equations for simple geometries in terms of gas properties. Also, it


67

is important to notice the linear behavior based on pressure, which is not the usual case

when considering heat transfer by gas conduction at fluid regime (not vacuum).

For the cryostat design of a superconducting magnet refrigerated by liquid Helium,

these assumptions will be taken:

Most of the gas molecules inside the cryostat will be pumped out by the

vacuum pump at ambient temperature (before cooling down the casing).

Those molecules not extracted by the vacuum pump will be cryoadsorpted by

the cold surface once it reaches the temperature/pressure values

corresponding to the vapor pressure curve for each gas (Fig. 4.7, extracted

from [83]).

Even Hydrogen molecules (H2) will be finally adsorpted when surface reach

temperatures in order of few Kelvins, so that at nominal temperature, Helium

will be assumed to be the only one responsible for residual gas conduction.

Moreover, as there will be Helium inside the cold mass (refrigeration circuit),

a small but a finite leak of Helium could be expected in the insulation vacuum

volume around the cold mass.

Fig. 4.7. Vapour pressure of different gases at cryogenic temperatures


68

According to these assumptions, residual gas composition will be almost pure Helium.

Equation (9) (residual gas conduction) does not include the distance between surfaces. It

could be taken into account by parameter α, but for the ideal cases of large planar surfaces or

concentric walls of similar radius, the thermal losses do not depend on the distance. This

result will be used to reduce as much as possible the distance between the different layers

inside the cryostat (providing that they do not touch each other) for a compact solution. For

the specific case of the AMIT cyclotron, gas conduction is achieved at vacuum pressures

below 0.001 mbar.

4.3.2. Supports

The second heat transfer mechanism to be considered is conduction through the

supports of the cold mass. As it is well known, Fourier’s law expresses the heat conduction in

solids as being proportional to a temperature gradient, the solid cross section (A) and the

material thermal conductivity (k):

(10)

At cryogenic temperatures, thermal conductivity is strongly non-linear, and values for

typical structural materials cover a wide range of several orders of magnitude, as shown in

Fig. 4.8, from [84]. Typical approaches for designing supports and vessels for cold mass inside

a cryostat include some assumptions on the heat transfer, and they use the integral thermal

conductivity [59]. It is defined from the total heat conducted from one side at warm

temperature T2 to the opposite side at cold temperature T1. This procedure provides a simpler

classification and evaluation scheme for the total heat losses at different temperatures: for

instance, integral thermal conductivity from 20 K to 80 K can be found just as the integral

from 0K to 80K minus the integral from 0K to 20 K. Some selected values can be found in

TABLE XI.

Then, according to Fourier’s law, in order to reduce thermal losses up to certain value

Q, the length to cross surface ratio of the support will be designed according eq. (11).


69

(11)

Fig. 4.8. Thermal conductivity of selected materials

This procedure, which is quite simple and easy for a rough estimation or first concept

design, can be improved by the addition of a thermal intercept. As it will be clearly stated at

the refrigeration scheme chapter, the amount of available cooling power depends strongly on

the temperature. Because of basic thermodynamic principles (Carnot efficiency), each cooling

watt becomes more and more difficult to be generated as temperature decreases. Thus,

given also that the thermal conductivity is decreasing with temperature, the result is that

using thermal intercepts is usually an improvement to reduce the necessary amount of


70

cooling power at the lowest temperature (even if the need for cooling power at intermediate

temperature increases, total input power of cooling cycle will be reduced for a given cold

mass heat loss).

It is within the scope of this work to get an optimized thermal behavior as one of the

main objectives, so that an improved method is proposed for the thermal design of the cold

mass supports inside a compact cryostat (in which the available length is limited).

As explained in the concept design of the magnet, there is a constraint on the

maximum diameter for the cryostat, which will be inside the magnet yoke. In terms of radius,

there is just 30 mm from the coils to the iron inner surface to allocate the cryostat, which is

clearly not enough for good thermal insulation for any support capable of withstanding the

expected forces from the combination of weight and magnetic forces.

TABLE XI INTEGRAL THERMAL CONDUCTIVITY FOR SELECTED MATERIALS [W/m]

From 0K up

to… 20 K 80 K 290 K

OFHC copper 11000 60600 152000

DHP copper 395 5890 46100

1100

Aluminum 2740 23300 72100

304 Steel 16.3 2420 3060

G10 composite 2 18 153

The distance between coils and yoke (30 mm) is definitively too short for any support

with low thermal losses. The supports can be made longer and, consequently, more efficient,

while keeping the overall size of the cryostat, if they protrude from the cryostat but not from

the iron. Thermal intercept is also studied to improve the efficiency of cooling power at low

temperature.

In general, the choice of the temperature of the thermal intercept is usually the

boiling temperature of one of the common cooling fluids (Nitrogen). In our case, as pointed

out earlier, it is the temperature of the first stage of the cryocooler.


71

Usually thermal intercepts are designed in one of these two ways:

1. There is some good conductor thermally anchored to the intermediate

temperature refrigerator and then somehow attached to the right place at

certain position of the support.

2. The support is in fact split in two parts, being inserted an intermediate part,

which is a good conductor and possibly directly attached to the intermediate

refrigerator.

The second one provides the best solution as it really intercepts all the heat coming

from the warm side before getting into the cold side of the support. Temperature

homogeneity of this intermediate part is the key compared to the first solution, in which the

inner section of the support may be not enough refrigerated.

4.3.3. Thermal shield

In addition, the thermal intercept for the supports can be designed for reducing the

radiation losses of the whole cold mass (i.e., casing and coils). Radiation heat transfer

between two grey surfaces forming an enclosure is governed by eq. (12):

(

)

(12)

where T is temperature, A is surface, ε emissivity, σ is the Stefan-Boltzmann constant

and F is the view factor. And index numbers 1, 2 and 12 mean surface 1, surface 2 and from

surface 1 to surface 2, respectively.

According to this equation, the heat transfer is proportional to the difference of the

fourth power of temperatures. Thus, it is very convenient to set up one or more intermediate

temperature layers between the ambient temperature cryostat wall (300 K) and the outer

surface of the cold mass cryostat (4 K). This is a quite usual procedure in cryostat design, and

they are called thermal shields. These could be actively refrigerated to evacuate the heat

more efficiently or just passive parts in thermal equilibrium by radiation heat transfer:


72

- If passive thermal shields are used, which is a quite economical and easy way to

reduce the radiation losses, more than one layer is usually included. This is the operational

concept of multilayer insulation (MLI), for which a reduction of the heat losses in the order of

1/(N-1) can be found when N layers are applied [59] compared to the situation without

shield.

- If an actively refrigerated thermal shield is used, steady-state temperature and heat

losses values can be custom designed. Physical parameters (e.g., emissivity, view factor) and

cooling capabilities become important for the actual performance of the thermal shielding.

An additional advantage of using one active shield is the possibility of coupling the thermal

intercept of the support to this shield, so that the refrigeration power applied to the shield

will be used to reduce also the conduction heat losses of the support.

- It is quite common to combine both types of radiation thermal shields. One layer of

a refrigerated thermal shield is then covered by MLI. This is usually the best solution in terms

of efficient radiation insulation, while the preferred option, if there are no concerns on the

available space or limited thermal power, is using just MLI.

Ideally, the material of one actively refrigerated thermal shield should satisfy two

conditions: good thermal conductivity and low emissivity. At the same time, its design should

provide a homogeneous temperature in the surface to reduce temperature gradients in the

cold mass. The simplest design to meet these specifications is to provide a path for the

cryogen to refrigerate the whole thermal shield, for example by a bath cooling concept. If

there is no available space for this bath, channels or pipes can be used to refrigerate the

shield as homogeneously as possible.

4.3.4. Cold mass design

In the particular case of the AMIT cyclotron, there is not enough distance between

the casing and the thermal shield and between the thermal shield and the cryostat to

efficiently use MLI, so that a cooled thermal shield is needed. It will be used to refrigerate

also the thermal intercepts of the supports.

Moreover, in this Thesis, a proposal for a slightly different concept of thermal

intercept is presented. In terms of thermal efficiency, it is exactly the same as a common split


73

support, but in case that the forces to be supported by the cryostat are high, this procedure

can improve the overall performance of the cryostat. This will be clearer after the study on

the loads to be supported.

Finally, as a further step to improve accuracy, this method takes into account not only

conduction heat losses through supports, but also radiation. The radiation term is not the

main one but it can lead to a different temperature distribution along the support, so that the

classical integral conductivity parameter is not valid anymore (it is calculated under the

assumption that conduction is the only term for heat flow between cold and warm bodies).

Including radiation will be important also for the real temperature distribution in the thermal

shield.

Then, a FEA (Finite Elements Analysis) model should be evaluated including the real

conductivity and emissivity curves for the materials (as a function of temperature). In this

situation, a parametric study is suggested. This parametric study will help for finding the

optimum design (in terms of geometry) and yield also a sensitivity analysis.

A sensitivity analysis is important for example for the emissivity values. They are quite

sensitive to actual material (procured material, manufacturing method, etc…) and assembly

process. Some improvements are possible, for example using some low-emissivity coatings

but real values for emissivity even for these specific materials are strongly dependent on the

manufacturing/assembly procedures and cleaning procedures.

In the case of the AMIT cyclotron, this procedure was followed. The FEA model used

is shown in Fig. 4.9. Radiation and conduction terms for heat transfer were included and

nonlinear patterns for material properties were used. Thermal constraints for the model

included uniform temperature inside the casing (4.5 K, liquid Helium temperature), and

outside the cryostat (295 K, ambient temperature). It is not simple to refrigerate both sides of

the thermal shield because there is no enough clearance between parts to accommodate a

pipe or to design a flow path for the cryogen around it. Because of this reason, just one side

of the thermal shield is in contact with the cryogen. The most convenient place to refrigerate

the thermal shield is at one of the upper or lower plates. Cryogen flow and piping design will

determine which one is better, but for the present thermal calculations this selection is not

important, and the lower plate is selected by now. Temperature for this lower plate of the


74

thermal shield is coming from the CSS operational point, which is about 50 K for the first

stage (at nominal conditions). The output values from these evaluations are the heat flows at

casing and thermal shield (demanded cooling power from each cryocooler stage).

Fig. 4.9. Thermal FEM Model of cold mass support.

Parameters included in the study are the material, cross section of the rods and the

length of the supports.

Regarding the material for the rods, two main possibilities are commonly used in

cryostat design: glassfiber and titanium alloys:

G10-CR (Cryogenic grade fiberglass) is an established and well known

material used for low-loss supports at cryogenic temperatures. This material

is usually available and supplied as plates or round bars. As the machining of

composite materials (this one is made of thermoset epoxy resin and glass

fibers) is not easy, expensive and even not recommended because its

mechanical properties can be degraded [85], these two common shapes are

retained as candidates.

Ti6Al4V (Titanium alloy) is also used for cryogenic supports because of its

combination of high strength, low contraction and low thermal conductivity

(for a metal). Moreover, they are made of a homogeneous material instead of


75

a composite. According to the properties found in [86], ratio between

strength and thermal conductivity integral is not as good as G10-CR one.

Therefore, it is used when strength and/or thermal contraction properties are

critical (very high loads or dimensions). For the AMIT compact cyclotron,

thermal properties are the most important ones so that G10-CR glass fiber is

selected.

Rod cross section shape was fixed to a rectangular one for this particular case based

on the following rationale:

1. It provides higher flexural stiffness. The mechanical design will try to

minimize any other loads than tension, but some flexion could arise from

friction or misalignments of the rods. In this case, the rectangular shape (in

which the axis with higher flexural stiffness is properly aligned) is better

suited to deal with it compared to the round one.

2. The rectangular cross-section can be easily adapted to a compact design.

Nevertheless, the volume occupied by the supports will go through the iron

yoke, so the amount of ferromagnetic material is reduced and it will finally

spoil the magnetic performance of the magnet. The thinner the rod, the

smaller the volume of removed iron.

This FEM model will also provide information on the need (or not) of covering cold

mass with low emissivity material. It is easily understood that although emissivity values for

all the surfaces in radiative contact are included in the equation for the total heat transferred,

the cold mass one is the most important. For this reason, liquid Helium cold mass surface

(casing) will be covered by low emissivity foil, and also the outer surface of thermal shield,

which will be in radiative contact with the cryostat wall (ambient temperature).

The results of the FEM model show an optimum length for the thermal anchoring

despite of the simplified first approximation given by eq. (11). In that first approximation, the

longer the support the better, but in this specific case of the AMIT cyclotron, the temperature

of the thermal anchor will be higher when the length is increased because refrigeration is

applied at the bottom side of the thermal shield. Also, the radiation heat from the thermal


76

shield will be higher as surface increases. The results are shown in Fig. 4.10, where the

minimum can be found at 68 mm. Total support length is 230 mm.

Fig. 4.10. Heat load to casing

Finally, additional heat should be included coming from particles radiation. When the

accelerated particles hit the target, radiation is generated besides radioisotopes. Some

amount of this radiation will deposit some energy on the cold mass and hence it is an

additional heat income to refrigerate. The evaluation of that heat is not easy as it strongly

depends on the target design, radioisotope to be produced, beam current, beam energy and

radiation shield design among other parameters. The calculation of this radiation level is out

of the scope of this thesis, so the value obtained by Monte Carlo simulations done at Medical

Applications unit of CIEMAT [87] is directly included in the thermal budget of the cyclotron

magnet.

TABLE XII summarizes the thermal losses for the magnet design when casing is at 4 K

and thermal shield is refrigerated at 60 K.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

0 20 40 60 80 100

He

at L

oad

(W

)

Distance to Thermal Shield Anchoring (mm)

Heat load to Casing from Conduction & Radiation


77

TABLE XII CYCLOTRON MAGNET: THERMAL BUDGET OF COLD MASS

Contribution Heat (W)

Loads @ Casing 4K

Conduction (supports) 0.52

Thermal radiation (from thermal shield) 0.05

Target radiation to casing[86] 0.12

TOTAL at 4.5 K 0.69

Loads @ Thermal Shield 60K

Conduction (supports) 0.9

Radiation (from cryostat) 10.5

TOTAL at 60 K 11.4

The emissivity value of casing surface covered by low emissivity tape was considered

as a pessimistic value of 0.02. If no covering was added, expected emissivity of 0.05 to 0.1

could be found as reasonable for the stainless steel casing and, as a result, heat losses from

radiation could increase up to 0.15 W.

Heat losses from residual gas conduction can be neglected as far as vacuum level is

preserved at 1e-7 mbar or better, as shown in TABLE XIII. This vacuum level is considered

achievable with a proper design of the cryostat and the connection box.

TABLE XIII RESIDUAL GAS CONDUCTION

Pressure

(mbar)

Heat to Casing from-

Thermal shield (mW)

Heat to Thermal Shield from

cryostat (mW)

1e-4 529 1960

1e-6 5.29 19.6

1e-7 0.53 1.96

1e-8 0.05 0.19


78

4.4 Refrigeration system

As seen in previous chapters, a warm iron superconducting cyclotron refrigerated

using liquid Helium flow provided by an autonomous CSS is the preferred solution for this

specific radioisotope producer.

Under this premise, the CSS will liquefy the needed Helium by means of a cryocooler,

send it to the magnet and then recover it in a closed loop to liquefy it again. Room

temperature Helium gas will be needed to be liquefied, but the cooling scheme is similar to

the classical situation of using liquid cryogens. Cryogenic equipment can be kept far away

from the radioactive environment to provide safer operating conditions and easier access for

maintenance. Regarding the drawbacks of this concept, there is some inefficiency due to the

transportation of this cryogen (not the whole cooling power generated by the cryocooler is

transferred to the coil) and there are also some additional devices and electric input power

needed for pumping and controlling the closed loop system.

This work will use this refrigeration scheme, partially based on the work developed in

collaboration with CERN [62], shown in Fig. 4.11. There are three heat exchangers, HX1, 2 and

3. The first one starts to cool down the Helium gas with the cold gas coming out from CSS.

The second heat exchanger is directly connected to the first stage of the cryocooler, while the

third one is linked to the cold head. The cold gas coming out from HX2 is sent to cool down

the thermal shield and the connection box, and then enters HX3 for further cooling. The liquid

Helium exits from HX3 and is sent to cool both coils sequentially, so there could be a small

temperature difference between them. There is a pump at room temperature to compensate

the pressure drop of the circuit. Besides, this scheme allows working at different mass flows,

that is, different cooling capacities depending on the operation mode.

Because of the limited cooling capacity of the cryocooler, the cooling down time of

the cold mass is quite long. Assuming a weight of the cold mass around 300 kg of copper, it

can be estimated that 5 days are necessary to reach an operating temperature around 5 K if

all the cooling power could be transferred to the coils. The heat capacity of copper can be

evaluated using properties from [86], to be around 50 kJ/kg when cooled down from ambient

temperature to 5 K. From the reference load map of Sumitomo for the cryocooler [61], a


79

mean value of 40 W cooling capability along the whole temperature range could be defined

to obtain the rough estimation of 5 days for cooling down. In addition, stable temperature

would be reached once that the Helium liquefaction has finished. This second stage of the

cool down process is very sensitive to the actual thermal losses of the system and the total

volume to be liquefied, but about 10 days could be a realistic estimation based on the first

test results on the CSS performance. During this first stand-alone test operation of CSS, an

additional 24 h were needed to liquefy and stabilize the temperature of the inner parts of the

CSS heat exchangers plus a dummy cold mass about 5 kg of copper [89].

However, the heat exchange between Helium and coils does not only depend on the

available cooling power. It depends also on the Helium mass flow, the pipe roughness, the

temperature distribution, etc. The cooling down time can be shortened by increasing the

Helium mass flow, which can be done by raising the pressure. Assuming a maximum testing

pressure of 6 bar and a safety factor of 2 for the components under pressure, it is reasonable

to choose 3 bar as the pressure set-point during cool down. Due to the unavoidable pressure

increase after quench, it is better to choose a steady state pressure close to 1 bar.

This cooling scheme includes some additional advantages:

- From the point of view of the cyclotron, it is not important at all how the liquid

Helium was generated or purchased, so the cyclotron design could be refrigerated by

the CSS, any other liquefaction system or even by a liquid Helium Dewar directly. The

cyclotron is therefore versatile and for example it could keep producing

radioisotopes with a secondary cryogenic supply during CSS maintenance.

- In order to minimize the volume, instead of a bath scheme, it is proposed a cooling

channel in which a forced internal flow of two-phase Helium will be in direct contact

with the coils. Besides, it is intrinsically more effective, given that the coolant is in

direct contact with the coil. This is a quite innovative concept and it will be tested in

a mock-up as a proof-of-concept check. The reduction of liquid Helium volume will

reduce the quench pressure while the casing compactness is optimized.

Therefore, thermal design must deal with just a low mass flow of liquid Helium. It is

so small that basically both coils will be refrigerated not by pure liquid, but by liquid-gas

mixture (two-phase flow). Such an innovative proposed solution is first checked by numerical


80

methods using liquid Helium correlations. In the next paragraphs, the thermal properties of

the coil will be evaluated for proper accuracy of the model.

Fig. 4.11. Cryogenic concept for AMIT cyclotron including the CSS system for autonomous

operation.

4.4.1. Liquid Helium flow model

For the numerical method to compute the Helium flow properties, a set of Matlab

functions were developed by the Author. A schematic overview about the basic flow diagram

for this code is shown in Fig. 4.12.

Multiphase prediction and simulation is not a simple issue, and great efforts have

been devoted to establishing a confident and general procedure to deal with it. Up to now it

is not completely solved, but some literature can be found for general attempts or particular

solutions [90].


81

The core of the program is a Matlab function (ForcedFlowN.m), which, is coded to

evaluate a general unidimensional straight pipe (Fig. 4.13). Input parameters are the cross-

section, length, surface roughness, orientation, temperature at the inlet and the wall, mass

flow, inlet pressure and quality factor (ratio of liquid mass over total mass in a biphasic flow).

This function will calculate the output temperature, heat flow, pressure drop and outlet

quality factor for a given gas flow. The heat input from the coil is considered to be

homogeneously distributed along the whole surface. The properties for each element can be

an input from the user or they can be evaluated as a compound value from the constitutive

materials according to the standard rule of mixtures [91]. Fluid behavior is modeled according

to the equations found in [92], taking into account the actual boiling regime. In order to

model heat transfer and pressure drop due to the two-phase Helium flow, a homogeneous

model was chosen. Thus, each phase is evaluated according to its mean fluid properties and

the frictional pressure drop is calculated with the typical Darcy-Weisbach factor. Convective

contribution is evaluated according to the homogeneous model, while for the boiling

contribution the Kutateladze correlation is used [92].

When one phase flow is to be evaluated (for example during the cool-down, when all

the Helium is evaporated before getting inside the cryostat), common heat transfer

correlations for forced one-phase flow were used [93].

The first function (CircuitGen.m) is responsible for the actual path definition, so it will

generate a matrix for saving the input and output values at each element. All the elements,

one immediately after the other, will define the whole path of the cooling circuit. This matrix

will include also information about the geometry, heat loads and initial conditions

(temperature, pressure). It will be saved as a Matlab file called Mesh. This function does not

include any calculation, as it will just set up the information into the Mesh file. Additional

functions for any other configurations than a single circuit could be easily developed if

needed in the future.

The main function (Circuit.m) takes the information from the Mesh file and sends it to

the core function (ForcedFlowN.m). Output values from each step and element are used to

refresh the input data for the next element and time step. This is managed by the main

function, as it sets the conditions for each element before it is evaluated. Some additional


82

management is also provided by this function, as it will be in charge of memory and

processing time management. Some basic post-processing is available at the end of the

simulation.

For more sophisticated post-processing including time variation of one element, or

temperature distribution in the circuit at a given time step, additional functions have been

coded. All of them are specific to this case, so they will not be explained here. The

comparison between the simulation results and the measurements are shown at each test

independently in the next Chapters.

This program is very versatile. It will be used for cooling performance prediction not

only for the liquid Helium, but also for the tests with liquid nitrogen. It may be used for any

path, gas or flow. There are just four limitations on its applicability:

1. It cannot be more accurate than the empirical correlations on which it is based.

Fluid correlations are typically fitted from smooth linear paths and completely

developed fluid flow regime. In the case of complex paths, variable cross-

sections and curved pipes with short straight sections, the actual fluid behavior

could be significantly different from the numerical result.

2. The length of each element, as in any other FE method, is important to achieve

good results. As this is a transient simulation, time step is also an important

parameter. Reducing too much both of them will lead to the need of a high

amount of computational resources and calculation time.

3. This set of Matlab functions was developed for the particular situation of a

cryogenic circuit in which the mass flow is constant. This means that the

amount of cryogen flowing through one element will flow through the next one

(even if the cryogen changed temperature or quality factor). This has two

consequences: firstly, no additional cryogen can be included at an intermediate

point of the circuit; secondly, no cryogen can be stored or removed at any

element. This is of course true in a closed system at steady-state, but some

transient effects cannot be evaluated (for instance, the fluid dynamics during

the first seconds of a quench). This limitation is also related to the constraints


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of the correlations, which are not fitted for this kind of transient effects

anyway.

4. This program was developed for the simulation of the thermal transients of a

superconducting magnet, so that the cryogen circuit is composed as the serial

connection of individual elements. Therefore, it is not possible to simulate

thermal links between two non-adjacent elements. This is the situation of a

heat exchanger: two different elements of the circuit are thermally coupled

while there is no mass transfer between them. The conclusion for this

limitation is that the CSS cannot be simulated. Anyway, there is not enough

public information about the cryocooler performance at intermediate

temperatures, which is mandatory for a realistic model. In the future, when

some experimental data about the cryocooler performance will be available, it

could be possible to implement the capabilities of heat exchangers modeling as

an interesting future development of the program.

On the other hand, this method is much simpler and more efficient than complete FE

modeling of real fluid flow inside the pipe. Just one element is used for each partition of the

path, while a complete fluid model by means of a commercial FE program will require at least

thousands of 3D elements for each partition and each time step. One must keep in mind that

empirical validation will be needed in both cases.

The developed custom program was first cross-checked during the thermal test of the

coils assembly using liquid nitrogen. As test results were very close to calculations, the

validation mock-up for the refrigeration scheme was first simulated by using this program and

then checked empirically as exposed in Chapter 5.4.


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Fig. 4.12. Schematic flow diagram of the custom code to compute the cooling fluid behavior

Fig. 4.13. Schematic of dedicated FEM model for transient calculations (ForcedFlowN.m)

4.4.2. Overpressure in case of quench

It is also necessary to check the stress distribution in the cooling circuit in case of a

quench. A custom made Excel spreadsheet was developed to compute the peak pressure

distribution along the whole transient. Classical equations of fluid motion inside a circular

pipe and Helium properties were used ([82], [94]). The casing, from the mechanical design,


85

was checked up to 7 bar. The maximum pressure allowed in case of quench was fixed at

5 bar. This number is proposed as a compromise between the maximum operational pressure

of Helium during cooling down (3 bar, which will be justified later) and the maximum testing

pressure for the pressurized elements at manufacturing (6 bar, assuming a safety factor of 2).

The dimensions of the pressure relief pipes were designed by this procedure. The maximum

pressure in the worst point and circumstances after a coil quench was found to be 4.6 bar for

the final design, including 16 mm pipe diameter and safety valves. Time evolution of pressure

after quench (quench @ t=0) is shown in Fig. 4.14. It includes the very first moments after the

quench, which are in fact the most dangerous ones in a flow refrigeration concept. Relief

valves cannot be close to the liquid Helium because they would increase thermal losses too

much. Therefore, these relief valves are outside the cryostat, at ambient temperature. It was

evaluated that about 1 second is needed for the pressure to reach the relief valve position. In

order to increase the system safety, two operational modes will be defined: First one with a

pressure relief valve set at 1.5 bar (for nominal conditions in which pressure will be 1.3 bar)

and a second one set at 4.5 bar (for the 3 bar during cool down).

Fig. 4.14. He Pressure evolution in case of a quench.


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4.5 Alignment and support system

As it was explained in previous chapters, there is a need for a supporting structure of

the cold mass that complies with several requests:

- It should hold the own weight of the cold mass while keeping low thermal losses.

- It must allow aligning the cold mass in the six degrees of freedom respect to the iron

yoke, considering the cold mass as a rigid body.

- It should be compatible with thermal contractions in any temperature distribution,

not only during nominal operating conditions, also during the cooling down at different

speeds.

Magnetic forces on the coils were already shown in Fig. . The important point is that

the nominal magnet design, where the coils are perfectly centered with the iron, is unstable.

If there is any misalignment in any direction, a magnetic force will appear that tend to

increase even more the misalignment.

Thus, the stiffness of the support system is very important, and the measurement of

the actual forces during operation is critical. The strategy proposed here is to develop a

supporting system which includes alignment capabilities. Therefore, it will be possible to

increase the current by small steps while fine tuning of the alignment is done.

The overall view of the supporting system is shown in Fig. 4.15. The proposal includes

rods at both upper and lower halves of the cold mass. In this specific case, given the large

expected magnetic forces and to improve the iron symmetry (which is important for the

stability of the particles), four rods are proposed from the upper side and four from the lower

one. Strictly speaking, only rods at the upper side are necessary to hold the weight, but those

on the lower side are needed to hold the magnetic forces. There is one joint at every rod end.

This joint will give free rotation to the rod end, so the transmitted load will be purely

tensional. Of course, the torsion around revolution axis is not free and it will produce some

flexural loads on the rods, but due to the symmetry of the system, this kind of torque is not

expected by design.


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The thermal shield is linked to the intermediate metallic piece of each support, where

two joints provide flexibility to the assembly, which is very convenient during cooling down

and powering. Besides, those joints avoid that the supports take any compressive load, which

is not adequate for the rods, since they are made from G-10CR. The positioning system is

based on a thread at the tip of the support. A nut is pressing a surface on the iron yoke so it is

possible to enlarge or reduce the distance between the cold mass and the iron yoke. As

thread pitch will be designed for about 1 mm, alignment and positioning sensitivity will be in

the order of 0.1 mm in the direction of each support.

It is worth to notice that eight rods under tension behave as a hyperstatic structure. It

means that in order to shorten the distance between the iron and the cold mass in one

direction, the rods on the opposite side must be loosen to allow that movement. This strategy

is a bit slow procedure, but it is expected to be necessary only during commissioning.

Fig. 4.15. Supporting concept: detailed view of one support (left) and overall view (right).

A FE model in Ansys was developed to evaluate all the loading cases:

- Ambient temperature for all elements in the cryostat.

- Nominal temperatures at cold for casing and shield.

- Custom defined temperatures distribution (all the parts at intermediate

temperatures, as expected during cooling down).

At nominal temperatures, a number of additional simulations were performed:

- Effect of misalignments in all directions (one each time and a combination of them).

STEEL

G10-CRCu

Adjustment Nut(Rod Axis Displacement)Possibility of fine tuning at cold;Once adjusted, they keep positionwhen thermal cycles occur


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- Effect of operation on the adjusting nut.

Some of the results are shown in the next images. The first structural simulation is

shown in Fig. 4.16, which presents the total deformation of the cold mass after cool-down. In

this figure, the grey shadow is the initial position at warm and the strain is amplified in the

picture for clarification. There are some important features to be pointed out:

- Upper rod tips remain at their initial positions because they are attached to the

warm wall of the cryostat, hold by the magnet iron.

- Lower rods are moving axially upwards guided by the joint on the cryostat. This joint

is flexible so that shrinkage does not lead to thermal stresses at any temperature.

- At this position, lower rods can be fixed, so that they can contribute to support

magnetic forces, but they are still free for elongation in case of warming up the cold

mass.

- The whole cold mass is hanging from the upper rods, so that when it is cooled down,

the cold mass moves almost 2 mm upwards. This movement should be foreseen in

the design as there would be two consequences: The first one is that the clearance

between cold mass to upper wall of the cryostat will be reduced in 2 mm compared

to the warm dimensions. The second, the mean plane of the coils will be shifted,

also, if compared to the iron and the fixed nuts of the supports. The manufacturing

drawings and parts dimensions at warm will include these details.

Fig. 4.16. Directional deformation (axial displacement) of casing after cool down.


89

The results on the equivalent stress of the G10-CR parts of two rods at cold condition

are shown in Fig. 4.17. They are holding the weight of the cold mass and thermal stresses, but

no magnetic forces are included. In this case, every rod is just under pure tensional loads,

because of the revolute joints at each end.

As an example of the situation under non-centered magnetic forces, the case of 1 mm

of horizontal misalignment is shown in Fig. 4.18. Horizontal magnetic forces are probably the

most dangerous ones, because of the inherent instability of magnetic forces.

Moreover, horizontal magnetic forces are the only ones that could lead to a non-pure

tensional load on the rods. In fact, any rod which is not perfectly aligned to the magnetic

force direction will suffer some bending. This is the main reason for using as many as eight

rods. The larger the angular separation between rods, the higher the probability of a

magnetic force misaligned respect to the rods appearing, and the higher the bending loads

applied to the rods. This concept including four supports, equally distributed along symmetry

axis, and each one including two rods has been found in this specific case as an optimal

compromise in terms of low thermal losses, high stiffness and moderate stresses for any

expected magnetic misalignment.

Fig. 4.17. Stresses at supporting structure while casing is cold


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Fig. 4.18. Supporting structure under horizontal magnetic misalignment (1 mm)

4.6 Current leads

There are several possibilities for current lead design [95]. Generally speaking, a

moderate current, below 600 A, allows to keep low heat losses. There are two main options,

vapour-cooled and HTS (High Temperature Superconductor) current leads. The latter are

more expensive, but thermal losses are intrinsically much lower. Furthermore, they are more

compact, since they are shorter and the Helium gas does not need to flow through them, so

they can be placed horizontally.

HTS current leads are usually conduction-refrigerated, so they are not in direct

contact with the Helium, but both ends must be refrigerated to a temperature lower than the

critical one. Commercial HTS current leads cover a wide range of nominal currents, and they

minimize thermal losses at liquid Helium temperature from temperatures up to about 100 K

at the warm end. Then, a resistive stage (usually a copper plate) is needed for the current

path from ambient temperature to the warm end of the HTS current lead. HTS current leads

will be refrigerated by cryogenic flow of Helium coming from the CSS first stage (“warm” end)


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while the “cold” end is refrigerated by the liquid Helium used for the coils (see Fig. 4.19). The

cold end of the HTS current lead is connected to the coil leads with a custom made

feedthrough, tight to liquid Helium.

Fig. 4.19. Current leads concept design

Heat exchangers were designed for the refrigeration of both HTS ends. The warm

heat exchanger (designed for temperatures from 40 to 80 K) is shown in Fig. 4.20. It includes

57 drilled holes of 4 mm diameter to optimize the heat transfer in a compact design of 75 mm

length. This heat exchanger is connected to the HTS by means of flexible cooper braid to

avoid thermal stress at cold temperature.

The superconducting wires are going through the connector of the cold exchanger, so

there is not any splice at the feedthrough, which is a significant advantage. These NbTi wires

will be duplicated to increase operation safety and margin in temperature. This is because

they will not be directly cooled by the liquid Helium once that they are out of the

feedthrough, so their temperature will be slightly higher.


92

Fig. 4.20. Temperature distribution at the conduction heat exchanger for the warm end of HTS

current leads.

Also, a protection system is proposed as a backup circuit for the current to flow from

the power supply to the coil. In case that one or both HTS current leads fail (typically due to a

quench or thermal runaway), an alternative path for the current is provided by means of a

shunt. The geometry and material for this shunt will be designed according to the following

assumption: The temperature distribution and time evolution in the proposed shunt is shown

in Fig. 4.21, which corresponds to the most pessimistic scenario, that is, including adiabatic

boundary conditions and completely open circuit at the superconducting current leads. The

maximum temperature is reached after 30 s from the quench, and its value is 275 ºC,

considered safe for a stainless steel shunt. This hot spot occurs because Joule heating exceeds

the heat diffusivity capability of the shunt at that point. The current decay is very slow

because of the large time constant of the magnet, about 30 s. The variable geometry of the

shunt, taking advantage of the nonlinear effect of temperature on the thermal properties of

the shunt, led to a just 40 mW thermal loss added by both shunts.


93

Fig. 4.21. HTS Current leads shunts for quench protection: temperature distribution (left) and

time evolution (right) for the most pessimistic scenario

4.7 Vacuum system

The vacuum pumping system schematic is shown in Fig. 4.22. A turbomolecular pump

was selected according to conventional guidelines that can be found for example in [96]. The

vacuum level at ambient temperature should be 0.001 mbar or better, and the minimum port

diameter (to minimize vacuum impedance) was set, for this specific situation, as ISO-K 160

flange size. Both turbomolecular (f) and primary (j) pumps require limited background

magnetic field as proper working conditions (in the order of 10 to 50 mT). Because of that,

the turbomolecular pump will be connected to a “long” pipe coming from the magnet

cryostat. It can be checked that the longer the pipe the higher the impedance, so a

compromise length in the order of 400 mm was found to be the best design in this case.

Additional magnetic shield will be needed at this distance, which will be done by covering the

pump by two layers of MuMetal [97]. A primary pump was designed for serial connection to

the turbomolecular one, but it can be directly connected to the cryostat (a) for the initial

rough pumping by means of the bypass valve (g). It is important to provide a backup system

for limited damage in case of, for example, power failure. In this case, a normally close gate

valve (c) was placed at turbomolecular pump inlet flange. In case of a power failure, this valve

gate will close protecting both the pump and the cryostat vacuum. Additional vacuum


94

sensors, at high (b) and low (i) vacuum levels are needed for the control system to set the

optimum speed of the turbomolecular pump.

Another pressure release valve (e) is included also at the cryostat through one of the

connection box walls. All the cryostat should be in vacuum in normal operation, but in case of

a Helium leak from the inner pipes or a quick rise of the temperature levels of the cold mass

(when all the cryoadsorpted gases at the cold surfaces are released), the pressure level in the

cryostat could increase dramatically.

It is a relatively complicated vacuum system with many safety valves, but the

prototype performance should be robust under any circumstances, preventing damages to

important components.

Fig. 4.22. Vaccum system scheme

4.8 Cryogenic Supply System

As previously mentioned, the CSS was developed in collaboration with CERN. The first

objective was to prove the capability to liquefy Helium gas and to check the efficiency of the

system to transfer the cooling power of the cryocooler to the cryogens. In consequence, the


95

first CSS layout was not directly suitable for integration with a cyclotron (Fig. 4.23). These

steps were carried out by CERN:

- It was first tested and commissioned in stand-alone situation, using dummies to

model the thermal loads (coils and shield). The pipes for Helium delivery were cut

when the dummies were removed. Additional piping was needed to match the

transfer line for cryogen delivery to the cyclotron. The results were presented by the

authors in reference [62]. The CSS was able to provide 1.4 W at 4.5 K in the second

stage while the first stage was providing 25 W at 40 K.

- Additional features were also included in the sake of test flexibility. The main one

was the insert of a Joule-Thomson valve just before the output pipe. This

modification was not part of the collaboration agreement so it was removed before

the transfer to CIEMAT. When it was removed, one of the internal pipes coming from

a heat exchanger was also cut.

Fig. 4.23. a) The Cryogenic Supply System (artistic view). b) 3D model of the low loss Transfer

Line.

Once that this CSS was received by CIEMAT and CERN finished the collaboration, the

Author decided to include some improvements, as can be checked in Fig. 4.24:


96

- In order to improve its cooling capabilities, an additional port should be included for

the optional injection of liquid Helium from an external Dewar, which can be seen

schematically in Fig. 4.24 as a red line. It is designed as a double wall pipe. The outer

channel is in high-vacuum, while the inner one is the pipe to guide the injected liquid

Helium into the circuit. These pipes are made of stainless steel and thin walls were

used to reduce thermal losses. It is also as long as possible (between the main upper

flange and the output pipe of liquid Helium of the CSS. Additionally, thermal

anchoring is applied to the outer pipe at CSS thermal shield temperature. This will

increase thermal losses at the first stage, but the losses at the second stage which

are much more critical, are minimized.

- Due to the so long piping of the final real system, expected pressure drop will be

high. At nominal conditions, mass flow is very low so pressure drop will not be a

problem. However, during cooling down when Helium is not cold enough, the

maximum mass flow delivered by the pump is limited. Moreover, thermal power of

the first stage is higher than the second stage but the mass to be refrigerated at the

second stage is much larger (coils and casing). Because of these reasons, a manual

valve was developed. It was done to bypass the second stage heat exchanger during

the cool down until low temperatures are reached (green lines in Fig. 4.24). This

bypass was added to the original CSS by means of a manual needle valve to keep low

thermal losses.

- The Helium circuit was also revised to increase safeness and robustness of the

system in case of quench.

A schematic of the complete Helium system is presented in Fig. 4.25. Inlet and

vacuum ports are needed for cleaning the circuit by purging cycles. Helium purity is critical for

liquefaction, as present impurities could solidify and block the Helium flow. These cycles can

be done manually, starting by pumping air out from the circuit. Then, pure Helium gas from a

commercial bottle is used to fill in the circuit. Afterwards, the Helium is pumped away and

filled again with new pure Helium. The amount of air or any other gas species inside the

circuit is dramatically reduced at each cycle. Usually, a number from 3 to 5 cycles is

considered enough, depending on the actual initial situation of the circuit. The proposed


97

procedure is to use 4 cycles under normal conditions, but up to 8 cycles if it is the first use

after a change in the Helium circuit which includes important modifications (e.g., brazing

pipes, new filter). The purging procedure can be easily done with the manual valves MV2 and

MV3.

A manual valve bypassing the compressor (MV1) is used to reduce the pressure at the

inlet. As it is a volumetric compressor, it provides a constant volumetric flow. At warm

conditions the pressure drop in the system is so high that the inlet pressure would increase

too much for the nominal volumetric flow. By partially opening this valve, the net volumetric

flow provided by the compressor is reduced and so the needed pressure.

The safety system for overpressure management in case of quench, already explained

in the previous chapter, is shown also in Fig. 4.25 (PR1, PR2 and EV1). Under normal

operation it is not affecting the Helium flow at all because PR1 and PR2 are closed. Just in

case of overpressure these valves will open and then they will reduce the pressure at

cyclotron side. The pipes involved in this pressure relief system should be designed to be as

long as possible to keep low thermal losses. Nevertheless, it is providing a direct path for the

heat to flow from ambient temperature to the liquid Helium parts. The thermal anchoring of

this pipe to the thermal shield is also done to reduce thermal losses in the cold part of the

circuit.


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Fig. 4.24. Cryogenic Supply System schematics as supplied by CERN (left) and modified at

CIEMAT ready for cyclotron magnet cooling (right)

There are two buffers in the circuit. Both are needed to provide stable flow in the

circuit. Since just one compressor is being used, pressure is not really stable and this should

be corrected because of three main reasons:

- The MFC1 (Mass Flow Controller) is measuring pressure, temperature and mass flow.

It is acting on its internal valve to maintain the mass flow setpoint based on a closed

loop control system. If it is directly connected to the compressor outlet without a

buffer, the mass flow and pressure oscillations could result in a non-stable operation

of the MFC valve.

- It is a quite long piping system for the Helium to flow from the pump to the

liquefaction stage inside the CSS cryostat. On the other hand, once that liquid

Helium cooled the cyclotron coils, the resulting vaporized Helium is returning directly

to the return pipe of the circuit. Returning pressure variations from the compressor

coupled to the typical thermo-acoustic oscillations [98] when the Helium is being

liquefied could result in a non-stable operation or even the impossibility of Helium

liquefaction.


99

- The pressure regulator in the Helium supply bottle is not a precise valve.

Conventional industry grade was selected for this valve. When it opens or closes

according to the actual pressure level in the circuit, transient effects could be

noticeable. By using a buffer between the supply bottle and the circuit, any abrupt

pressure change in the pressure regulator is dumped in the buffer. The most critical

moment is during the cool-down stage, when liquid Helium starts to be liquefied.

Liquid density is much higher than gas density and therefore additional Helium is

injected to the circuit because of the constant volume of the circuit. This Helium

injection is not smooth or continuous, so that the buffer is responsible of dumping

the pressure oscillations in the system.

Fig. 4.25. Helium flow circuit


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4.9 Summary and Contributions about the Mechanical Design

In this Chapter, the Author describes a detailed procedure for the structural and the

thermal designs of superconducting magnets. Each possibility is briefly discussed and then a

decision is taken based on the particular specifications of the AMIT project and the state of

the art. The use of computation is needed for the optimization at several situations.

This chapter is one of the main contributions of the Author. It summarizes the

structural challenges of a superconducting magnet and the possible solutions. A conventional

design was selected from the state of the art while compactness is optimized. A procedure for

the thermal design is given in detail because it is critical for the efficiency objective. It

includes conventional procedures from the literature but also additional contributions from

the Author. One of them is the analysis of the radiation effects over the supporting rods and

the thermal shield to optimize the thermal anchoring position on the rods.

Some custom-made innovative solutions from the Author are also detailed in this

Chapter. They were developed to solve specific challenges of AMIT project:

- A simple code for rough calculations of thermal transients (which is useful when the

cooling power available is low compared to the mass to be cooled down)

- One example of a flexible heat exchanger between one gas pipe and a solid part for

low mass flow conditions when compactness and efficiency are critical and thermal

strains are expected between them.

- Procedure to design and optimize current leads shunts, taking advantage of the

material properties at each temperature.

Finally, the contribution about the adaptation of the CSS is explained. The conceptual

design of these changes is not really innovative, as they were found in the state of the art, but

their adaptation to an innovative device such as the CSS required important changes and

considerations to the whole system as reported in this Chapter.

Some of the contents of this Chapter were shown in MT24, Seoul (International

Conference on Magnet Technology, 2015) [99]. It was also published in [100].

Chapter 5. Engineering Design

A general overview of the proposed method to design a magnet for compact

superconducting cyclotrons has been shown, including some of the explicit choices for a

number of specifications. This chapter will provide additional information on the solutions for

the specific case of the AMIT cyclotron, as a reference for any other superconducting

compact magnet. It is important to depict how the theoretical concepts can be converted into

solutions which, even if sometimes are not optimal, they are a balanced trade-off.

5.1 Connection box

The necessity of connecting the cryogenic pipes while preserving the tight constraint

of the compactness was solved by designing a connection box. It will be placed below the iron

yoke (Fig. 5.1).

For the AMIT cyclotron, cryogens delivery is done through a 2.5 m rigid transfer line.

In order to minimize thermal losses while keeping the possibility of disassembly, special and

custom design was developed based on a retractable cover (A). All the elements and piping

inside the connection box are allowed to move to cope with the differential thermal

contractions, including those of the transfer line. Besides, all these elements are supported by

low losses rods, which are optimized while they allow easy access for maintenance.

This connection box is also a good place for adding some other ancillary components

which should be close to the cold mass and the coils but cannot be fitted inside the cryostat

due to space restrictions for enhancing the magnet compactness: for instance, the pressure

management components. Therefore, the vacuum pump will be connected to it (B), but also

the pressure relief system in case of quench (C).

Moreover, this connection box is also very convenient for the connections of all the

instrumentation wires coming from the casing and thermal shield, including thermal sensors

and voltage taps. Every connection should be done with high vacuum feedthroughs (D).


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The current leads will be installed inside this connection box, so that the cryogenic

pipes will also refrigerate the current leads heat exchangers. The power supply feedthroughs

are also included in this component (E).

Fig. 5.1. Connection box: front (top) and rear view without main flange (bottom)


103

5.2 Material choices

In the next paragraphs, the material selection for the main components of the

cyclotron magnet will be reviewed:

- Iron yoke: Usually, pure iron is selected for a superconducting magnet

with a nominal field of 4 T, because of the high saturation field. However,

pure iron is very soft and so its machinability is poor. Therefore, a low-

carbon steel (ASTM 1010) was selected because its magnetic properties

are quite similar to pure iron, while improved mechanical properties can

be reached. Magnetic properties of ASTM 1010 up to a field about 2 T can

be found from the supplier datasheet. On the other hand, the actual

properties of the material are sensitive to the exact amount of

components and the manufacturing process, so it was decided to

measure the magnetic properties of the actual material before the final

design. Moreover, microstructural characterization was done in the

samples to check any anisotropy due to the manufacturing process (grain

anisotropy) or the machining of the samples (surface effect). No

evidences were found by the Materials unit of Ciemat about different

magnetic behavior at different directions or surfaces. The magnetization

curve used in the simulations is shown in Fig. 5.2. High field

magnetization values (from 2 to 12 T) were included by extrapolation and

the whole curve was smoothed to speed up the simulations. This steel is

relatively soft, so that a surface treatment (bluing) was applied on every

part. This is also convenient to prevent corrosion damage, for instance.


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Fig. 5.2. Magnetization curve of the procured ASTM 1010 material

- Casing: 316L: Stainless steel is preferred for the casing and cryostat for

several reasons including weldability, thermal contraction coefficient

close to that of the superconducting coils, corrosion resistance and

emissivity at low temperatures. Additionally, it is important that the

material for the casing and cryostat is as much insensitive to the

magnetic field as possible. For this later specification, austenitic stainless

steel is the best choice. In terms of fracture toughness, some austenitic

steels tend to become brittle at cryogenic temperatures. The best

common austenitic steels for low temperatures while preserving ductile

behavior are the series 304 and 316. Finally, by considering the best

combination of low temperature behavior, stress limits and corrosion

resistance, 316L was selected for this specific device. 316LN would be a

slightly better choice from the point of view of magnetic permeability,

but it is very difficult to be found in the market for such a small quantity

and complex geometries. The casing is split in several welded parts to

ease assembly and also machining, taking into account the size and shape

of the available commercial steels. For reducing the radiation heat input

into the casing, surface finish is important, but in order to minimize as


105

much as possible this heat, low emissivity foil will be wrapped around the

whole outer surface of the casing. The filler selected material was

ER316LMn [101] which is a commonly used material for cryogenics. It can

be used for TIG and MIG welding applications for austenitic steels where

two important properties are required: impact toughness at low

temperature and non-magnetic weld. ER316LMn is a low carbon material

resulting in a completely austenitic weld so magnetic susceptibility is low

and it is not affected by the welding process.

- Rods: As explained in the thermal design chapter, a thermal insulating

material is needed for supports. It must be non-magnetic (same reason as

for the stainless steel) and as stiff as possible (to reduce displacements of

the coil if magnetic forces change after a current variation). In this case,

the figure of merit for candidate materials could be stress limit vs.

thermal conductivity. Glass Fiber Reinforced Composites (GFRC) are the

best option. Being these composites made of glass fibers and a thermoset

polymer, they are electric insulators and besides a small cross section can

withstand a high load. Being thermal insulators with small cross section,

thermal losses will be very low. Of course GFRC are, because of their

constituents, intrinsically affected by low temperatures, and most of

them become brittle when cooled down. Fortunately, there are some

specialized composites in the market designed for cryogenic

environments. They are labeled as CR, being G-10CR and G-11CR the

most common grades. Market availability is a very important constraint

for using composite materials: the cost of producing custom material is

very high, while machining of the composites is risky and not

recommended so material as close as possible to the final shape should

be purchased [85]. For the AMIT cyclotron, G-10CR was found in proper

geometry (plate) and then the final section was designed according to the

available thickness plate. It is also important to take into account the

orientation of the glass cloth to keep the mechanical properties of these

composites.


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- Thermal Shield: Oxygen Free Copper. The most important parameter for

the thermal shield material is its thermal conductivity. As it will be

refrigerated from just one side (lower side), while the heat coming

through the thermal intercepts at the rods and radiating from cryostat is

distributed around the whole shield, the higher the conductivity the

lower the temperature gradient. The thermal conductivity should be high

even at low temperatures (it will be cool down to 50 K). Mechanical

properties are not so critical, but of course it must be feasible to

manufacture such a complex geometry, being supported at the thermal

intercepts, without large deformations. Copper capabilities for being bent

or soldered are useful for the design. Moreover, it can be bought in a

wide range of thickness and, even more, if is thick enough, some threads

can be machined. The oxygen free grade is preferred for improved

thermal conductivity at low temperatures. The surface roughness of the

thermal shield is important for the emissivity, even critical for the outer

surface which will be in radiative contact with the cryostat wall. The

cryostat will work at vacuum conditions, but to prevent the risk of

corrosion-induced increase of emissivity and to minimize the actual

value, the external surface will be covered by cryogenic low-emissivity

foil. Because of the reduced available space, and to ease vacuum

pumping, low emissivity foil RUAG Coolcat 4K has been used instead of

classical multilayer insulation.

5.3 Fabrication techniques

Some special fabrication techniques are included in this proposal for a compact

superconducting cyclotron magnet. While most of them are well known in the state of the art

for this kind of cryogenic devices, a few are more innovative to reach certain particular

specifications.


107

High-vacuum environment

The magnet cryostat, transfer line and CSS need high vacuum for optimal thermal

insulation. Therefore, clean surfaces are needed for all the parts in vacuum. Dust or grease on

the surfaces is a potential problem for the vacuum system and the minimum achievable

vacuum level. When possible, closed volumes should be avoided, as the trapped air will

require a lot of time to be evacuated. For example, for joining two parts, it is preferred a bolt

and a nut rather than a bolt screwed to a threaded blind hole.

Welding preparations will be designed in such a way that small volumes or clearances

are not in the vacuum side of the assembly. There will be structural welds, but most of them

will be vacuum-tight. The manufacturing procedures will include leak and pressure tests at

intermediate steps of the fabrication and assembly to be sure that the welds are compliant

with the vacuum specification.

Besides, welding preparations must induce small deformations. For example, the

casing parts are welded around the coils, keeping them aligned while minimizing the gaps to

avoid unexpected movements of the coil inside the casing during powering. As shown in

Fig. 5.3, there are machined channels close to the weld to reduce the heat conduction to the

coils, which are very close, and to minimize deformations. The low thermal conductivity of

the stainless steel is a further advantage. At the same time, the coils must not undergo a

dangerous overpressure during welding. The overpressure is limited by parts of the casing

which are not welded and machined with high accuracy, behaving like a hard stop.

Fig. 5.3. Welding preparation (casing)


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Assembly procedure

As a consequence of the compactness objective, integration procedure is challenging.

Even more, it is very likely that disassembly (at least, partial) will be needed in this first

prototype. Since many parts are welded, disassembly is complicated, but always kept in mind

during the engineering design of the magnet.

As the cryostat was designed to be as compact as possible, its assembly is required to

be done directly on its final position (around the casing). Then, the cryostat was designed as a

group of nested layers.

The thermal shield around the cold section of the supports is made of the same

oxygen-free copper but it was found that its stiffness could be a problem for the supporting

concept. These supports can just hold pure tension, and the joints are free to rotate but

under certain conditions the shield stiffness could restrict the proper rotation of the rod as it

will be shown in Fig. 6.8. It illustrates how some grooves were designed at the shield to

reduce its stiffness so that the rod can rotate and accommodate much better any possible

movement of the casing, like, for example, the obvious thermal contractions when cooled

down. The angle of the rods would change depending on the temperature of the cold mass,

but the supporting system is always working under a purely tensional load.

Similar situations can be found for pipes inside the cryostat; some of them must be

finally joined or bent after preassembly with adjacent parts to check actual position. In such a

confined volume, there is no place for long pipes capable to absorb small geometric

discrepancies. For instance, when two parts are connected by a pipe, this pipe will be tuned

after a preassembly check. For these joints, it could happen that welding is too dangerous

because of the high temperatures. Brazing techniques are proposed in these situations. There

are some other advantages for brazing compared to welding like its capability for dissimilar

joints. It is recommended for joining stainless steel to copper. From the thermal conductivity

point of view, copper is better than steel, but some parts must be made of stainless steel,

mainly connectors for vacuum or pressure gaskets.

For example, a removable joint is preferred at certain points for inspection, allowing

access to check/repair feedthroughs or wires, removable filter maintenance or even easier

assembly. In all these situations, a removable cryogenic hermetic sealing is needed. There are


109

some commercial solutions available from the market. For the AMIT cyclotron the ones

selected were Kenol fittings [102] and CF fittings [103]. Both of them require stainless steel

flanges. The reason is that the fitting is actually made by pressing two steel flanges (hard) on

a soft metal seal. There is a range of materials available for the seal, the most typical ones

being Cu, Al or In. Suppliers provide some guidance for selecting the most proper one for

each application in terms of temperature, pressure and expected time before reopening

[102]. For the challenging case of liquid Helium, aluminum gaskets are recommended for a

mock-up which is expected to be re-assembled several times, but stainless steel ones are

better for the definitive assembly.

Another important check to be done in cryostats concerns thermal induced stresses.

As some parts are at ambient temperature and some at cryogenic temperature, the

supporting scheme must cope with thermal strains for releasing any possible thermal stress,

both during transient and permanent modes of operation. In the case of the AMIT cyclotron,

there is one pressurized circuit inside a vacuum vessel. The design should ensure a proper

contraction and positioning of cold parts without producing thermal stresses. Sometimes a

flexible part is needed for this requirement to be met. Metallic bellows can be found in the

market, and they are usually made of stainless steel and can be procured including desired

flanges for installation. However, they are not adequate for high overpressures.

In a similar way, the iron yoke will be assembled around the cryostat. It has been split

in several parts, compatible with the existing plates of low carbon steel in the market, and the

complex geometry of the cryostat, mainly due to the supports and apertures for vacuum

chamber and cryogenics. A cross-section is shown in Fig. 5.4, where three important

considerations were taken into account in the design:

- In order to maximize symmetries for beam dynamics stability, there are some

additional holes included in the iron which are idle, not used for any component. This is the

case of the four vertical holes going through the iron; two of them are needed to

accommodate the Helium pipes, while the other two are for radial symmetry and all the

upper side ones are drilled for the horizontal mid-plane symmetry.

- Poles should be easily dismountable in case that their profile must be tuned after

the first magnetic characterization. Mechanical tolerances (from manufacturing and


110

assembly) for the whole cyclotron (including not only the magnet but also the RF cavity, ion

source, target…) are critical for the actual extracted beam current. It is a quite common step

in every cyclotron to adapt the pole geometry after the real first measurements are done, so

this possibility cannot be excluded, even when the design will try to get it good enough for

specifications at first attempt. Because of that, this design was done to make pole

disassembly possible while the cryostat and other parts remain in place.

- For the first assembly, the iron will be mounted around the cryostat alone, but the

connection box will be assembled immediately afterwards, and welded to the lower end of

the pipes. Because of this, the iron partitions for accessing the poles are according to a more

complicated scheme than expected. General procedure for the iron design and assembly

should take into account any special needs for accessories and peripheral systems.

In addition to this, some other minor details are considered in the following

paragraphs.

When feasible, radial partitions are preferred as they have minimal effect on the

magnetic field map. Partitions in the iron yoke resulting in non-radial gaps could produce a

non-negligible effect on the total magnetic flux because of the differences in reluctance

between the air and the iron and how the magnetic field tends to follow the lower reluctance

path. Radial gaps can be easily avoided by the flux, while transversal ones are crossed by the

flux. Everywhere a transverse gap cannot be avoided, special care should be added to provide

as good contact as possible between the iron parts.

Some constraints coming from material procurement could affect the dimensions of

individual parts, in especial when the magnetic properties of the material are much more

critical than the geometry. Then, the final iron design should start just after the material is

checked. Maximum weight for a single part should be checked also in case that there is any

constraint for handling it for assembly or disassembly at final operation site. Additional

partitions could be needed.


111

Fig. 5.4. Cross section of the iron yoke. Each individual part is drawn with a different pattern.

Instrumentation

A prototype is typically equipped with many sensors to be able to understand the

performance and to identify the potential sources of problems. In this cyclotron magnet, we

will mainly need sensors to measure temperatures, voltages and stresses. Vacuum and

pressure gauges are also necessary but they are commercial, so no special attention is

devoted to them.

Cernox sensors are selected for this magnet as they are able to measure with good

accuracy temperatures of few Kelvins in a high background field. Cryogenic temperature

measurements are not simple because of several reasons. For instance, the measurement is

very sensitive to the quality of the bonding between sensors and the part to be measured

(risk of high contact thermal resistance). In addition, the intrinsic low thermal capacity and

conductivity of the materials at cryogenic temperatures yield in large temperature oscillations

and temperature gradients, even for low input heat values. The input heat from the wires

which connect the sensor to the monitor is also important to be considered when designing

the system. General guidelines on cryogenics measurements should be followed to minimize

these difficulties, which can be found for example in [104]. Temperature sensors are bonded

using Stycast 2850 FT which features a good balance of thermal conductivity and electrical

insulation at low temperatures.


112

The voltage taps allow measuring the voltage drop at each coil, which is the best

method to detect quench propagation. In the order of milliseconds, a quench can be detected

and protection systems can be fired. It was decided to put a voltage tap at the splice between

both coils. It is a risk, because that is the point with the highest voltage to ground in the case

of a coil quenches, but it allows to distinguish which coil is quenching and early detection of

the quench, because subtraction of the voltage drop of each coil eliminates the inductive

component of the voltage. Special care needs to be taken to insulate that voltage tap, which

has very difficult access indeed. The voltage drop at the superconducting and resistive

sections of the current leads can be also monitored to detect a quench or a thermal runaway.

The measurement of the mechanical load in the rods, as stated in the previous

chapter, is critical. A number of strain gages will be bonded to the end part of the cryostat

supports. Since the rods are round, two flat planes will be machined at opposite sides. Four

strain gauges will be glued, two on each opposite plane, one for transverse strain and the

other one longitudinally. They will be calibrated and tested to increase accuracy.

This configuration will yield the best sensitivity for tensional strain measurement. The

relationship between load and strain measured by the gages should be first evaluated, from

analytical equations or FEM methods. Once the expected range of measurement is known,

the gain of the acquisition electronics can be tuned to achieve the highest sensitivity.

Gages selection is very important for a proper measurement, so typical

recommendations should be followed [105]. The method of installation proposed in this work

includes some advantages compared to other possibilities:

- The parts on which the gages are bonded are at ambient temperature, so no special

adhesives are needed to operate under cryogenic temperatures.

- The gages are outside the cryostat, so there is no need for vacuum feedthroughs.

Access for maintenance is easy.

- As a complete bridge is bonded at each rod, the ambient temperature variations are

self-compensated. Besides, bending torques are also compensated between the opposite

gages, and only pure tensile longitudinal stresses are measured.


113

On the other hand, as the rods are tested and calibrated before installation, special

care should be taken to not damage the gages or the bonding adhesive during the welding

operation of the near parts of the cryostat.

During calibration of gain and offset of each bridge, it is important to use the same

cable to the converter. In the case of this cyclotron, the control rack will be far away from the

cyclotron to reduce the footprint, then, long cables will be used which will affect the

measurement. Moreover, calibration should be done with the same cable, being this cable

made of paired shielded wires to minimize electromagnetic interferences (EMI).

5.4 Validation of the refrigeration scheme.

The total power needed for the AMIT magnet is detailed in TABLE XIV. These are the

thermal demands to be satisfied by the refrigeration scheme. These numbers depend on the

actual temperatures of both stages, so they are valid just for the indicated temperatures

(4.5 K and 60 K).

As described in earlier sections, the proposed refrigeration scheme is based on small

mass flow of liquid Helium in intimate contact with the coils. Thus, it is an internal biphasic

forced flow system. Therefore, it is a quite complicated system for an analytical study

because all the heat transfer modes should be included in the equations: convection,

buoyancy, viscosity, conductivity, etc. Moreover, liquid Helium properties are quite special

compared to other fluids. For the specific situation of coil refrigeration by using the liquefied

Helium from the CSS, the fluid path is very complex and usual relations for fully developed

flow regime cannot be directly applied. Based on these circumstances, the proposed model of

using empirical correlations for liquid Helium flow needs to be checked by empirical

validation for the specific situation of the AMIT cyclotron.


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TABLE XIV CYCLOTRON MAGNET: THERMAL BUDGET

Contribution Heat (W)

Loads @ 4.5K

Casing 0.69

Cold mass at connection box 0.05

HTS Current leads (cold side) 0.07

Pressure relief system 0.1

TOTAL at 4.5 K 0.91

Loads @ Thermal Shield 60K

Thermal shield (cryostat) 11.4

Thermal shield (connection box) 5.3

HTS Current leads (warm side) 10.5

Pressure Relief system 7.0

TOTAL at 60 K 34.2

The cooling concept of the cyclotron magnet is based on a liquid Helium flow in direct

contact to the coils through a channel. In the sake of compactness, the inlet of the coolant

will be below the iron yoke, where there is space for the connection box. The coolant will go

to the lower coil and then cool the upper one, going back to the connection box and, finally,

to the CSS for cycling. The size of this channel is designed as a compromise: The channel

should be small enough to fit into the casing in a compact design but also to provide a

turbulent flow in nominal conditions (0.1 g/s, ambient pressure) for enhancing the convection

heat transfer from the coil surface. On the other hand, this channel should accommodate

higher mass flow during the cool down stage (considered up to 0.5 g/s and 3 bar) without

significant pressure drop (in the order of mbar). A cross section of 10 by 2.5 mm was found

adequate for the casing wall thickness of 8.5 mm, while the Reynolds number for nominal

flow is about 5000 and thus, it is turbulent according to [106], which states that turbulent

flow is reached for a tubed-shaped geometry for ReD>4000, being ReD the Reynolds number

based on the hydraulic diameter of the cross section geometry. The expected pressure drop

during cool down with this channel is in the order of 10 mbar if one complete turn is done at


115

each coil so no additional turns are included. As liquid Helium absorbs heat from the coil, it

boils. Thus, a mixture of vapor and liquid Helium (two-phase flow) is the actual cooling fluid

and the channel was designed in helical shape to facilitate the vapor flow.

To prove the feasibility of the solution adopted to cool the coils, some tests were

carried out in a mock-up setup, as published in [107] and shown in Fig. 5.5.

Fig. 5.5. Cross sectional view of coil and casing mock-up

A coil made of the same wire and number of turns as the cyclotron one was

manufactured. It has exactly the same cross section, in order to study the transverse thermal

conductivity from the cooling channel to the outermost coil turns. The external diameter was

halved in order to accommodate it inside the available cryostat in the existing test facilities.

Only one coil is present to ease the fabrication and assembly of the casing. Materials to be

used are the same as the final cyclotron magnet, including low magnetic permeability

stainless steel and Cernox sensors [108] for precision low-temperature measurement in high

background magnetic fields.

The conceptual design of the test is depicted in Fig. 5.6, while Fig. 5.7 shows pictures

of the casing and the insert. The testing setup includes a portable LHe (Liquid Helium) Dewar

as reservoir of cryogen instead of the CSS system and several controlled valves to regulate

and measure the flow. Pressurization of Dewar is achieved with a Helium gas high pressure

bottle controlled by a flow controller equipped with a pressure gauge (forward configuration)

[109]. Instead of using a thermal shield cooled by cool Helium gas, radiation shielding is


116

cooled by a liquid nitrogen reservoir. This is much easier in terms of mock-up design and

manufacturing, while the actual power refrigeration supplied by the Helium flow is not

affected at all. Besides, the validation calculations are simpler because the temperature of

the thermal shield is very uniform.

Helium mass flow is measured at the outlet by two mass flow controllers. The first

controller measures the Helium flow through the current leads (vapor cooled) while the

second controller measures the Helium flowing out at the main port of the cryostat upper

flange. Long pipes at the outlet including optional heaters to control the temperature of the

exhaust are mounted. As a result, the Helium flow is measured at ambient temperature

which is an important constraint to get accurate values from the mass flow gauge.

Fig. 5.6. Test setup for cooling scheme validation


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Fig. 5.7. Coil inside the casing for test. Insert ready to be installed in the test cryostat

The analytical calculation of stress and temperature distribution for this single coil

test were reported in [76]. In order to include in the model the mechanical and thermal

anisotropy of the coil and the cryostat thermal losses distribution, FEM models were

performed using ANSYS (Structural and Thermal) modules coupled with Ansoft Maxwell

(Magnetic). Results from these models are summarized in Fig. 5.8 (temperature distribution)

and TABLE XV (heat losses), where K is the integral conductivity. About 1.5 W of heat losses

are expected for this setup, while the real cyclotron magnet needs just 1 W. Calculations and

models can be extrapolated to the cyclotron magnet even if they are not fully compatible.

The main reasons for this discrepancy are the inlet and outlet Helium pipes inside the cryostat

and the Helium vapour cooled resistive current leads. Also, radiation shield temperature for

this setup is about 77 K, instead of 60 K for the real cyclotron magnet.

The first parameter to be checked, which is directly linked to the temperature

distribution shown in Fig. 5.8 (a), is the coil thermal conductivity. Real constituent materials

are included in the coil model, so that the actual properties of the composite material (Cu,

NbTi, insulation and epoxy) are evaluated in each direction. The temperature distribution in

the cryostat can be also checked in Fig. 5.8 (b). Liquid Helium flows inside the casing and the

evaporated Helium is evacuated through the outlet pipe while it is used for cooling the

current leads. A high vacuum level is kept in the cryostat around this assembly to avoid

thermal losses by convection.


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Fig. 5.8. Temperature distribution computed by FEM simulations on Coil(a) and cryostat (b).

The refrigeration capabilities of the fluid inside the circuit, both steady state and

transient, were modeled by using the custom-made FEM code based on dedicated Matlab

functions developed by the Author and explained in Chapter 4. Limitations on the physics

included in this model were already exposed, but in this validation test a good match was

found compared to the experimental results.

The temperature evolution of the coil was measured at a constant LHe flow input to

check accuracy of heat loads and fluid flow models. Temperature sensors were placed in the

flat side of the coil, near the outlet of the casing while the calculated temperature shown in

Fig. 5.9 is the coil mean value. An unexpected fast cool down was found at the beginning of

the process. The whole casing was cooled using liquid nitrogen which was then evacuated

prior to the liquid Helium flow. Once the Helium mass flow is kept constant (450 g/h), the

simulated and measured temperature evolutions is very close.

On the other hand, real final temperature of the coil is not 4.2 K as expected for this

450 g/h LHe flow. This can be explained as the effect of an important heat loss (about 4 –

5 W) at the LHe transfer line connecting the Dewar and cryostat. Since no information about

the transfer line performance was available, a second test was performed.


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TABLE XV STEADY STATE HEAT LOSSES CALCULATIONS FOR SINGLE COIL TEST

Heat loss contribution Main parameters

Value

(W at 4,2 K)

Conduction (Rods) K 0-77K = 18 W/m 0.028

Conduction (Inlet) K 0-77K = 3000 W/m 0.42

Conduction (Outlet) K 0-77K = 3000 W/m 0.08

Conduction (GHe In) K 0-300K = 27 W/m 0.027

Conduction (GHe Out) K 0-300K = 27 W/m 0.3

Radiation to casing Cold Surface 0.16 m2 0.57

Current Leads 100 A (nominal) 0.1

Conduction Vacuum 0.07 W/m2 0.012

TOTAL 1.54

Once the mock-up coil was filled with liquid Helium and the temperature was stable,

the transfer line was removed. In those conditions, the vaporization rate of LHe was 220 g/h,

or 1.3 W. This value is very close to the predicted one, 1.5 W.

For the third test, the equilibrium temperature of the coil as a function of LHe mass

flow was measured. Results can be found in Fig. 5.10. Simulations were performed including

additional heat losses to model the effect of the transfer line based on the results of the

second test. In this case, the best correlation with experiment data was found for high mass

flow if including 5 W of heat losses at transfer line as shown. The lower the mass flow, the

lower the losses from the transfer line, which can be explained from the depressurization of

the Dewar [110]. In fact, the increment of the flow is achieved by pressurization of the Dewar.

The difference between heat losses with and without transfer line confirms a real heat loss of

4.5 ±0.5 W in the transfer line and 1.3 W in the casing. And so, the cooling scheme was

proved to be feasible for this cooling power and temperature. During the first stage of this

test (Fig. 5.9), additional mass flow was injected into the system (higher than 450 g/h) due to

an operational mishandling. This is the reason of the punctual increment on the cooling rate.

The simulation was performed for a constant mass flow.


120

Finally, a training test of the superconducting coil was performed. No quench was

triggered before reaching nominal critical current considering the load line of the wires for

this mock-up (169 A). Both the first and second quenches reached the same value, that is, no

degradation took place. Then, the coil manufacturing techniques, including winding,

impregnation, splicing and concept design of the aluminum shrinkage cylinder, were validated

with this setup before moving to the production of cyclotron coils.

Fig. 5.9. Coil temperature during cool down on Single Coil Test: comparison of measurements

(dots) and calculations (dashed line)

Fig. 5.10. Equilibrium temperature of coil as a function of LHe mass flow: comparison of

measurements (dots) and calculations (solid line) assuming 5 W of heat loss in the transfer line

for any mass flow.


121

As a result of this test, the following conclusions must be outlined:

- It has proved that the proposed cooling scheme of the cyclotron, based

on a helical channel, is sufficient to evacuate about 1 W of heat at 4,2 K

while keeping a compact design. Potential risks come from the transfer

line and possible mechanical overpressures after quench, where pipes

are longer that in this test setup. Overpressure management should be

designed carefully as described in previous chapters.

- Fluid dynamics modeled from empirical relations related to non-

dimensional parameters are not perfect but good enough to roughly

describe the behavior of the system. The custom-made FEM model has

been proved to be very effective in terms of accuracy versus

computational resources ratio. The presence of not completely

developed flow is probably one of the most limiting assumptions in the

model.

- Electromagnetic and structural design procedures have been proved to

be adequate in terms of specifications and performance. Fabrication

techniques are validated prior to the manufacturing of the larger coils of

the cyclotron.

5.5 Summary and Contributions about the Engineering Design

In this Chapter, the Author describes important features involved in the development

of a superconducting magnet. Discussions about materials, welding procedures and high-

vacuum environments are detailed. Innovative solution for assembly procedure is presented

to fit the compact design from previous chapters.

In addition, the validation tests for the refrigeration concept are detailed and their

results are shown. They were published in [107] and presented in EuCAS15, Lyon (European

Conference on Applied Superconductivity) [111].

Chapter 6. Fabrication and Assembly

In the previous chapters, design tools and manufacturing procedures have been

proposed for compact and efficient superconducting magnets for cyclotrons. Some of them

have been detailed for the particular specifications of the AMIT cyclotron. This chapter will

provide additional information on how they can be used in real life and how it was done for

this cyclotron project. The superconducting coils and the Helium vessel were manufactured at

ANTECSA [112] (Portugalete, Spain) while the thermal shield and the cryostat were

manufactured at “The Vacuum Projects” [113] (Paterna, Spain). Iron assembly and further

tests were done at CIEMAT-CEDEX facilities in Madrid.

6.1 Superconducting coils

The superconducting coils are basically two solenoids with nearly square cross-

section. Epoxy resin is used to provide mechanic stability. Wet impregnation is more

economical than vacuum one, both from the point of view of tooling complexity and

manpower. However, it is a challenging technique for a coil with such a high number of turns

because of the short pot life. A collapsible mandrel allows easy disassembly of the winding

tooling. Tight tolerances of this tooling guarantees good dimensional accuracy of the finished

coils. Its feasibility was proved with the fabrication of the coil for the validation of the

refrigeration concept.

In order to manufacture a robust coil with good electrical insulation, in special during

handling and assembly into the casing, glass fiber foils are added onto the flat sides and

prepreg clothes wrapped around the last layer of wires.

Finally, a thermoretractable tape was wrapped around the coil. When the coil is

introduced into the oven to cure the resin, the wires are pressed, which improves their

compactness and dimensional accuracy. Of course, additional paths are provided for the

excess of resin to flow outside the coil while the pressure is applied. Some steps of the

manufacturing process are shown in Fig. 6.1.


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Fig. 6.1. Coil manufacturing: Winding (Left) and thermoretractable tape wrapping (Right)

Once that the coil is cured, and the tape and mandrel dismantled, the coil is perfectly

characterized by metrological verification. The assembly of the coil with the shrinking cylinder

demands a very accurate and polished coil outer diameter. The glassfiber bandage is

machined in a lathe to accomplish the tight tolerances for its dimension and circular shape.

Special care must be taken to not machine or damage any of the outermost wires.

The first manufactured coil was made of copper wire for the validation of the

refrigeration concept to check the winding procedure. Its diameter is halved compared to the

cyclotron coils, but any other parameter is identical (e.g., number of turns, wire diameter,

manufacturing process). Therefore, it is used as a Reference Coil (RC) for electrical

comparison at warm temperature. Four superconducting coils were manufactured and

tested, named from SC1 to SC4. Three of them were considered successful. A selection of the

most relevant parameters from the metrological measurements is shown at TABLE XVI.


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TABLE XVI COILS METROLOGICAL MEASUREMENTS

Nominal value SC1 SC3 SC4

Coil Ext. Diameter 525 526.00 525.55 524.95

Cilindricity <0.1 0.069 0.064 0.058

Int. Diameter 413.72 413.56 413.55

Height 5 56.13 56.09 56.01

Some problems when manufacturing coils can arise, such as damage of the wire

insulation (see Fig. 6.2). In this case, in the second coil, SC2, it occurred at the inner diameter

during the demoulding of the coil from the mandrel.

Fig. 6.2. Insulation defect at the inner diameter of SC2 coil

Sometimes damage like the one reported above can be repaired, but, unfortunately,

this kind of damage is not always visible. Moreover, the quality of the coils is critical for the

magnet performance. For instance, one short circuit between two adjacent wires will reduce

accordingly the total ampere-turns in the coils, and so the magnetic field provided by the coil.

Besides, there is a significant risk of major damage at that weak point in case of a quench.

Electrical measurements are the best option to check coils integrity along the

manufacturing processes. The leakage current between the wires and the tools is

continuously measured during winding. After curing, a complete set of electrical

measurements should be done to check the integrity of the coil. There is also a risk of


126

damaging the coil, for example, during the assembly of the aluminum shrinkage, so

measurements should be repeated after each assembly step.

The electrical insulation measurements are carried out at ambient temperature and

they are better evaluated by comparison against a reference coil. When there are just few

turns in the coil, this kind of damage can be easily detected by direct resistance or inductance

measurement, or discharging a capacitor. In the specific case of such a large number of turns,

there may be not enough accuracy in these measurements.

Another possibility is to measure the resistance and the self-inductance as a function

of frequency by means of an impedance analyzer. It provides information on the electrical

behavior of the coil when compared to a reference coil. This technique is usually sensitive

enough for detecting faults and it is very powerful if combined with a common ground

insulation measurement. Applied voltage for this insulation measurement should be high

enough to cover any possible situation of the coil during its application. In the specific case of

these coils, they were tested up to 500 V, as explained in the quench protection chapter.

TABLE XVII shows the impedance measurements for direct current and three

frequency values. The measurements obtained for the reference coil when one short-circuit

was intentionally induced in one layer show that the highest sensitivity can be found at the

resistance value for high frequencies. This is shown in the table in bold numbers.

In general, the electrical parameter shift is a complex function of several variables:

coil geometry, wire specifications, number of turns and of course the kind and severity of the

damage.


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TABLE XVII COILS ELECTRICAL MEASUREMENTS

Reference Coil

(Cu, half

diameter)

Ref. Coil

(1 layer short-

circuited)

SC1 SC2 SC3 SC4

R (Ω) 87.66 87.78 225.82 222.2 224.15 227.78

100 Hz

L (H) 3.366 3.36 12.46 12.42 12.43 12.44

D(R/X) 0.042 0.043 0.030 0.029 0.029 0.029

Rs (kΩ) 0.089 0.091 0.236 0.224 0.229 0.230

s (kΩ) 2.115 2.111 7.828 7.80 7.81 7.815

1 kHz

L (H) 3.456 3.44 17.96 19.1 18.01 17.679

D 0.0089 0.0238 0.0098 0.011 0.013 0.0122

Rs (kΩ) 0.193 0.514 1.09 1.22 1.53 1.270

s (kΩ) 21.715 21.614 112.7 113.6 113.1 111.08

10 kHz

L (H) -1.96 -2.098 -0.46 -0.414 -0.458 -0.482

D 0.0802 0.159 0.02 0.0206 0.023 0.024

Rs (kΩ) 9.877 20.96 0.6 0.56 0.62 0.7

s (kΩ) -123.151 -131.82 -27.9 -25.99 -27.71 -29.101

The electrical measurements shown in TABLE XVII were done before the assembly of

each coil with aluminum shrinkage. All the coils seemed to be perfect, because there were no

major changes in the values when comparing the coils. When the next assembly step was

performed, the shrinkage of the aluminum cylinder, the insulation of the SC2 was degraded

and therefore it was discarded. Concerning the SC3 coil, a variation in the value of RS was

found for 1 kHz (1.53 %), but not as high as for the reference coil, so this test is not decisive in

this case. In order to clarify it and to understand the reason, a further step was taken on the

frequency analysis of the coils. A complete scan of impedance measurements from 5 Hz to

10 kHz was carried out. The measurements are presented in Fig. 6.3, which include the

impedance scan of coils SC2 and SC3. In addition, two artificial short-circuits in a single turn

were induced sequentially by soldering a copper wire (0 Ω) and a 0.5 Ω resistor between two


128

adjacent turns of the SC2 coil. External insulation damage on SC2 should not affect the

resonance, and using SC2 as the reference for a short-circuit is very convenient.

After performing the complete frequency scan, the reason was found and so, this

work proposes to directly test the superconducting coil with this procedure.

The coil is not a perfect inductor: it has some stray resistances and capacitances

associated to it. The inductive and the capacitive components of the impedance are related to

the frequency according to eq. (13). The Self Resonant Frequency (SRF) of the coil is the

frequency at which these two components are equal. Thus, the equation for the resonant

frequency is the same as a general RLC system, this is eq. (14).

2

2 (13)

2 √

(14)

The inductive value L, for a coil, is always known from design because the main

contribution is the actual inductance of the coil (for example, parasitic inductive effects of the

leads are negligible). On the other hand, the value of the reactance C strongly depends on the

parasitic components.

In general, the value of the resonant frequency of a coil is very high, so that it works

well below the resonance and the higher frequency, the higher the sensitivity to check coil

integrity, as explained before.

In the particular case of the AMIT cyclotron, the SRF is about 1765 Hz and the

spectrum is quite narrow. Thus, both measurements done at 1 kHz and 10 kHz were outside

the resonance effects. This “low” frequency could be explained from the induction value L for

the coil, which is quite high (12 H).


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Fig. 6.3. Inductance spectrum of SC2 and SC3

In conclusion, these measurements are the example of how the impedance can be

used for checking the integrity of a coil if the measurements are done close enough to the

resonance frequency. In this case, SC3 seems to suffer a lack of insulation between loops. It is

clear that there is no direct short-circuit, but a contact value in the order of 1 Ω has been

measured for one wire loop.

Two coils are perfectly compliant with the specifications (SC1 and SC4) while a third

one just partially (SC3). The next step is the assembly of the aluminum shrinking ring. There is

some interference (0.2 mm in diameter) at ambient temperature as it was designed and

explained in the previous chapter. The safest procedure for the assembly is by temperature

difference. The aluminum ring was heated up 100 ºC and the coil was cooled down to -20 ºC.

For a reference temperature of 20 ºC, and thermal expansion coefficients of 2.4e-5 K-1 for

Aluminum and 2e-6 K-1 for the coil, the shrinkage diameter increases 0.30 mm and the coil

reduces 0.042 mm. This is enough for the interference plus a clearance of 0.1 mm, needed for

the assembly.

Finally, the coils were tested at nominal temperature and current. Strain gages were

measuring the stresses at the shrinkage cylinder and current was increasing up to the value

which provided the nominal working point on the load line. Nominal design current is


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different because these tests were performed without the iron yoke. Therefore, the magnetic

field is not the same in the coils for the same current and the consequence is a shift on the

load line margin. In order to test the coil at an equivalent margin, the current in the tests

must be increased. Nominal equivalent current for the tests is 125 A (70 % working point),

and current up to 150 A (80 % working point) was the maximum value for the tests. Some

information about these tests can be found in TABLE XVIII. All these three coils reached

successfully the ultimate current in few quenches. They were tested immersed in a liquid

Helium bath at CERN facilities (SM-18), because there are cryostats large enough to

accommodate these coils. In fact, two coils were placed at the same time in a vertical

cryostat. Electrical connections with the cryostat current leads were done carefully because

the induced flux in the idle coil can change rapidly during the quench, yielding large induced

voltages. SC1 showed worse memory than SC4, but is was considered valid since it reached

again the ultimate current in few quenches and none of the quenches was below nominal

current. SC3 reached the maximum test value at first power up.

TABLE XVIII COIL TRAINING

Equivalent Nominal Current: 125 A, Tests up to 150 A

Run SC1 (A) SC3 (A) SC4 (A)

1 130 >153 119

2 136 148

3 152 >150

Retraining

1 138 153

2 143

3 152

To summarize, four coils were manufactured. All of them were characterized and this

is the decision on which ones are used: The SC1 coil was used as lower coil and SC4 as the

upper coil. SC3 remained as a spare part because of the medium detected short circuit, and

SC2 discarded as previously explained. The manufacturing of the shrinking aluminum

cylinders and the casing parts was tailored to the actual dimensions of the selected coils to be

used.


131

The final step is the installation of the temperature sensors (Fig. 6.4). As it is very

important to monitor coil temperature with accuracy, sensors must be installed directly on

the coil surface. Even more, two sensors are placed closely for redundancy, since there is a

high risk of damaging the wires during the assembly of the coils with the casing and the

cryostat.

Fig. 6.4. Superconducting coil including temperature sensor

6.2 Helium vessel

The Helium vessel, or casing, is the hermetic container for the coils as explained in

the previous chapters. It must provide the path for the liquid Helium to flow in intimate

contact to the coils, and also withstand the magnetic forces between them. Its main features

were already explained, but some other minor (but important) details should be highlighted.

Additional grooves were designed in the inner surface of the casing to accommodate the

temperature sensors, their wires and the protective resin.

In order to limit the deformation of the casing because of the welds, they are made in

lengths of few centimeters, alternatively from one position to the opposite one, to balance

the induced stresses. An intense Argon gas flow is introduced in the casing to protect from

oxidation and to help to evacuate the heat, protecting the coils from high temperatures.

Some details of the welds are shown in Fig. 6.5. It is also important to notice that these


132

fabrication techniques were validated during the fabrication of the casing for the single coil

test.

There is also a risk of heating too much the wires, both the superconducting and the

instrumentation ones. For the latter, as they are much lighter and then their heat capacity is

very limited, a protective shielding of glass fiber plates was included between the wires and

the welding part (when applicable).

Some of the welds are structural, but others should be vacuum-tight to Helium. A leak

test was performed to guarantee the tightness. Additionally, a cold test with liquid nitrogen

was done to check the proper behavior under thermal contractions at low temperature, since

most of the contraction between room and liquid Helium temperature is already present at a

liquid nitrogen bath (see Fig. 6.6). The leak test was repeated at that moment, to guarantee

that there was not any leak at cold, which would be very risky, taking into account the very

difficult disassembly of the finished cyclotron magnet. The temperature sensors were

measuring properly. The electrical insulation to ground was also performing well.

Fig. 6.5. (left) Coil assembly inside the casing. (right) Detail of the welds


133

Fig. 6.6. Setup for leak test at 77K of the finished casing

Finally, the casing was cleaned in an ultrasonic bath to be ready for working in a high

vacuum environment. A foil of low emissivity aluminum was wrapped around to decrease the

heat losses by radiation (see Fig. 6.7).

Fig. 6.7. Casing covered by low emissivity aluminum foil


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6.3 Cryostat

The manufacturing of the cryostat covers a lot of work and steps. It starts with the

assembly of the thermal shield around the casing, then the supports and, finally, the cryostat

wall around them.

The thermal shield was split in several parts to allow the proper installation of such a

complex geometry around the casing. Metrological control was always in mind at every step

due to the small gaps between the three layers of the cryostat. Tolerances specified in the

manufacturing and assembly drawings were carefully evaluated in advance. This process was

quite time consuming as there are complex geometries, small clearances, at least eight

degrees of freedom (arbitrary tensions can be applied to each supporting rod) and the

complete range of temperatures should be taken into account (nominal temperate

distribution but also intermediate temperatures during transient operation)

The thermal shield is basically assembled by screws, and cryogenic grease (Apiezon N)

with good thermal conductivity was applied to all the surfaces in contact to reduce thermal

contact resistance.

A challenging problem is how to hold the casing while the thermal shield is being

assembled. Of course, the thermal shield is completely covering the casing in all directions,

but there is no support yet for the casing. The assembly (see Fig. 6.8) was done by using a

custom table. This table had some holes to accommodate screws and custom cylindrical rods

as temporary feet for the casing. The casing is then lying on these feet at certain height from

the table level. On the other hand, the thermal shield was split from the design at the specific

places that the feet are. These holes at the bottom plate of the thermal shield were not

directly covered by bonding or soldering a disk. They were covered by a shifted and larger

circular copper part for additional reasons: the radiation from cryostat to casing is covered

(there is no direct radiative contact between both parts) but the vacuum conductance

between inner and outer side of the thermal shield is improved. As the vacuum level to be

reached is so high, it is important to keep in mind during the whole design and manufacturing

processes of any high-vacuum device that the lower the vacuum impedance at any volume of

the cryostat to the vacuum pump, the better the vacuum achieved, and the lower the time


135

needed to reach steady state. In this case, these windows are located in fact facing the port

where the vacuum pump is connected.

Fig. 6.8. Preliminary fitting of thermal shield around casing(left). Detail view of rods thermal

anchoring manufacturing: slots improve flexibility (right)

Besides the dimensional control, electrical measurements were done to check the

fabrication quality (see Fig. 6.9). The three layers of the cryostat were electrically insulated by

the G10-CR rods. The position of the casing was changed with respect to the cryostat wall

using the adjusting nuts while monitoring the electrical continuity. In that way, the full range

of movement was validated. Furthermore, the electrical insulation between the casing, the

thermal shield and the cryostat wall was successfully tested up to 500 V.

Finally, the casing was cooled down with liquid nitrogen (Fig. 6.10) while the cryostat

was under vacuum, to test the leaks, the relative movements and the overall behavior of this

complex structure. The test was successful. No contacts between the parts were detected.

The cooling time was close to expectations.


136

Fig. 6.9. Electrical measurements on cryostat after rods assembly

Fig. 6.10. Thermal testing of the cryostat with liquid Nitrogen


137

6.4 Iron assembly

The iron assembly process starts once the cryostat is finished. Due to the complex

geometry of the cryostat, the iron was split in parts to make it possible to mount, as

explained in the previous chapter. Fig. 6.11 shows intermediate steps of the assembly. The

gaps between the cryostat and the iron were checked with the help of precision gauges,

together with the expected range of movement of the cryostat respect the iron (+/- 1 mm).

The first test is to check if the upper supports measure the weight of the cryostat,

equally distributed between the four arms. The second step is to test again the range of

movement of the casing respect the cryostat, and to inspect if there are contacts between

the different layers (by means of electrical continuity).

Fig. 6.11. Iron assembly (left) and pole assembly (right)


138

6.5 Connection box assembly

The connection box was manufactured on-site after the iron assembly (Fig. 6.12).

Liquid Helium pipes are the first ones to be mounted because the wires from the coils and the

temperature sensors will be installed through a custom-made feedthrough.

Later on, thermal anchoring for the low temperature end of current leads is installed

at the same time than safety shunts made of stainless steel. Once that resistive side of

current leads is installed, and all the contacts are electrically insulated using kapton, pressure

relief pipes and lateral walls of the cryostat are assembled. Same for the wires of thermal

sensors and voltage taps.

Fig. 6.12. Dedicated supporting structure during connection box manufacturing


139

Fig. 6.13. Input pipe assembly. (left) Welding setup. (right).

The thermal shield for limiting radiated heat losses from all the cold parts in the

connection box is hold by screws. It is not fixed to the box to minimize thermal stresses from

contractions. Nevertheless, the pipes are quite long so the expected displacements are in the

order of millimeters.

As a rough number, the transfer line is 2.5 m long since that is the distance between

the CSS and the cyclotron. The cryostat is at ambient temperature but inside there will be

stainless steel pipes at liquid Helium temperature. Given an integral contraction around

3 mm/m, there will be a total displacement of 7.5 mm if one side was free and the other one

completely fixed.

The thermal shield is simply supported by three rods made of stainless steel. Once

the casing is cooled down, the whole thermal shield will be free to move upwards, since it is

attached to the cryostat thermal shield and therefore, to the same supporting rods as the

casing.

The last part to be installed is the lower wall which is the one for the vacuum pump

connection. The completely mounted connection box can be seen in Fig. 6.14.

Finally, the integration of the cyclotron and the CSS will be done through the transfer

line. Fittings between them are Kenol connectors, and thermal protection will be wrapped

around them: Multi-Layer Insulation (MLI) is covering both low-temperature fittings and a

second MLI is connecting both thermal shields, the one at the transfer line and the one from


140

connection box. This process is presented in Fig. 6.15, where the intermediate step is shown

(low-temperature pipes are covered, Helium gas pipes are not covered yet).

Fig. 6.14. Connection box ready to be closed

Fig. 6.15. Intermediate assembly step of the connection box cryogenic connection to rigid

transfer line


141

6.6 Summary of Fabrication and Assembly Chapter

In this Chapter, the Author describes the steps during the manufacturing stage of the

superconducting magnet designed along the previous chapters. Some of the most important

quality controls and acceptance tests are shown and discussed. In addition, general advises

and recommendations from the experience are supplied. The Author was in charge of the

follow-up of fabrication and assembly.

Chapter 7. Magnet Testing

This work has proposed a procedure for designing and developing compact and

efficient superconducting magnets for accelerators. As a case study, a superconducting

magnet according to the AMIT project specifications was presented. This chapter provides

information on the tests and results obtained with the actual manufactured magnet.

7.1 Pre-cooling tests

The first test of the magnet was carried out by cooling it with liquid Nitrogen from a

Dewar. For the second one, it was cooled by liquid Helium from a Dewar. This strategy was

preferred in order to perfectly check the magnet behavior before using the CSS, which is a

complex system and, as such, it also needs its own commissioning.

A dedicated transfer line was developed by the Author in order to provide a path for

the Nitrogen to be pumped through the cyclotron coils (Fig. 7.1). The cryogen mass flow was

controlled with a mass flow controller just before the warm exhaust venting outlet.

Every temperature inside the magnet, as well as strain gauges, Helium mass flow and

any other measured variable were monitored, stored and/or controlled by the actual final

cyclotron control system for its validation. The results can be also found in [114].

Fig. 7.1. Setup for first magnet cooling test


144

Testing the system by using LN (Liquid Nitrogen) allows checking the cool down

process, the steady-state temperatures and the flow control. This test provides important

data on the thermal performance of the cyclotron in an economically and easily way

compared to using liquid Helium (LHe) as cryogen. With this test, the achievement of the

superconducting state at the HTS current leads can also be verified while its main limitation is

that the coil is not able to become superconductor. Fig. 7.2 shows the cool down curves,

according to the expected values in terms of time and cryogen flow requirements (29 h

expected for 3 kg/h LN flow, instead of 27 h). In the simulation, the coil temperature and its

properties are evaluated in the refrigerated surface as a weighted mean of its constituent

materials. This is not accurate if the temperature distribution in the cross section is not

homogeneous because of the non-linear behavior of the materials. Thus, some discrepancies

can be found during the cool-down transient. Besides, a thermo-camera was employed to

check the cryostat during the cool down; no cold regions were detected at the surface.

Fig. 7.2. Cooling down curves using liquid Nitrogen

Additionally, this test allows another interesting measurement. Since the coil is fully

immersed in LN and its electrical resistance decreases with the temperature, cooling the

magnet down with LN allows to power the coil, in a non-superconducting state, up to 0.65 A,

a limit given by the maximum voltage that the available power supply is able to provide,

150 V. Although this current is really small compared to nominal current, it allows checking


145

the magnetic field map at very low currents, which provides valuable information from the

point of view of electromagnetic model verification.

The magnetic field is measured using the test bench developed in collaboration with

ALBA (Fig. 7.3). It is based on three Hall sensors, perpendicularly oriented, attached to a non-

magnetic carbon fiber strip, which can be moved with high accuracy in the region of interest.

An important point about this magnetic measurement is to set the procedure on the

alignment and reference frames between the magnet and the magnetic test bench. This

procedure was carried out by using a FARO Arm attached to the mobile part of the test

bench, so that the magnet was measured at different positions and the information was

correlated to the internal encoders of the test bench. This allows knowing not only the

translation vector from Hall sensor absolute position to magnet measuring point but also the

rotation matrix from one coordinate frame to the other.

Fig. 7.3. Setup for magnetic measurements

The magnetic field distribution for 0.65 A test current can be checked by comparing

simulated and measured values (see Fig. 7.4). Deviation is less than 0.5 % for any position in a

circle of 150 mm diameter centered with the iron poles. This is a good partial result, but the

iron is not saturated as at nominal condition so that the magnetic field profile is very different

compared to the nominal one.


146

Fig. 7.4. Magnetic field measured at low current (0.65 A)

7.2 Liquid Helium tests

Once that the magnet and the set-up were successfully checked down to LN

temperature including its control system, sensors and valves, a test with LHe was carried out.

After precooling the magnet with LN, the Nitrogen was removed and then purged

with several cycles of Helium gas flow and vacuum, alternatively. Then, LHe from a

pressurized Dewar was injected into the system through the same dedicated transfer line

used for the pre-cooling test. The flow was controlled by a mass flow controller at the outlet

in a similar way as it was done in the previous test with LN. The amount of LHe required and

the cool down curves were measured to evaluate the thermal performance of the system.

The cooling down curve when using LHe and the amount of Helium required is

presented in Fig. 7.5. The expected time from the simulation done with the developed Matlab

program, for a 0.8 kg/h mass flow, was around 11 h, while casing reached liquid temperature

in just 9h. It is important to notice that the two cooling circuits are directly connected, so

temperatures at the second stage are lower than the ones expected at working conditions

with the CSS. Moreover, the Helium enthalpy is not completely used, as the Helium gas in the

exhaust pipe is at 34 K. At nominal conditions using the CSS, this enthalpy will be recovered

by precooling the incoming Helium gas to the cryocooler.


147

Steady-state conditions were achieved when the whole magnet circuit was full of

LHe. Measuring Helium temperature at the outlet of the cyclotron allows evaluating the

enthalpy change of the Helium used for refrigerating the magnet. It results in an overall

required power of 31 W, while the design value was 34 W according to TABLE XIV. It is also

important to notice that in this test the temperature distribution is not the same as nominal

because the second circuit is cooled with the evaporated liquid Helium instead of being

cooled with gas Helium at 60 K. In addition, there is no current through the current leads

during the cool down process. Future tests will be aimed at separating the power

requirements for each circuit from the Helium quality factor data coming from the first

circuit.

Fig. 7.5. Cooling down curves using liquid Helium from a Dewar

Once the temperature was stabilized, magnetic measurements were performed. The

current was increased by steps, measuring the magnetic field at some points for each current

value. This strategy allowed checking the magnetization properties of the iron and its effects

on the magnetic field in the mean plane of the cyclotron. Fig. 7.6 shows the magnetic field in

the center of the cyclotron compared to the simulation value at the same point. Measured


148

values for magnetic field at magnet center are slightly bigger. Iron properties were measured

just up to 2 T and the magnetic test bench was calibrated up to 2 T, so the iron properties

used for calculations and the calibration curve of the sensors above 2 T are extrapolated and

this could be the reason for this difference.

Regarding the magnetic field mapping of the iron, it was first performed at 30 A, and

some interesting results were drawn. Measured values are very close to the calculated ones.

However, one should notice that not only the absolute value of the magnetic field is different

when comparing powering the magnet at 30 A or at nominal current, but also the shape

differs slightly. This small difference on the field shape and particularly on the fact that the

magnetic field at the center is not a global maximum value represents a major difference in

terms of particles stability. This means that higher values of current are needed not only to

reduce the bending radius of the particles, but also to achieve beam stability with this iron

pole profile.

Another important check to be done is to confirm the azimuthal symmetry of the

magnetic field. It will greatly affect the beam stability and the beam current reaching the

target. The magnetic field at 30 A at a cyclotron diameter in the mean plane is shown in

Fig. 7.7. While the nominal curve is fully symmetric, the measured curve shows a 0.5 %

deviation on this symmetry. As particles will be moving in circular paths, they will feel

alternative oscillations in the magnetic field at a given radius and this should be considered

for the beam dynamics and its stability. It can be easily proved by simulations that the real

position of the coils cannot explain this effect, so the iron properties, its dimensional

accuracies (small irregular gaps) and the misalignment between the hall probes and the

cyclotron magnet should be the reason for this non symmetrical behavior.


149

Fig. 7.6. Magnetic field measured at cyclotron center

When increasing the current to higher values, specifically at 75 A, several supports of

the cold mass were broken. It was a problem with the measurements of the corresponding

strain gauges. The most likely reason for this problem is related to the bonding of the strain

gauges. A picture showing the situation of one strain gauge proves that it was not perfectly

bonded to the surface of the rod (Fig. 7.8).

Fig. 7.7. Magnetic field measured at low current (30 A)


150

Fig. 7.8. Broken strain gauge after powering tests

All the strain gages were tested after bonding them to the supports, so it is clear that

this strain gage was broken after the magnet assembly. The most critical step for the gages

integrity is the welding of the stainless steel parts which are close to the rods in the cryostat.

The gauges were covered for protection and Argon flow was applied inside the cryostat to

keep the high temperature levels far away from the gages, but at least one of them

definitively suffered damage. Furthermore, the damage was not so severe to be noticeable at

low strain levels, but high enough to completely break the bonding at medium strain levels

and, as a result, uncontrolled misalignment of the coils occurred, breaking some supports at

their weakest point: the neck besides the joint at warm side.

Fortunately, the magnet was quite well aligned and the supports which survived were

enough to hold the cold mass up to 65 A. The tests continued using 30 A as the maximum

current for magnetic measurement because of safety reasons.

The reparation of these supports and their strain gages to control and to monitor the

misalignment of the coils is under development. The magnet disassembly for a full repair has

been discarded for the time being, because of the long dead time and cost.


151

7.3 Modified CSS tests

Prior to test the autonomous operation of the cooling system, it was needed to

evaluate the CSS performance after the modifications and adaptations for using it with the

AMIT cyclotron.

Two dummy loads were included inside the CSS vacuum vessel (dummy thermal

shield and dummy coil). Temperature sensors and heaters were installed in both dummies.

The results for the modified version of the CSS are presented in Fig. 7.9. This test will also

allow checking the Helium pumping system and its control system (see Fig. 7.10).

Regarding the time needed for cooling down, it was reduced from about 11 h ([89]) to

9 h thanks to the implemented bypass valve. The results were considered satisfactory, as the

time needed for cooling down the dummy load was improved by 18 %, while small

increments of thermal losses were measured (see TABLE XIX). As the reference value of the

CSS for the second stage is 1.40 [email protected] K, it can be interpolated from the measured

conditions of the modified version that it can deliver 1.27 [email protected] K. Alternatively, it can be

compared for the same heat load at the dummy coil, and then the interpolated working point

is 1.4 [email protected] K. Thus, the effect of the modifications (bypass valve, injection pipe and the

filter) can be quantified as 0.13 W or 0.27 K when compared to the first version of the CSS.

The effect of the bypass valve on the cool down time is related to the amount of mass

to be refrigerated by the second stage of the cryocooler, so that the expected improvement

of 25 % for the cyclotron magnet seems to be feasible. In addition, no problems were

detected because of the added filter for impurities in the cold return pipe of the CSS.

mailto:[email protected]

mailto:[email protected]


152

Fig. 7.9. Autonomous cool down of AMIT cyclotron with a dummy load

TABLE XIX MODIFIED CSS PERFORMANCE

Reference

(Original CSS) Modified CSS

Dummy Shield (K) 40 36.5 37.4 50.4

Heat Load

Dummy Shield(W) 25 20.7 13.1 32.4

Dummy Coil (K) 4.6 4.32 4.7 5.1

Heat Load

Dummy Coil (W) 1.40 1.15 1.32 1.50

Mass Flow (g/s) 0.08 0.067 0.073 0.061

On the other hand, the possibility of LHe injection was not tested at this stage. This is

a much more complex situation in terms of the control system, so it was decided to postpone

this test until the whole facility is checked in nominal operation. The nominal operation

requires one mass flow control to be regulated (the amount of Helium flowing through the

system), while for the injection three mass flow controllers should operate at the same time:

the amount of circulating Helium, the amount of injected Helium and the amount of Helium

to be released in order to reach the steady-state.


153

Fig. 7.10. (left) Modified CSS tests and dummies. (right) He pump circuit

7.4 Autonomous operation of the cyclotron

The proposed method in this work is to sequentially test each single component

before installing it as a part of a more complex assembly. Once that both the cyclotron and

the CSS are tested independently, the autonomous operation of the AMIT cyclotron

refrigerated by the modified version of the CSS is shown in this chapter. The overall picture

for this test is presented in Fig. 7.11. Helium mass flow was about 10 g/min while the bypass

was opened and 0.1 g/min once that it was closed.


154

Fig. 7.11. Autonomous operation of AMIT cyclotron

Fig. 7.12. Autonomous cool down of AMIT cyclotron


155

TABLE XX THERMAL PERFORMANCE OF THE CYCLOTRON REFRIGERATED BY THE CSS

Temperature Heat losses Expected

First Stage 85 K

CSS+T.F. 97 K 7.9 W -

Thermal

Shield 167 K 44 W 35 W

Second Stage 7 K

CSS+T.F 10.5 K 3 W 0.3 W

Coils 11.2 K 1.4 W 0.9 W

The most important results are included in TABLE XX. The first noticeable fact is that

the nominal temperature of the coils cannot be reached by means of the CSS, but a number

of relevant conclusions can be extracted:

- Both the control system and the Helium pumping circuit were working as

expected when connected to the cyclotron. The additional pressure drop,

volume and mass coming from the cyclotron compared to the tested

dummies could be handled by the system without further problems, as

expected.

- The overall behavior of the whole system was stable and smooth when

cooling the magnet with gas Helium up to 11 K (Fig. 7.12).

- The expected rough number for the cool down time using the bypass

valve was in accordance with the actual time needed. It took 4 days to

reach 11 K.

- The second stage of the cryocooler reached its steady state operation but

the Helium returning from the heat exchanger was so warm that no

liquefaction was possible.

- Both the CSS and the cyclotron magnet were tested separately and no

major problems were detected. The actual conditions of these


156

independent tests were not exactly the conditions for the nominal

operation of the assembly, but this cannot explain such a large deviation.

Finally, it was concluded that there were just two possibilities to explain the lack of

liquid Helium inside the casing:

1. The cyclotron magnet or the CSS were damaged from the previous tests to

this one. This could have happened during handling for setting up the tests,

for instance.

2. There are unexpected thermal losses between the CSS and the cyclotron.

They could be in the connectors or the transfer line.

Detailed inspection of every part was done and no other evidences that could explain

this result were found. The procedure for assembly and disassembly each part was also

revised reaching the same result.

Based on these premises, an additional test was proposed to check the transfer line

performance between the CSS and the cyclotron.

7.5 Transfer line tests

The transfer line was connected to the CSS but two dummy loads were cooled down

instead of the cyclotron. Each one of these dummy loads, made of copper, has a heater

attached at one side and a temperature cernox sensor on the opposite side. The first dummy

is connected to the first stage of the CSS as the cyclotron thermal shield. The second dummy

is connected to the second stage as the casing. First dummy actually covers the second one to

avoid radiation thermal losses in the second dummy. In addition, temperature sensors were

attached to the pipes of the transfer line to distinguish between the heat losses coming from

the connections and the losses coming from the transfer line.

The overall view of this test and the lowest temperature reached by the dummy as a

function of the thermal power supplied by the heater are presented in Fig. 7.13. The

conclusion is clear, the refrigeration capabilities of the system (CSS + transfer line) cannot

provide liquid Helium to the dummy if the required thermal power is higher than 0.2 W.


157

After additional post processing of the test data was done, a detailed view of the

thermal balance can be shown in TABLE XXI. It explains the situation and it proves that the

performance of the transfer line is far away from the specifications coming from the supplier

[62].

No clear explanation could be found for these values from the inspection of the

transfer line or from the revision of its manufacturing drawings. The most likely possibility is

that the transfer line suffered some mechanical damage on its internal parts during

transportation. It could also have suffered damaging when welding the outer pipe, which is

one of the most dangerous steps in its manufacturing process.

A new transfer line is being developed at this moment. Some changes were

implemented in this second version by the Author to achieve a more robust transfer line,

including its manufacturing processes and disassembly capabilities for further inspection, if

needed. As examples, the assembly of this new transfer line does not include any weld in the

outer pipe (which could lead to insulation damage) and the management of the thermal

strains was changed to avoid any possibility of thermal contacts at intermediate

temperatures during the cool down. The details are out of the scope of this Thesis, but it will

hopefully provide the required low-loss cryogenic connection between the CSS and the

cyclotron for autonomous operation of the magnet.


158

TABLE XXI TRANSFER LINE THERMAL BALANCE AT SECOND STAGE

Contribution Expected (W) Measured

From Cryocooler to the LHe Outlet 0.15 0.29

From GHe return pipe to HX3 0.08 0.17

TFL LOSSES 0.05 0.66

Thermal losses/efficiency inside CSS (HX3) Unknown 0.20

Dummy Losses (when the heater is OFF) 0.03 0.06

TOTAL at 4.5 K 0.3 1.32

Fig. 7.13. CSS and transfer line with dummy load test

7.6 Summary of Magnet Testing Chapter

In this Chapter, the Author describes a proposal for testing superconducting and

autonomous prototype magnets. Several steps are described to test subsystems in sequence

before the complete system test. The Author was in charge of the tests.

The results and the discussion in this Chapter were published in [114] by the Author

and presented in MT25 Conference in Amsterdam [115].

Chapter 8. Conclusions. Future developments

Literature about superconducting magnets can be broadly found and some good

examples were used for this work. Most of them are related to applied superconductivity

while some others to accelerator magnets, but none of them focused on compact and

efficient magnets. Scientific papers, including both reviews on the state of the art and

descriptions about a single machine are not so exhaustive or just fragmented information is

provided along sequential papers.

A complete and exhaustive procedure for compact accelerator magnet design has

been developed and exposed in this Thesis. Each decision based on a rationale. An actual

project has been chosen as a practical application case. Its main requirements were

compactness and efficiency.

This procedure can be easily adapted for the development of a number of different

magnets and applications which could benefit from these design criteria. There are some

fields which are nowadays developing or considering superconducting magnets as an

emergent technology, like (but not limited to) renewable energy production, transportation

and energy storage [116].

All the three main objectives of this work can be considered fully completed (Fig. 8.1).

About the last main objective, manufacturing one magnet following this procedure to

test its feasibility and its actual performance, it was done under the AMIT project

specifications and AMIT consortium/CIEMAT financial support. The magnet was tested at

nominal temperature but it did not reach the nominal field because of a problem with the

strain gauges measurements. Once it is repaired, the magnetic characterization will be

performed till nominal field. Therefore, additional work and time are still needed for future

steps before the magnet can be used for radioisotopes production because some of them

could not be completed by the time of writing this work. They are related to the secondary

objectives.

Chapter 8. Conclusions. Future Developments

160

The optimization of the magnetic field according to beam dynamics is a secondary

objective not completed yet. It should continue in the future. A complete campaign of

magnetic measurements is needed, so that the stable operation of the superconducting coils

should be achieved. Cooling down the coils by means of a Helium Dewar, as shown in

previous chapter, is the first step but it is too expensive to keep it for a long time.

Fig. 8.1. Thesis objectives: completed (green) and partial/in progress (orange)

This is in fact the other secondary objective: the autonomous operation. It is well

advanced. Most of the subsystems, like power supply or control, are autonomous, but not the

cooling system. The CSS is commissioned, but a new low-loss transfer line, designed by the

Author is in fabrication. Once it is ready, the cooling system will be completed and the

commissioning will lead to full autonomous operation.

On the other hand, alternative objectives were completed. The magnetic field

measurements were carried out successfully at several field levels. The positioning system for

magnetic calibration worked as expected.

A hybrid solution for the cryogenics was already included as a possibility from the

very beginning of this Thesis because of the challenging and risky specification of refrigerating

the whole cyclotron magnet using just one cryocooler which is 2.5 m far away from the

cyclotron. This hybrid solution was successfully added by the Author to the CSS designed by

CERN, so that this alternative objective is considered also completed.

Chapter 8. Conclusions. Future Developments

161

Besides these proposals for the enhancement of the performance of the first magnet

prototype, a second prototype based on this procedure and the experience gained can be

envisaged:

- Alignment concept for the magnet could be improved by increasing the sensitivity of

movements of the rods. Also, strain gages should be moved to a different location in

which they could be more easily checked or replaced in case of problems or

malfunction.

- Pressure management system in case of quench could be revised based on the

experience of this prototype to reduce thermal losses. Design was quite conservative

in terms of maximum pressure and time of transient so that thermal losses were not

completely optimized in this first design.

- Compactness could be improved even more if cryogenic system is located close to

the magnet. It could reduce the height of the magnet because no connector box and

transfer line are needed. On the other hand, cryocooler would be inside the

radiation shield, so maintenance could be more difficult and/or expensive.

Another possible future step in this technology could be the development of a

superconducting system for different applications for example for energy storage, energy

production or others. Superconductivity could provide essential advantages but nowadays it

could not be profitable because of additional weight, volume or operational costs compared

to resistive technology. This work could be used as a baseline for these developments while

further work on the specific details of each application will be of course necessary.

List of Publications

Main contributions by the Author from this Thesis

[100] J Munilla et al., “Development of a Superconducting Magnet for a Compact Cyclotron for Radioisotope Production,” IEEE Transactions on Applied Superconductivity, vol. 26, no. 4, pp. 1–4, Jun. 2016. Presented at International Conference on Magnet Technology 2015, MT24 in Korea by F. Toral.

[107] J. Munilla et al., “Validation Test of the Forced-Flow Cooling Concept for the Superconducting Magnet of AMIT Cyclotron,” IEEE Transactions on Applied Superconductivity, vol. 26, no. 3, pp. 1–4, Apr. 2016. Presented at European Conference of Applied Superconductivity 2015 (EuCAS15) in Lyon by the

Author.

[114] J. Munilla et al., “Cold Tests and Magnetic Characterization of a Superconducting Magnet for a Compact Cyclotron for Radioisotope Production,” IEEE Transactions on Applied Superconductivity, vol. 28, no. 3, pp. 1–5, Apr. 2018 Presented at International Conference on Magnet Technology 2017, MT25, Amsterdam by L.

García-Tabarés.

Secondary and/or coauthored contributions related to this Thesis

[117] J. Munilla, “Superconducting Compact Cyclotrons for Isotope Production,” in Academia-Industry Matching Event on Superconductivity, CIEMAT, Madrid, 2013. Presented by the Author.

[64] C. Oliver, J. Munilla, C. Vazquez, F. Toral, and D. Obradors, “Optimizing the radioisotope production with a weak focusing compact cyclotron,” 2013. Cyclotrons conference 2013, Vancouver. Presented by C. Oliver

[118] D. Obradors et al., “Characterization of the AMIT Internal Ion Source With a Devoted DC Extraction Test Bench”, International Particle Accelerator Conference, 2017, Geneva. Presented by D. Obradors.

164

Additional contributions by the Author partially related to the contents of this

Thesis.

[119] J. Munilla, “Mechanical design of the common-coils option,” in FCC Week 2017, Berlin, 2017. Presented by the Author

[120] F. Toral et al., “EuroCirCol 16 T Common-Coil Dipole Option for the FCC,” IEEE Transactions on Applied Superconductivity, vol. 27, no. 4, pp. 1–5, Jun. 2017.

[121] F. Toral, J. Munilla, and T. Salmi, “Magnetic and Mechanical Design of a 16 T Common Coil Dipole for an FCC,” IEEE Transactions on Applied Superconductivity, vol. 28, no. 3, pp. 1–5, Apr. 2018.

[122] J. Munilla et al., “16T dipoles for FCC,”

Presented at the North American Particle Accelerator Conference (NAPAC19), in Michigan, 2019

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