1

Use of Niobium for Fabricating Superconducting Radio

Frequency Cavities*

A.T. Wu

Thomas Jefferson National Accelerator Facility, 12000 Jefferson Avenue, Newport

News, VA 23606, USA

Abstract:

Since the pioneer work done by the High-Energy Physics Lab at Stanford University in

1965, superconducting radio frequency (SRF) technology has been developing steadily up to

now. Demanding on niobium (Nb) has been increasing constantly, since more and more particle

accelerators select Nb based SRF technology as a key part of their accelerator constructions. For

example, the proposed International Linear Collider (ILC) that will probe new physics using TeV

collisions of electron and positron beams will need approximately 17,000 1-meter-long Nb SRF

cavities. Others such as x-ray free electron laser (XFEL) at DESY in Germany, energy recovery

linac (ERL) at Cornell University in USA, the new Spiral 2 facility in France, the isotope

separation and acceleration (ISAC) II in Canada, and the 12 GeV upgrade of CEBAF at Jefferson

Lab in USA will all require Nb. This popularity in Nb can be, at least partially, attributable to

the unique physical and mechanical properties that Nb possesses --- the highest superconducting

transition temperature of 9.25 K and the highest superheating field of 0.23 T among all available

pure metals with excellent ductility that enables machining to be done relatively easily. In this

chapter, the use of Nb for fabricating SRF cavities is reviewed, giving particular attention to

some examples of important new developments in the past decade on reducing the production

costs and increasing the throughput of high quality Nb SRF cavities. Some R&D examples on

the study of the requirements in the physical, chemical, metallurgical, and mechanical properties

of Nb for the applications in particle accelerators based on Nb SRF technology are updated and

reviewed. This chapter also includes some unpublished experimental results from my own

research. Hopefully this review can be served as a useful reference for new researchers who

want to use Nb for their various R&D projects in particle accelerators and for Nb suppliers and

manufacturers who want to provide the best and the most economic products to be used in

particle accelerators.

2

1. Introduction

The use of SRF technology in particle accelerators was started by the pioneer work done

at Stanford University in 1965 where electrons were accelerated in a lead-plated resonator1.

Since then, Nb has been gradually taking over as the main metal for fabrication SRF resonators

or cavities due to its unique physical and mechanical properties. Although the major

consumption of Nb is as an alloying element to strengthen high-strength-low-alloy steels for

building automobiles and high pressure gas transmission pipelines and to provide creep strength

in superalloys operating in the hot section of aircraft gas turbine engines, demands on Nb from

SRF community have been increasing steadily over the last couple of decades due to the fact that

many current and future particle accelerators select Nb based SRF technology as one of the key

parts in their accelerator constructions for the applications in nuclear physics, high energy

physics, and free electron laser. Some examples of large scale particle accelerators that utilize

Nb based SRF technology include CEBAF2, KEKB

3, RIA

4, TESLA

5, SNS

6, SPIRAL2

7,

ISAC28, XFEL

9, and ILC

10 etc. For instance, the proposed International Linear Collider (ILC)

alone will require roughly 520 tons of Nb by assuming a production yield of 80% and a thickness

of 3 mm for the Nb sheets. This is quite significant since the total production of Nb in the world

in 2008 is only 60000 tons11

.

As the popularity in Nb-based SRF technology grows, more and more people are

involved in the R&D of the technology. One example of the growing in this field is the

evolution of the previous SRF workshop to SRF conference in 2009 due to the increasing

numbers of participants and contributed papers. For instance, the number of the contributed

papers has grown from 127 for the SRF workshop in 2001 to 223 for the SRF conference in

2009. This is an increase of more than 75%. More and more consensuses have been reached

regarding what the requirements are on Nb for the applications in the SRF field. In this chapter,

I will try to update and review some selected aspects of the developments in cavity fabrication

process and some R&D examples on the requirements in physical, chemical, metallurgical, and

mechanical properties of Nb for fabricating Nb SRF cavities. It represents only my personal

interests and viewpoints and it is by no means the complete coverage of the entire R&D activities

in this field.

This chapter will be organized in the following way: Section 1 gives a brief introduction

to the several important fundamental parameters employed in SRF technology. This defines the

3

terminology that we will use in this chapter. The benefits for selecting Nb as the material for

SRF applications are discussed in Section 2. Section 3 summarized the steps in fabricating Nb

SRF cavities with emphases on some newly emerged techniques in the fabrication process

aiming at reducing the production costs and increasing the throughput of high quality Nb SRF

cavities. Some examples of R&D on exploring the requirements in physical, chemical,

metallurgical, and mechanical properties of Nb are shown and discussed in Section 4. Section 5

presents a summary and a very brief perspective in the application of Nb for fabricating SRF

cavities.

Section 1: Some Fundamental Parameters Employed in SRF

Technology

There are two different groups of particle accelerating structures commonly used for

particle accelerators, depending on the velocity (V) of the particle to be accelerated. Normally, a

parameter β that is defined as V/C where C is the velocity of light is used to separate the two

groups.

When 0.5< β <1, this is typical for accelerating electrons with kinetic energy of a few

MeV and protons with 100 MeV. A typical Nb SRF cavity for this group of particle accelerating

structures is shown in Fig.1 where a chain of five coupled RF cells are resonating in the

transverse magnet (TM010) mode. This accelerating structure is the topic of the discussions for

this chapter. In this field configuration, the longitudinal electric field is maximized along the

axis of the cavity and the RF phase between adjacent cells is 180 o as schematically illustrated in

the upper part of Fig.1. In this way, a particle with velocity close to the speed of light will

experience the maximum acceleration in each cell of the cavity.

When β < 0.5, this is the typical for accelerating ions from helium to uranium with

kinetic energies from a few to 20 MeV per nucleon. For this group, a variety of different

accelerating structures may exist such as, for instance, coils, helix, spoke, and crab shapes tec.

Fig.2 shows an example of how the cavities look like from this cavity group. Based on the past

experience, we know that the major consumption of Nb is from the first group of accelerators

where 0.5< β <1.

Normally, the performance of a SRF cavity is characterized by an excitation curve where

the quality factor Qo is plotted as a function of the accelerating gradient Eacc. Therefore, we need

4

to define first several fundamental parameters used often in SRF technology including the

quality factor Qo, the accelerating gradient Eacc, the surface resistance Rs, and the RRR value.

1.1 Quality Factor Qo

The quality factor Qo of a cavity is defined as the ratio of stored energy (U) in the cavity

to the dissipated power (P) through the cavity walls per radian per second

Qo=ω (U/P) (1)

Where ω=2πf. f is the frequency of the stored RF power. Qo is inversely proportional to

the Rs of the wall material

Qo=G/Rs (2)

Where G is called the geometry constant. Here it is assumed that the surface resistance

of the wall material is homogenous over the whole interior surface of the cavity.

P can also be expressed in the following way:

P=

Rs ׀ ׀

2 ds (3)

Where H is the local magnetic field and the integral is taken over the entire interior cavity

surface.

1.2 Accelerating Gradient Eacc

The accelerating gradient Eacc is defined as the maximum energy gain for a charged

particle when traveling through a cavity divided by the length of the cavity and the charge of the

particle. It is can be expressed as

Eacc (4)

5

1.3 Surface Resistance Rs

Surface resistance Rs of the wall material of a SRF cavity is related to the power

dissipation P through equation 3. For a superconductor such as Nb at a temperature below the

superconducting transition temperature (Tc), Rs can be expressed as

Rs=Rbcs+Rres (5)

Where Rbcs is called the BCS resistance and Rres is called the residual resistance. In the

superconducting state, there is only a limited depth (called penetration depth λ) on the surface of

the wall material that the stored RF power can penetrate. For Nb cavities operated at 1500 MHz,

typically λ is about 50 nm. According to Bardeen-Cooper-Shrieffer (BCS) theory12

, Rbcs

decreases with temperature below Tc in a fashion as given in equation 6 as long as the frequency

of the stored RF field in the cavity is as compared with the gap energy frequency that is about

700 GHz for Nb:

Rbcs

exp(-

) (6)

Where A is a constant, depending upon the material parameters of the superconductor,

such as the λ, the coherence length ξ, the Fermi velocity VF, and the mean free path L. 2Δ(T) is

the energy gap of the superconductor.

Noted here that unlike Rbcs that is temperature dependent Rres is not. Ideally, Rres should

be zero. But we know that it is impossible in the real world. Various defects and imperfections

on the surface in a depth of the λ contribute to Rres. This will be discussed further in the

following sections. Empirically, Rres is found to be proportional to the square root of the normal

state conductivity of the material.

Fig.3 shows a typical measured surface resistance of a 9-cell TESLA cavity.

1.4 RRR Value

RRR is the abbreviation for Residual Resistivity Ratio. RRR value is defined as the

ratio between the resistivity at 300 K to the residual resistivity at a low temperature when

6

the material under the measurement is in normal state. It can be expressed mathematically

as:

RRR=

(7)

For Nb, this is usually done either by the measurement at a temperature just before

the superconducting transition temperature Tc and extrapolation to 4.2 K or at a specific

temperature below Tc where an external magnetic field is applied just enough (a

homogeneous magnetic field up to 1 Tesla parallel to sample) to drive Nb to lose its

superconductivity at 4.2 K.

RRR is a parameter that can be used to quantify the general impurity content of a

material. The higher is the RRR, the lower is the impurity content of the material. It is

important to point out here that there are background contributions to RRR value from

phonons and grain boundaries. Calculated theoretical limit13 for RRR for Nb is 35000. In

practice, the highest RRR ever achieved14 for Nb is 33000.

Generally speaking, a good Nb cavity should have Eacc>35 MV/m and Qo > 1010.

This would imply that the surface resistance sh uld be less than a few tens f nΩ. RRR

value is larger than 300.

Section 2: Why Is Nb The Material of Choice

To answer this question, first we have to answer why the cavities have to be

superconducting. For any particle accelerators, the operation cost is one of the most important

factors that require careful consideration before construction. Selection of superconducting

cavities over normal conducting cavities as the accelerating structure of an accelerator can result

in huge savings in the operation cost. This is especially true for accelerators in a continuous

wave (cw) mode or at a high duty factor (>1%).

Take copper (Cu) cavities as an example. For cw operation, the power dissipation

through the walls of a Cu cavity is huge. This is due to the fact that the dissipated power per unit

length of an accelerating structure is given by the following formula:

7

=

(8)

Here ra/Qo is the geometric shunt impedance in Ω/m, and it depends primarily on the geometry of

the accelerating structure. For Cu that has a resistance that is typically 5 orders of magnitude

higher than that of a microwave surface resistance of a superconductor, Qo is typically 5 orders

of magnitude lower. Some simple calculations can show that if CEBAF used Cu cavities and

operated at cw mode with an accelerating gradient of 5 MV/m, the dissipated power for each

cavity could have been near 450 kW. This already exceeds the 100 kW power dissipation limit

for a Cu cavity since above which the surface temperature of a Cu cavity will exceed 100 oC.

This will cause a number of unwanted effects such as, for instance, vacuum degradation, stresses

in the Cu, metal fatigue due to thermal expansion. Therefore for Cu cavities, high accelerating

gradients larger than 50 MV/m can only be produced for a period less than a few microseconds

before the RF power needed becomes prohibitive.

In contrast, the same CEBAF machine based on Nb would need to dissipate power of

only a few watts. Of course, one have to consider also the cost of cooling a superconducting

material down to a temperature below Tc and normally the efficiency of refrigerators that are

used to cool down the material is low. Nevertheless, for CEBAF operated at 2 K a reduction in

the operation cost by a factor of 0.01 to 0.001 can be realized.

Apart from the general advantages of reduced RF capital and associated operation costs,

superconductivity offers certain special advantages that stem from the low cavity wall losses.

Because of superconductivity, one can afford to have a relatively larger beam holes in

superconducting cavities than for normal ones. This significantly reduces the sensitivity of the

accelerator to mechanical tolerances and the excitation of parasitic modes. Also larger beam

holes reduce linac component activation due to beam losses. Superconducting cavities are

intrinsically more stable than normal conductor cavities. Therefore the energy stability and the

energy spread of the beam are better.

We know that there are many superconductors in the world. Why do we select Nb as the

major material for building SRF cavities? Apart from some historical reasons, the first obvious

answer is that Nb has the highest Tc of 9.25 K among the all available elements in the period

8

table. This makes the requirement for cooling the cavities down to a temperature below Tc a

relatively easy task.

Furthermore, since Eacc is proportional to the peak electric field (Epk) and peak magnetic

field (Hpk) on the surface of a cavity, one has to be sure that the material that is used to make the

cavity can sustain large surface fields before causing significant increase in surface resistance or

a catastrophic breakdown of superconductivity (called quench). The ultimate limit to

accelerating gradient is the theoretical RF critical magnetic field that is called the superheating

field Hsh. Nb has the highest Hsh of 0.23 T among the all available metal elements.

Another advantage of Nb is that it is relatively easy to be shaped into different structures

due to its outstanding ductility and the fact that it is relatively soft (see Table 1). Nb can be cold-

worked to a degree more than 90% before annealing becomes necessary. This property is

responsible for the recent new developments on fabricating seamless Nb SRF cavities by

hydroforming and spinning as described in the following section.

Although there are other superconducting compounds that have higher Tc or higher Hsh,

they either are not having the three mentioned characters in a superconductor or were discovered

much later as superconductors than Nb. It is fair to say that so far Nb is the most investigated

material for SRF applications and the major material used in particle accelerators based on SRF

technology.

Only limited research effort has been put on other superconductors such as NbN (Tc=16.2

K), NbTiN (Tc=17.5 K), Nb3Sn (Tc=18.3 K), V3Si (Tc=17 K), Mo3Re (Tc=15 K), and MgB2

(Tc=39 K). Interested readers for this topic are referred to an overview paper15

from Valente-

Feliciano.

It is worth mentioning that recently some groups16

have started to revisit the Nb on Cu as

an alternative for making SRF cavities by taking the advantages of the good superconducting

properties of Nb and good thermal conductivity and cheap Cu substrates.

I am not going to discuss high temperature superconductors such as, for instance, MgB2,

YBCO, BICCO, etc here since they are not related to Nb that is the topic of this book.

Section 3: Fabrication of Nb Cavities

In order to understand and improve the use of Nb in fabricating Nb SRF cavities, we have

to know the typical procedure for doing it. Fig.4 shows a flow chart for a typical procedure of

9

Nb cavity fabrication. This procedure has been well established in the past couple of decades

and does not various much from one lab to another. However, significant amount of new

developments have taken place in the last decade or so on how each step is done in reality. For

instance, Step 15 in Fig.4 can be done in many ways, including Buffered Chemical Polishing

(BCP), ElectroPolishing (EP), barrel polishing, or Buffered ElectroPolishing (BEP). In this

section, I will first give a general description of the fabrication steps with emphasis on some

selected examples of new developments at some steps of the typical cavity fabrication process.

3.1: Typical Fabrication Steps for a Nb SRF Cavity

Normally the as-received Nb sheets from suppliers are either 3 or 4 mm thick. To

prevent them from damages and contaminations during transportation, the Nb sheets are covered

by sticking tapes that can be peered off. The first thing to do after receiving Nb sheets is to

check whether they meet the material specifications. The specifications typically cover RRR

value, grain size, impurity tolerance, surface finish, yield strength, and flatness. The sheets are

then either deep drawn or spun to form half cells. Trimming is then done to the half cells to

remove any irregularities and undesirable features. Normally a lathe or a computer controlled

milling machine is employed for the trimming. Since trimming may introduce some

contaminants on the surfaces of the half cells, degreasing is then needed. Degreasing normally

takes place in soap and water in an ultrasonic tank and then a light BCP of 5 µm is done to the

further remove any undesirable residuals on the surfaces of the half cells from the previous

fabrication or handling steps. After cleaning and visual inspection for surface scratches, defects,

and rust, electron beam (E-beam) welding on iris can then be performed. It is a good practice to

do some grinding on the welded region to make sure that inner surfaces are smooth. Then a light

BCP of 5 µm is done again before E-beam welding on equator takes place. Note here that all

welding should be done in vacuum at a pressure less than 10-5

to avoid significant impurity

intake during these steps. After these steps, we obtain Nb cavities. The cavities are then

chemically polished again for about 100 µm in order to make sure that it is completely clean.

Then cavities are normally baked at a temperature between 1350 to 1400 oC in a titanium

enclosure in a high vacuum furnace for a few hours to purify the cavities. Titanium is a good

getter for oxygen, nitrogen, and other gases. This process also serves as an annealing process to

remove some of the defects such as edge or screw dislocations generated during the previous

10

fabrication steps, especially from depth drawing and spinning. Then the final and the most

important surface Nb removal of 150 µm is followed. This step removes the mechanically

damaged layer as well as any evaporated niobium scale deposited on the surface during welding.

Then tuning to the correct frequency and field flatness is needed, since a thickness of 260 µm of

Nb has been removed the inner surface of a Nb cavity. At this stage, additional 5 µm Nb can be

removed from the inner surface of the cavity, but is not necessary. Final rinsing and cleaning are

then performed before assembling end flanges and couplers in a clean room. Finally the cavity is

pumped down and then baked at 120 oC for 48 hours before RF test.

3.2: Examples of Innovative Techniques for Fabricating Nb SRF Cavities

Recently, Nb SRF cavities have been also fabricated by hydroforming17

and spinning18

to

create seamless cavities.

Fig.5 shows the principle of hydroforming technique. This technique starts with Nb tubes

with a diameter half way between the iris and equator. The diameter at iris has to be reduced

while the diameter at equator has to be expanded. Since the ratio of the diameters between

equator and iris for a typical elliptical cavity is ~3, any attempt to form seamless cavities from

tubes is a significant challenge. Once has to balance the hardness and roughness introduced by

the diameter expansion at equator and the diameter reduction at iris to the inner surface of a

seamless cavity. It was found17

that a starting tube diameter between 130 and 150 mm is optimal

for a 1.3 GHz cavity. This technique can be used to produce single cell and multi-cell cavities.

The very first Nb double cell cavity produced by hydroforming is shown in Fig.6. The highest

Eacc for a single cell seamless cavity reaches 43 MV/m as shown in Fig.7.

It was demonstrated18

by Palmieri that single cell and multi-cell Nb and Cu seamless

cavities could also be fabricated by spinning. Fig.8 shows the progressive steps during the

fabrication of a single cell seamless Cu cavity by this technique. After spinning, cavity has to be

tumbled and mechanically ground for at least 100 µm to remove surface fissures before any

further chemical treatment. Typical excitation curves at 1.6 K for a spun seamless single cell Nb

cavity are shown in Fig.9. Several single cell cavities reach an accelerating gradient of 40 MV/m

with a decent Qo. Fig.10 shows the first nine-cell Nb cavity manufactured by the spinning

technique. Unfortunately the cavity was damaged before a RF measurement was done. One

challenge for this technique is that cavities can be quite thin after fabrication.

11

Although there are still many technical problems waiting to be resolved for the seamless

cavity formation techniques, the exclusion of welding steps from cavity fabrication process is a

very significant progress. These new developments also alter the flow chat shown in Fig.4.

Another interesting idea for fabricating Nb SRF cavities is that after Step 9 in Fig.4 Step

15 is followed. Then the polished Nb dumbbells are E-beam welded to form muilti-cell cavities.

Followed either by a light BCP removal of 5 µm + high pressure water rinse (HPWR) or just

simply HPWR before being evacuated for RF tests. The attractiveness of this idea is that the

final chemical treatment on multi-cell cavities can be avoided, which makes the life in this SRF

world much simpler and easier. This can be very important for electropolishing if it is employed

as the final chemical treatment, since I personally believe strongly19

that the cathode shape

matters during electropolishing on Nb. The size of a cathode is limited by the size of the beam

tube of a cavity if electropolishing has to be done on the cavity. This development is on-going

through acollaboration between Peking University (PKU) and JLab. Nb dumbbells will be

polished by BEP at PUK employing a shaped aluminum cathode fabricated at JLab (see Fig.11).

The thus formed cavities will be treated by HPWR and then RF-tested at JLab. It has been

demonstrated20

by PKU that bright and shining Nb dumbbells can be fabricated by BEP via a

shaped Al cathode as shown in Fig.12.

3.3: Examples of New Developments in Inspection Technique

In my view, there are several key steps in cavity fabrication process that deserve strong

attention to ensure the outcome of good cavities with decent excitation curves. The first of such

key steps is to make sure that the as-received Nb sheets meet the specifications.

3.3.1: Eddy Current and SQUID Scanning

One significant new development in this respect is the application of eddy current and

Superconducting QUantum Interference Device (SQUID) scanning21,22

to check the defects on

the as-received Nb sheets. This allows the removal of some defective starting materials before

going into cavity fabrication.

Eddy current scanning is a nondestructive technique. The working principle of this

technique is schematically shown in Fig.13. A double-coil sensing probe is used to detect

inclusions and defects embedded under the surface by detecting the alternation of the eddy

12

currents. It is important to keep the distance between sensing head and the sample constant

during scanning. Typically it takes about 15 minutes to scan a sample size of 300X300 mm2 and

a line width of 1 mm. Defects deeper than 0.1 mm from the surface are typically undetectable.

This can be improved by replacing the double-coil sensing probe by a SQUID detector. The

working principle of the SQUID scanning technique is shown in Fig.14 where the SQUID is

used to detect the secondary magnetic field of the eddy current. Probing depth can be improved

up to 2.8 mm for a SQUID scanning device operated at 90 kHz with an excellent signal noise

ratio. Latest developments on eddy current scanning can also be found at Reference 25.

3.3.2: Kyoto Camera

Checking whether the welding is done satisfactorily and how the welding area and areas

close to the welding area look like especially on the inner surface of a cavity are also very critical

during cavity fabrication. In the past, this was usually done by a borescope, long distance

microscope, or a well-lit angle mirror. All of these were inaccurate and not quantitative. It can

become quite a challenge to apply the devices to a multi-cell cavity. Recently a new camera

based system26

was developed jointly by Kyoto University and KEK, which can be used to

examine the inner surface quality even for a 9 cell cavity. The schematic diagram of this so

called “Kyoto Camera” is shown in Fig15a and the associated key components are shown in

Fig.15b. During inspection, the cavity is rotated and moved while the cylinder is kept stationary.

The inner surface of the cavity is reflected by the mirror and the image is captured by the

camera. A real setup of this system is shown in Fig.16. The resolution of Kyoto Camera can

reach 7.5 µm per pixel.

By dividing the electro-luminescence sheets into strips to form a strip illuminator and by

turning on and off each illumination strip and then following the movement of the bright point

across a defect, it is possible to obtain quantitative information regarding the defect structure on

the surface via simple geometric consideration. A typical example of this is shown in Fig.17.

Further information about Kyoto Camera and examples of real observations can be found also in

Refs 25 and 27.

3.3.3: Replica Technique

13

Typically Kyoto Camera cannot detect surface geometric defects with a size smaller than

5 µm. Some surface replica techniques have been developed to visualize surface geometric

defects with a better resolution for single cell and in some cases multi-cell cavities. The first

reported results from the replica technique are from S. Berry et al (see Refs. 28, 29). They

carried out the replica in two steps: a) A negative replica of the inner surface of a cavity was

made by a siloxane polymer (vinylpolysiloxane+hydrogenated polysiloxane) that can solidify in

15 minutes. b) Positive replica was done by a bi-compound mixture of polyurethane held under

a pressure of 1-2 bar for 15 minutes. A typical result from this replica technique is shown in

Fig.18 where the inner surface of a cavity at the quench size was replicated. The resolution of

this technique can be less than 1 µm. It was found that the residuals after the replication process

on the cavity can be completely cleaned by ultrasound in 65 oC basic bath.

Similar technique with a different replica material was also developed by Fermi Lab30,31

.

In this case, they used a silicon rubber (two-component translucent silicone RTV compound

manufactured by Freeman Mfg. Inc. V3040) to make negative replicas. Then positive replicas

were done using a second RTV compound (Momentive RTV630) or an epoxy. The advantage of

this technique is that it does not need any releasing agent and it leaves no residuals on the

surface30

. However, the silicone rubber must be evacuated in vacuum before pouring into a

replicated area. It takes more than 18 hours to cure. The resolution of this technique is

comparable to the previous one.

• JLab recently has applied a simple replica technique32

to Nb SRF cavities with a

resolution of a few nanometers. This technique is done in the following way: Depending on the

size of the features that are needed for replicating, either commercial cellulose acetate films of

35µm (for features smaller than 1 µm) or triphan foils of 100µm thick (for features larger than 1

µm) are employed. Negative replica can be obtained by applying a couple of acetone drops on

the observed surface and then put a film on the surface. Lift the film in 10 minutes. Then the

film is put on a flat surface and coated by palladium for 5 nm (see Fig.19 for an example).

Positive replica can be obtained by dissolving the acetate film by acetone. In fact, we really

don’t think that it needs to make positive replicas in the all replica techniques described here,

since this can easily be done by some suitable software for image processing. The advantages of

this JLab technique are that it is fast and simple and has a resolution basically limited only by the

size of the molecule of the film or the foil. It can transform a curved surface to a flat one. The

14

latter point can be extremely useful for subsequent observations or measurements since some

instruments (for instance an optical microscope) are very difficult to be applied to a curved

surface. The replicas can be examined not only by an optical microscope and a profilometer, but

also by a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

One challenge for the above mentioned replica techniques is how to effectively apply

them to multi-cell cavities when hands are not long enough.

3.4: Examples of New Developments in Final Chemical Treatment

Step 15 in the Fig.4 is perhaps the most critical one in determining the performance of a

Nb SRF cavity. It is normally done by either BCP employing the acid mixture of phosphoric,

hydrofluoric, and nitric acids with a ratio of 1:1:1 or 2:1:1 or by EP employing the acid mixture33

of hydrofluoric and sulfuric acids. There are quite some new developments on this topic in the

last decade. Most of them are done along the line of modifying or improving the existing

techniques. Due to the limited space, here I will discuss only a few new developments that have

shown some promising signs. One34

is BEP. Others include: a) Nb polished35

by ionic liquids at

a temperature higher than 100 oC, b) Plasma Etching

36, and c) FARADAYIC Electropolishing

37.

3.4.1: Buffered Electropolishing (BEP)

BEP experiment was initiated at JLab in early 2001. Some preliminary results were

published in Ref. 38. BEP uses the acid mixture of hydrofluoric, sulfuric, and lactic acids as the

electrolyte at a volume ratio of 4:5:11. Here lactic acid acts as a buffer in a similar way as what

H3PO4 does in BCP. By replacing the majority of H2SO4 in the conventional EP, BEP treatment

reduces the aggressiveness of the electrolyte significantly. It has been demonstrated that BEP

can produce the smoothest39-42

Nb surface ever reported in the literature. Smoother inner surface

of a Nb SRF cavity is known to have positive effects on its RF performance. Experiments also

show39,43 that Nb removal rate can be as high as 4.66μm/min. This is more than 10 times faster

than 0.38 μm/min of EP. Faster Nb removal rate can contribute significantly to the reduction of

the capital costs of the surface treatments for Step 15. For instance, conventional EP will take

typically 23622 seconds to get Step 15 done. With BEP, it takes only 1931 seconds. Other

benefits of BEP as compared with EP include: a) acid mixture is much safer to handle, b) the life

of the acid mixture is longer, c) acid mixture is cheaper, and d) less or no sulfur precipitation.

15

CH3

O

O-

O-

CH3

O

O-

O-

Fig.20 shows a quantitative comparison on the surface smoothness for Nb treated by BEP, EP,

and BCP as measured by a precision 3-D profilometer34

. The RMS of the smoothest Nb

surface39

treated by BEP is 20 nm over an area of 200X200 µm2. To the best of my knowledge,

this is also the smoothest Nb surface ever reported in the literature.

Early experiments38

done on Nb half cells have implied that cathode shape plays an

important role during BEP treatments on curved surfaces. This is confirmed later by

experiments at other labs20,42,44

. Fig.21 shows a typical example of the surface finish for a Nb

half cell treated by BEP for 1800 seconds without electrolyte circulation when the shape of the

cathode is changed in such a way to allow a more homogenous electric field distribution inside

the half cell.

At this moment, the polishing mechanism responsible for BEP is not completely clear.

It has been suggested44,45

that apart from HF lactic acid may also participate in the Nb polishing

process through the following reactions:

(9)

or

Nb2O5+4 +6H+ 2 +H2O (10)

or

Nb2O5+6 +6H+ 2 +3H2O (11)

3-

_

16

This may explain why BEP can polish at a much faster way than conventional EP.

To investigate whether BEP was governed by diffusion of fluorine ions, experiments42

with a rotating disc electrode were carried out at CEA Saclay in France. The result is duplicated

in Fig.22. where polishing current (I) was found to be not proportional to the square root of the

angular speed (ωa) of the rotation disc, implying that the diffusion is not the only process taking

place during BEP. If the polishing is diffusion limited, I is related to ωa in the following way46

:

I=6.2x10-4

nFSD2/3

ν-1/6

ωa1/2

C (12)

Where n is the number of electrons in the electrochemical reaction; S is the surface of the

electrode (cm2); D is the diffusion coefficient (cm

2/s); ν is the viscosity (St); C is the

concentration of the active species (mol/l).

Further measurements42

with Electrochemical Impedance Spectroscope (EIS) seemed to

indicate the polished surface might be covered by a porous film whose resistance increased with

the applied potential.

BEP has been applied to treat Nb SRF single cells both vertically in JLab and

horizontally at CEA Saclay for comparison. In fact, the first very vertical electropolishing on a

Nb SRF cavity was carried out at JLab via BEP in early 2002 as reported in Ref.47. Since then

many improvements48

have been made to the system, including a more reliable acid circulation,

more accurate acid flow control, more efficient heat removal from the treated cavity, external

cooling, and an automatic data acquisition system. The modern system is shown in Fig.23.

The major advantages of vertical EP as compared with the horizontal one are: 1) Easier

for draining. 2) Easier for making the electrode contacts, since cavity rotation is not necessary in

vertical EP. 3) Easier to incorporate other cavity treatment techniques into a vertical setup such

as, for instance, HPWR, BCP, BEP, drying, baking, etc. Of course hydrogen removal is more

difficult for a vertical system, which is especially true for treatments on multi-cell cavities.

So far the best results were all obtained with the vertical system at JLab and shaped

cathodes. Typical excitation curves49

for a CEBAF shape regular grain single cell cavity are

shown in Fig.24. It is noted from Fig.24 that Qo is improved significantly after baking at 120 oC.

The best result was achieved on large grain cavity where Eacc reached 32 MV/m. It is worth

17

pointing out here that the post cleaning procedure for all BEP treated cavities is identical to that

used for conventional EP. Improving on the post BEP cleaning may be one of the keys to open

the door for a better performance for BEP treated Nb cavities. Optimization process on BEP is

now underway.

3.4.2: Other New Developments to Remove Nb

Currently, Step 15 in Fig.4 is normally done with an acid mixture involving HF. HF is

well-known to be very nasty in terms of its effects on the health of human beings. A non-HF

electrolyte for polishing on Nb is a very attractive idea. There are many HF-free recipes for

polishing Nb as listed in Ref.35. Many of them are even most toxic or dangerous than HF is35

.

Two different ionic liquids have been explored to electropolish Nb at temperatures higher than

100 oC at University of Padua. One

35 consists of choline chloride and urea at a ratio of 4:1 plus

sulphamic acid in a concentration of 30 g/l. The other50

is a mixture of urea and choline chloride

at a ratio of 3:1 plus ammonium chloride in a concentration of 10 g/l. Polishing has to take place

at 120 oC and 190

oC respectively

35,50. The highest Nb removal rate can be 12 times quicker than

that of the conventional EP. A typical Nb surface produced by this technique is shown in Fig.25.

Plasma etching on Nb is a R&D project through the collaboration51

between University

of Old Dominion and JLab. The idea here is to use some reactive gas species36,51

such as, for

instance, Ar/Cl2 or BF3 to chemically react with Nb under the discharge from a DC or RF source.

This will generate some volatile Nb compounds that can be pumped away. An experimental

setup employing microwave glow discharge is schematically shown in Fig.26. It was

demonstrated36

that an etching rate of 1.5 µm/min can be reached when Cl2 was used as the

reactive agent. The surface finish produced by plasma etching appears to be comparable to that

by BCP at the present stage of development. This technique is attractive since it does not

employ HF and it can produce a decent etching rate.

Faradayic Electropolishing was developed37

by Faraday Technology Inc. The novelty

here is that instead of using the chemical mediated method to remove oxides from Nb surfaces,

Faradayic Electropolishing employs electrically mediated approach to perform this removal. The

electrolyte they used52

is 20% H2SO4 solution. Here sulfuric acid acts as an oxidation agent.

The key here is that the applied electric field is not constant. It is pulsed and modulated in a way

18

to optimize the polishing. For instance, during Faradayic Electriopolishing chemical reactions

can take place in the following way:

2Nb+5H2O Nb2O5+5H2 (13)

Nb2O5+10H++10e

- 2Nb+5H2O (14)

Reaction formula 13 is the same as that of EP and BEP. However, now by controlling

the electric potential applied to the surface, before the oxide is formed completely as an

insulating layer on the naked Nb surfaces the applied electric field can be either reduced or shut

off to allow newly formed oxides (for instance the pent-oxides) to diffuse away from the

polished surface. After the oxide concentration is returned to its original value, the electric field

will be turned on or ramped up. A typical setup is schematically shown in Fig.27. This

technique can produce a polishing rate as high as 5 µm/min. A typical Nb surface treated by

Faradayic Electropolishing is shown in Fig.28 at a polishing rate of 2.7 µm/min.

It is worth pointing out that any new development is not as simple as it appears to be.

Take BEP as an example, by adding only one additional acid into the electrolyte of the

conventional EP a new whole set of problems emerge. It takes more than 8 years to reach the

stage that it is now. Although the cause of this can always be attributed to external and

environmental factors including lack of manpower and the smartness of the persons involved in

the R&D (easy target), there are perhaps intrinsic reasons due to the nature of any new

development. For instance, how many years does it take to develop the conventional EP? We

are still trying up to now to improve it and to understand the mechanism responsible for it19

.

Therefore one has to be extremely careful before taking on a new development, especially on

something that is completely new.

3.5: Examples of New Developments in Final Cleaning Technique

HPWR with ultra pure water is the process that is normally done in between Steps 17 and

18 as the final cleaning before the assembly and low temperature baking. This final cleaning is

critical since it determines whether there are still contaminants or chemical residuals on the inner

surface of a Nb cavity. These contaminants or chemical residuals can have detrimental effects

19

on cavity performance. Here I will focus on two examples of these new developments. One is

Gas Cluster Ion Beam (GCIB) technique53

. The other is Dry Ice Cleaning (DIC) technique54

.

3.5.1: GCIB Technique

In contrast to HPWR where a mechanical effect is the main cleaning means, as should see

in the following GCIB offers both mechanical and chemical effects in the cleaning. The

application of GCIB technique to Cu RF cavities and Nb SRF cavities was first done55,56

by

Swenson et al at Epion Corporation to mitigate high voltage breakdown through reducing the

surface roughness of oxygen-free Cu via GCIB. Later on, collaboration was established between

Epion Corporation, JLab, Fermi National Accelerator Lab, and Argonne National Lab on a R&D

project to investigate the application of GCIB technique to Nb SRF cavity both experimentally

and theoretically. The results of this collaboration and the current status of the application of this

technique to the treatments of Nb SRF cavities are summarized in Ref.53.

The working principal of GCIB is schematically illustrated in Fig.29. Various types of

gases can be used for GCIB treatments. The gases can be inert such as Ar, Kr, Xe etc. or

chemically reactive such as O2, N2, CO2, NF3, CH4, B2H6 etc. that may react with the surfaces

under treatments depending on what the application one has in mind. After selecting an

appropriate gas species, the gas is forced through a nozzle that has a typical pressure of 7.6X103

Torr on one side and a vacuum of 7.6X10-3

Torr on the other side. Therefore the gas undergoes a

supersonic expansion adiabatically that slows down the relative velocity between the atoms of

the gas, leading to the formation of a jet of clusters. A typical cluster contains atomic numbers

ranging from 500 to 10,000 that are held together by van der Waals forces. A skimmer is then

used to allow only the primary jet core of the clusters to pass through an ionizer. The clusters are

ionized by an ionizer via mainly electron impacts and the positively charged clusters are

electrostatically accelerated via a typical voltage ranging from 2 kV to 35 kV and focused by a

beam optics. Monomers and dimers are removed from the beam by a dipole magnet before the

beam is neutralized with an electron flood. The aperture in Fig.1 after the neutralizor is used to

collect the monometers and dimers. Surface GCIB treatments are done through mechanically

scanning an object. Typically, the impact speed of the clusters to the surface of an object under

GCIB treatements is 6.5 km/s, and the current of a gas cluster beam can be as high as 1 mA.

20

The selection of an appropriate gas species for doing GCIB treatment is very important.

When an inert gas is chosen, the major effects on the treated surfaces are smoothing and asperity

removal due to lateral sputtering. Chemical gases, on the other hand, can produce some

additional effects such as, for instance, doping, etching, and depositing, etc. depending on the

properties of the treated object and the gas species selected. Implantation is only limited to the

top several atomic layers during GCIB treatments due to the low individual atomic energy. One

can also combine the use of different gas species in a specific order for a particular application,

although less work has been done in this research direction so far.

Only Ar, O2, N2, and NF3 have been used in the GCIB treatments on Nb. Ar was selected

because of its smoothing effect. O2 GCIB is interesting due to the possible chemical reactions

between O2 and Nb and so is true also for N2, although in case of using N2 there was a hope that

NbN could be formed on the treated surface since the superconducting transition temperature

(Tc) is 16.2 K that is much higher than 9.2 K for Nb. NF3 is expected to have a relatively higher

etching and removal rates on Nb than those from other chemically reactive gas species.

The main objective for the final cleaning is to remove or suppress all sources that can

cause field emission and degrade the cavity performance. GCIB was found to be able to

suppress field emission significantly57

as shown typically in Fig.30 through measurements using

a scanning field emission microscope (SFEM)47

. The cleaning effect of GCIB is done through

controlled removing or smashing of any residuals or contaminants through the bombardment

using one appropriate gas species or a combination of several gas species. In fact, three effects

on Nb surfaces after GCIB treatments have been identified53

. One is the smoothing effect.

GCIB can remove sharp tips or edges as demonstrated through mesoscale modeling58

shown in

Fig.31. This is also verified experimentally57

through observations by a scanning electron

microscope (SEM). The second is the smashing effect. After GCIB treatments, it was found that

residuals or contaminants were broken into pieces as if they were stepped on by a heavy sumo

wrestler. The third effect is the modification of surface oxide layer structure of Nb59

. With the

selection of a right gas species and treatments parameters, GCIB can increase the thickness of the

surface pent-oxide layer. This can lead to the increase in the threshold for field emission60

.

In fact, GCIB can also be used as the final chemical treatment technique in Step 15. It

has been demonstrated in Ref.61 that an etching rate larger than 5 nm*cm2/s is possible if

21

NF3+O2 are used as the gas species in GCIB treatments on Nb. Fig.32 shows the measured

etching rates on Nb for some selected gas species61

.

GCIB has been applied to the treatments of Nb SRF cavities. Due to the limitation of the

space here, interested readers are referred to Ref.53.

3.5.2: DIC Technique

The cleaning power54

of DIC is given mainly by a combined thermal and chemical effect.

A jet of pure carbon dioxide snow surrounded by supersonic N2 is released from a nozzle as

shown in Fig.33 and impacts on the treated surfaces. After leaving the nozzle, liquid CO2

relaxes spontaneously, resulting in a snow/gas mixture with 45% snow and a local temperature62

of ~78.9 oC. The supersonic N2 here functions in the following two ways: a) It gives an

acceleration and focusing of the jet. b) It prevents condensation of humidity on the cavity

surface. The cleaning effect is based on shock-freezing of the contaminants, strong impact of the

snow crystals, and an increase of volume by 500 times after sublimation. Contaminants or

residuals get brittle and start to flake off from the surface. It is very effective in removing

hydrocarbons and silicons, since liquid CO2 is a good solvent for non-polar chemicals. Cleaning

is usually done in a clean room of class 10 to minimize air-born contaminants to enter the cavity

during treatments. To achieve optimal cleaning, it is important to keep the cavity warm (20~30

oC) during cleaning. Fig.34 shows a photo of the cleaning setup and a schematic diagram of the

setup.

The DIC cleaning effect was further studied64

on polycrystalline Cu and Nb and single

crystal Nb flat samples by a SFEM and SEM equipped with an energy dispersive x-ray analyzer

(EDX). DIC was found to be able to suppress enhanced field emission from metallic surfaces

under a DC field up to 250 MV/m. It was found that that emitters down to a size of 400 nm

could be effectively removed and DIC also had a smoothing effect on surface protrusions.

Fig.35 shows field emission maps of a Nb surface before and after DIC.

RF tests have been done on the Nb single cell SRF cavities treated by DIC. In several

cases, performance was still limited by field emission54

. The best was achieved on a cavity that

reached 38.2 MV/m before quench. A strong high field Q slope was observed in all excitation

curves shown in Ref.54. It is noted here that it takes almost a decade for DIC to reach a stage as

described here (any similarities in development between DIC and BEP?).

22

Section 4: Requirements in Physical, Chemical, Metallurgical,

Mechanical Properties of Nb for Fabricating High Quality SRF Cavities

The topic of this section is huge. It would take the space of an entire book in order to

cover all the new developments in this field. Here I will try to limit myself to the discussions of

some fundamental requirements and some selected topics of the requirements that I feel are

important or may have immediate impacts on Nb SRF cavity fabrication.

4.1: Requirements in Physical Properties

Looking at this topic, the first image appears in my mind is how a typical particle

accelerator based on Nb SRF technology works. The particle accelerator is typically operated at

a temperature below the boiling temperature of liquid helium (abbreviated as LHe2 in the

following) that is 4.2 K or -269.0 oC and in a RF field. For instance, CEBAF of JLab is operated

at 2 K/-271.2 oC at 1.5 GHz. As stated in Section 1.2, the RF field only penetrates into Nb to a

depth of ~50 nm. Therefore surface properties of Nb are extremely important when we discuss

the requirements in physical properties of Nb for SRF applications.

To study the requirements, we need experimental tools. In the past decade, various

experimental tools have been employed to study the physical properties of Nb. Among them,

many are surface instruments. For instance, in 2003 a surface science lab47,65

(SSL) was

established at JLab to study various properties of Nb related to SRF applications. The SSL

contains typical instruments often used in the studies of the physical properties of Nb such as, for

instance, SEM and EDX, SFEM, a 3-D large scan area profilometer, TEM, secondary ion mass

spectrometry (SIMS), scanning Auger microscope (SAM), metallographic optical microscope

(MOM), electron backscattered diffraction (EBSD), and a well equipped sample preparation

room that allows all sample preparation required by the SSL. Other popular instruments that

have been used include x-ray photoelectron spectroscopy (XPS), atomic force microscope

(AFM), and scanning tunneling microscope (STM).

To use experimental tools effectively, it is important that we know the characteristics of

each experimental tool. For instance, if one wants to study the general surface roughness of a

polycrystalline Nb sample of an average grain size of 50 µm it is extremely unsuitable to use

AFM or STM since their maximum scanning length is around 50 µm. In this case, one AFM or

23

STM measurement can capture only a couple of grains. Therefore, it is critical to select the right

tools for a particular property that one wants to study. For the convenience to the readers, Table

2 summarizes the major characteristics of the popular experimental tools for studying the

physical properties of Nb for SRF applications.

4.1.1: Surface Oxide Layer Structure

Ideally the surface resistance of Nb should be less than 0.5 nΩ at the typical accelerator

operating temperature of 2 K and the accelerating gradient can reach a value that produces a field

close to the superheating field on the inner surfaces of Nb cavities. This has never occurred so

far, due to various sources of imperfections in Nb, which cause energy dissipations and

degradation in superconducting properties.

It is well know that Nb is a highly reactive metal. When it is in contact with air, chemical

reactions take place between Nb and the constituents of air such as, for instance, O2 and water

moisture, forming an oxide layer on its surface. To see how the oxide layer structure typically

looks like for BCP treated Nb surfaces, oxygen SIMS depth profile using the instrument

described in Ref.47 was done. A typical result is shown in Fig.36 together with the results

obtained on two baked Nb samples in air at 120 oC for 12 and 48 hours respectively. Not a sharp

decreasing in oxygen peak intensity was observed for the reference sample (BCP treated),

implying that the interface between the surface oxide and Nb might not be sharp. It is well know

that the oxide on the very top of the surface is Nb2O5 that is not harmful to the RF performance

of Nb. The fact that the interface between the top Nb2O5 and the underneath pure Nb is not sharp

can be bad for RF performance of Nb. This means that there are other oxides in between Nb2O5

and Nd. Since the other oxides may not be dielectric and are likely different in physical

properties from pure Nb, they may cause energy dissipations to Nb SRF cavities.

XPS is a powerful tool for studying differences in chemical states. An early study66

shows that the top of Nb is covered by amorphous Nb2O5 of a thickness of 6 nm. Later

experiments found67,68

that the thickness of Nb2O5 is around 3.0 to 4.4 nm on the surfaces of

BCP treated polycrystalline samples. The thickness increases with BCP acid agitation68

up to

1.3-1.4 times of the static one. In case of (100) orientated Nb single crystal69

, the thickness is

only 1.9 nm. Sub-oxides such as, for instance Nb2O, NbO, NbO2 are found68,70-73

at the interface

between Nb2O5 and pure Nb through deconvolution as shown typically in Fig.37. It is worth

24

noting here that surface roughness of BCP treated Nb shown typically, for instance, in Fig.20a)

can complicate the interpretation of XPS tremendously in consideration of the spot size of the x-

ray that can be 100x100 µm2 or larger. Another complication comes from the face that the

corrected signals in XPS measurements are the overlapped contributions from all oxides. A

program for deconvolution is needed in order to separate the contributions. Therefore more than

3 parameters have to be determined from the deconvolution process, which can be arbitrary. A

procedure74

for using principal components analysis has been proposed to deal with this problem.

Direct observation of the oxide layer structure by TEM with atomic resolution was

recently reported in Ref.75 on BCP, EP, and BEP treated Nb surfaces. A typical TEM cross-

section photo of a BCP treated Nb surface is shown in Fig.38. From this image, we can see that

the pent-oxide has a thickness of ~3 nm that is comparable with that obtained from XPS67,68

.

The sub-oxides region has a length of ~0.7 nm. No significant differences in surface oxide layer

structure are found between EP, BCP, and BEP from TEM, which is consistent with the results

from XPS.

Since the determination of the sub-oxides is not conclusive, in principal all or any stable

Nb sub-oxides can exist at the interface. To the best of my knowledge, stable Nb sub-oxides

include NbO2, Nb2O3, NbO (cubic), and Nb2O (tetragonal). Based on the experimental results

from XPS and TEM, the surface oxide layer structure of Nb can be tentatively represented in

Fig.39, if we can temporally neglect grain boundaries, possible interstitial atoms or secondary

phases or other defects and imperfections. Since the sub-oxides can be semiconducting or

metallic or superconducting at a lower Tc than that of pure Nb, RF losses can take place.

Therefore the Nb sub-oxides are undesirable for SRF applications.

4.1.2: High Field Q-slope

When discussing the requirements in physical properties of Nb for SRF application, one

cannot avoid the discussion of the high field Q-slope that is currently a challenge to the basic

understanding of superconducting properties of Nb in strong RF fields. In this subsection, I will

try to limit my discussion to the very fundamental aspects of high field Q-slope such as, what it

is about, how normally we deal with it, how this will affect Nb handling and cavity production

etc. The reasons for this selection are mainly the following two: 1) there are many updated

review papers76-79

on this topic, which are easily accessible for the interested readers and there is

25

no need to repeat here. 2) It is still not conclusive as to what the causes are for the high field Q-

slope. Although there are many models available in the literature attempting to explain the

origin of the high field Q-slope, none can explain all the major experimental results.

High field Q-slope refers to a sharp increase of the RF losses when the peak magnetic

field (Hpk) at the inner surface of a Nb SRF cavity reaches ~ 90 mT. This is better visualized by

looking at a typical excitation curve as schematically shown in Fig.40. 90 mT corresponds

roughly an accelerating gradient of 25 MV/m. The remedy to the slope is to bake the cavities at

120 oC in vacuum for 48 hours (Step 24 in Fig.4) (called “low temperature baking effect”). In

fact, it was found that baking in dry air could result in the same benefits76

. After the baking, high

field Q-slope is permanently cured. Subsequent exposure to air in clean room or HPWR will be

alter80

the baking benefits.

Since we know how to remedy the high field Q-slope, knowing the mechanism of how it

is created or not will not affect the production of high quality Nb. However, the baking time of

48 hours is time consuming and costly. It was found that a shorter baking time of 3 hours at 145

oC in vacuum

81 could produce the same result. This is, however, not confirmed by other labs.

Here I would like to mention two suggested origins for the high field Q-slope, which may

be useful from the cavity fabrication viewpoint. One is a group of models81-84

that try to relate

the low temperature baking effect to oxygen diffusion where it is suggested that the oxygen of

the sub-oxides that exist underneath the pent-oxide as discussed in the last subsection and/or the

oxygen of the pent-oxide itself starts to diffuse into the interior of the pure Nb. Fig.41 shows the

schematic for one of the oxide diffusion model. The other is the local defects model85

as

suggested by Romanenko. He did EBSD measurements on a region where the surface started to

show energy dissipation while the whole large grain cavity was still in superconducting state

(called “hot spot”). Then he compared the result with the EBSD measurements from the normal

region of the cavity. He found a huge difference in local misorientation maps between the hot

spot and the normal region as shown in Fig.42. The misorientation from EBSD measurements is

typically a reflection of the dislocations in the crystal.

4.1.3: Surface Smoothness

More and more experimental data have shown that a smoother Nb surface is beneficial to

the cavity performance. It is well-known by now that the EP treated cavities can routinely reach

26

an accelerating gradient of 40 MV/m as first pointed out by Visentin76

. The only major

difference that we have found so far between the surfaces treated by EP and BCP is the

roughness. EP treated Nb surfaces are much smoother (see, for instance, Fig.20). Currently, we

know that there are two clear benefits for having a smoother Nb surface. The first one is that it is

much easier to do cleaning on smoother surfaces. The physisorption force is much larger for the

particulates that sit on rougher surface than those that sit on smoother surface. It is been found60

by SFEM that there are fewer field emitters on smoother surfaces than those on rougher surfaces.

Therefore cavities with smoother inner surfaces can pass the obstacle of field emission relatively

easily. The second point is that rougher surface may create local enhancements in magnetic

field86

that has been suggested as one of the mechanisms responsible for causing energy

dissipations at high field.

A remarkable insight on this topic was presented by Saito in Ref.87 where he did

quantitative calculations by assuming a dependence of thermodynamic critical field on the

magnetic field enhancement factor. He was able to demonstrate that in order to reach an

accelerating gradient higher than 30 MV/m the surface smoothness much be better than 2 µm.

To the best of my knowledge, currently the smoothest Nb surface was obtained39-41

by BEP at

Peking University where a root mean square (RMS) of 20 nm was measured over a surface area

of 200x200 µm2.

To summarized up so far for this section, we know that high quality Nb cavities can be

manufactured if the cavities contain less Nb sub-oxides, have very smooth inner surfaces (should

be less than at least 2 µm), and should be fully annealed to remove defects among other

requirements that will be discussed in the following (In the chapter, I will not discuss cavity

optimization from geometry calculations).

4.2: Requirements in Chemical Properties

It is a common practice to give a specification on the chemical composition when

purchasing Nb from manufacturers. This is because the performance of a Nb SRF cavity

depends critically on the chemical composition of Nb. Surface oxygen contain discussed in the

previous section in one example. It is known that interstitial oxygen of several at% can strongly

depress superconductivity in Nb. Tables 3&4 are typical examples of the specifications for

impurity concentrations of Nb for spallation neutron source (SNS) project. To understand the

27

specifications better, I feel that it is very useful to know the production process of Nb. This can

be easily visualized by a Nb production flow chart as shown in Fig.43. Nb ingots are first

produced. Then they are forged and rolled to form sheets for delivering to SRF users. Typically

the thickness of Nb sheets delivered to JLab is either 3 or 4 mm.

4.2.1: Q Disease

One deadly element in Nb that can cause detrimental effect on its RF performance is

hydrogen. It is found from many cavity tests that slow cooling of a cavity in the temperature

range from 100 K to 150 K can degrade the quality factor up to two orders of magnitude. This is

caused by the formation of harmful η and ν phases of niobium hydrides (called Q disease).

The harmful niobium hydrides can only form when the concentration of hydrogen in Nb

exceeds 100-200 at ppm. From Table 4, we can see that normally the impurity contain of

hydrogen of the as-received Nb sheets is below 10 ppm. Why can the Q disease still take place?

This is due to the following two reasons: a) At a temperature between 100 and150 K, the

mobility of hydrogen atoms can reach 5 µm/min. A slow cooling through this region will allow

hydrogen to migrate and gather. At some localized regions the hydrogen concentration can

exceed 100-200 at ppm so that the harmful phases can be formed. b) Various cavity fabrication

steps in Fig.4 such as, machining, E-beam welding, and surface polishing can result in an uptake

of hydrogen to Nb, leading to an increase in hydrogen concentration in Nb. Normally the Nb

pent-oxide on the top of Nb surface can serve as a barrier for hydrogen to enter to the interior of

Nb. However, during handling and treatments the pent-oxide layer can be damaged or removed

(for instance, Step 15). Hydrogen can then start to move in. Hydrogen is also generated during

the reaction of Nb with water as shown in Formula 13. Therefore, the hydrogen concentration in

a fabricated Nb SRF cavity is always much higher than 10 ppm. Q disease was first reported88

by Bonin and Roth.

One can avoid Q disease by cooling through the dangerous temperature region of 100-

150 K quickly and warming up quickly too. Baking at an elevated temperature can remove

hydrogen too (Step 14 in Fig.4). For instance, at JLab cavities are normally baked at 600 oC for

10-12 hours to remove hydrogen.

4.2.2: Effect of Impurity Concentration on Thermal Conductivity

28

One of the major reasons for requiring high RRR Nb for SRF applications is the desire to

have a high thermal conductivity, since thermal conductivity (κ) of Nb at 4.2 K is related to RRR

approximately89

by:

κ = 0.25 (W/m-K)xRRR (15)

More precise relationship between κ and RRR in a wide temperature range can be found in

Ref.90. From Formula 15, we can see therefore high RRR Nb will allow the heat generated

during accelerator operator to be effectively removed by liquid helium. This will result in a

more stable accelerator that is more sustainable for thermal instabilities caused by microscopic

defects that are the major cause for low field quenches. The reason for this is the following:

Simple model calculations have shown that the maximum surface magnetic quench field (Hq) of

Nb is given91

by:

Hq =

(16)

Here Tb is the temperature of the bath to that the cavity is immersed, r is the radius of a

microscopic hemispherical defect, Rn is the surface resistance of the defect. Typical κ for RRR

Nb is 75 W/mK at 4.2 K.

So generally speaking, higher RRR and purer Nb are beneficial to the cavity

performance. A typical RRR value of 300 or higher is required currently for SRF applications

with Ta concentration below 500 ppm. However, the higher is the RRR, the more expensive is

the price, which is especially true92

for Ta since it is not easy to separate Ta from Nb due to the

similarity in many properties between these two elements. Reactor grade of Nb (RRR ~ 30) is

much cheaper than RRR Nb with a low Ta concentration. Therefore one has to ask the question

about whether the specification on Ta is too restricted and whether RRR has to be that high. To

answer these two questions, it is helpful to look at Table 5 where the contributions of different

impurities on RRR of Nb are shown as a percentage of the influence of nitrogen (from Ref.93).

We can see here that contribution from Ta is the second smallest and is twenty times less than

29

that of N. Furthermore, from Table 3 we can see that the specification for Ta concentration for

the Nb used for SNS is 1000 ppm and SNS has been running fine up to now.

In fact, a systematic study of the effect of Ta concentration on cavity performance has

been carried out94

. It was found that when 160 wtppm<Ta<1300 wtppm, 35.9 MV/m>Eacc>26.5

MV/m and 15x109>Qo>6.4x10

9 could be achieved after post purification at 1250

oC in a Ti box

for 12 hours and subsequently held at 1000 oC for 24 hours prior to cool down to room

temperature. A typical example of excitation curves94

measured on a single cell cavity made

from a Nb sheet of Ta concentration of 1300 ppm is shown in Fig.44.

The impurities in Nb can be reduced substantially by heating Nb to 1250 oC or higher

under ultra high vacuum and in a Ti enclosure for a period of time up to 24 hours. At DESY,

this is normally done at a temperature between 1350 oC to 1400

oC for 4 hours. Ti serves here as

a getter material for N, O, CO2, water vapor, and methane. Even at a temperature as low as 700

oC, Ti can start to absorb N up to 90 at% and O up to 50 at%. The absorption ability of Ti to N

increases significantly above 1000 oC due to a phase transition in Ti.

One would naturally concern about the possibility of contaminating Nb by Ti during the

purification process. As reported in Ref.95, the diffusion (D) of Ti into Nb is given by

D = 0.099 exp (-86930/RT) [cm2/sec] (17)

when 994 oC ≤ T ≤ 1492

oC. Here R is the gas constant 8.314 J/(mol K). Using Fick’s law, one

can calculate the concentration of Ti as a function of time and temperature. At 1250 oC, Ti

concentration has dropped to 10-5

in a depth of 5 µm by assuming 6 hours diffusion time. Such a

thickness can be easily removed by, for instance, BCP.

From the discussion in this section, we can see that the requirements in chemical

properties of Nb depend on the applications that one has in mind. If an application does not need

to have an accelerating gradient higher than 30 MV/m with a Qo around 1010

, Ta concentration

can be higher than 500 wtppm. On the other hand, RRR value does not have too much room to

be selected. As discussed above, RRR is related to the effectiveness of the heat transportation

between the SRF cavities and their cooling bath. RRR is also important for making the cavities

more sustainable to the effects from microscopic defects. Therefore, RRR values ranging from

30

250 to 300 or better are needed for most applications unless one wants to use the technique of Nb

on Cu. For SNS project, typical RRR of as-received Nb sheets ranged from 300 to 400.

4.3: Requirements in Metallurgical Properties

In my view, this topic has been a bit overlooked in the SRF field. Defects such as, for

instance, dislocations, stack fault, non-fully crystallized regions, non-uniform grain size, etc can

degrade cavity performance. One example is the hot spot discussed in Section 4.1.2 which is

found to be related to localized misorientations.

Normally, it is specified that the Nb sheets should be fully annealed before they are

delivered to users. However, the experience from myself shows that this is not always the case.

To illustrate the importance of this topic, I will go some into some depth in discussing a

softening problem that I encountered during SNS project at JLab. All the photos and data

discussed here were published only as a JLab technical note95

and have not been published

elsewhere.

For example, during SNS project I did cross-section examination using MOM on several

samples from different batches of as-received Nb sheets from two different suppliers. On one

sample from one supplier, I saw that it was not fully annealed at all. Only about 100 µm thick on

the surface I could see crystal structures as shown typically in Fig.45a). From the figure, the

trace of rolling process during the fabrication of Nb sheets (see Fig.43) can be easily seen. On

another sample from another supplier, I found that the crystal structure was progressively

becoming bad from the surface. At the region starting about 1 mm from the surface, significant

amount of the amorphous phase showed up as shown typically in Fig.45b). Therefore, if cavities

are made from such Nb sheets after Step 15, the exposed surfaces are either amorphous or having

a grain size significantly different from the surface before the treatment. This can certainly cause

significant scattering in the data of cavity measurements.

At Jlab, we used to do hydrogen outgasing at 800 oC for 1 to 3 hours. However, during

SNS project we found that the cavities under such a treatment were significantly softer than they

were before the treatment. This created significant microphonic effects among others and

needed to be remedied. MOM observations95

clearly showed that grain growth and hydrogen

outgassing were responsible for the softening. This conclusion was reached based on the

following two considerations: a) MOM observations showed that the grain size could grow from

31

45 µm to 85 µm after heat treatments at 800 oC for one hour. From any metallurgical text book,

we know that the yield strength of a polycrystalline metal is related to its grain size via the

following relationship:

σy = σi + 1.414σDl1/2

d-(1/2)

(18)

Here σy is the lower yield stress, σi is the shear stress resisting the movement of dislocations

across a particular slip plane, σD is the shear stress to unpin a dislocation, l is the distance from

the piled-up dislocations at the head of the plastic deformation front held up by a grain boundary

to the nearest dislocation source (called Frank-Read source), d is the diameter of the grain.

Therefore the larger is the grain size, the lower is its yield stress. b) From an early study of the

effect of carbon and silicon concentrations on the stress-strain curves of iron through elongation

experiments in 1988, it was found that the elongation curves could be controlled continuously

from a continuous yield to a yield with a yield plateau and the width of the yield plateau various

regularly with the concentrations of carbon and silicon. TEM observations on a sample during

elongation found that carbon and silicon could act as pinning center for dislocations, creating

therefore many short Frank-Read sources. Since σD in equation 18 is inversely proportional to

the length of a Frank-Read source, as a result the yield strength is higher. Similarly in Nb, 800

oC heat treatment is known to outgas H and other impurities. These H and other impurity

interstitial atoms previously could serve as pinning centers for dislocations were now removed

by the outgasing process, leading therefore to a lower yield strength.

Interestingly in some cases, not all the grains were fully grown and grain size could still

vary from the surface to the interior even after the heat treatment as showed typically in Figs.46

and 47. Some areas also showed preferential grain growth along the rolling direction as shown

typically in Fig.48. Another interesting thing to see is shown in Fig.49 where the arrow indicates

a grain with many pits implying that locally the stresses were not completely released by 800 oC

annealing for three hours and there were still many dislocations showing up as the pits on the

surface. Grain growth was found to be much less serious when heat treated at 600 oC

From here we can see that 800 oC heat treatment for 3 hours is too high for hydrogen

outgasing if the stresses stored in the as-received Nb sheets or in the fabricated cavities are not

fully released. This was the base why the 800 oC treatment for 3 hours was changed to 600

oC

32

for 6 hours after SNS project at JLab for hydrogen outgasing. In fact, it is better to outgas H at

650 oC since in this way one can remove unwanted stresses in Nb too while recrystallization can

be largely avoided (see Table 1)

Based on my experience through doing metallographic observations on Nb, I recommend

strongly to use MOM cross-section observation as a quality control tool for both Nb users and

manufacturers as I suggested in Ref.95. One should check whether a Nb sheet is fully annealed

and whether the grain size is uniform from the surface to the interior. This can save a lot of

troubles in cavity production and potentially can save a lot of money wasted in doing cavity

fabrications and treatments and testing.

Dislocations and other defects can also lower RRR leading to lower thermal conductivity

for Nb as pointed out in Ref.96. In my personal view, metallurgical properties of Nb have not

been given the attention they deserved in the SRF community. More work is needed in this area.

For more information about dislocation movements, slip systems, and other issues related to

metallurgical aspects of Nb, please read an excellent recent paper96

by Bieler et al.

4.4: Requirements in Mechanical Properties

This topic is, in fact, strongly coupled with the topic discussed above since grain size,

dislocations, interstitial atoms, and other defects can affect mechanical properties of Nb

enormously just as they do on any metal. Therefore it is also quite naturally to give

specifications on grain size and other mechanical properties of Nb when ordering from Nb

supplier. Table 6 shows an example of such specifications for SNS project.

Mechanical properties of Nb can change significantly with the impurity contents and

therefore RRR for the same reason as discussed in the previous section during hydrogen

outgasing process. A study97

of the variations of mechanical properties with heat treatments at

three different temperatures between 1100 oC and 1250

oC was done by Myneni and Umezawa

on reactor grade Nb. A significant variation in yield strength was found as reproduced in Table

7. The as-received Nb had a yield strength of 110 MPar that was reduced to 56 MPar whereas

RRR increased from 68 to 210. Apart from the effect due to RRR, grain growth after the high

temperature annealing could contribute to the reduction in yield strength as shown in Equation

18.

33

Wu and Myneni measured95

yield stress of Nb from two different suppliers as a function

of the inverse square root of the diameter of grain after heat treatments at 800 oC for 3 hours (one

supplier denoted as SNSw1 & SNSw2 RRR=~400, the other as SNSt RRR=~300) as shown in

Fig.50. From this figure, we may extract the following information: a) generally speaking, yield

strength decreases with increasing grain size, b) regular linear dependence between lower yield

stress and diameter of grain as shown in Equation 18 seems to be followed only when the grain

growth mechanism is the same.

For SNS project, I found that some grains could be huge after heat treatments at 800 oC

for three hours as compared with samples from normal sheets where the average grain size was

ranging from 45 to 51 µm that were a bit below the specifications (see Table 6). A typical

example is shown in Fig.51. Normally exaggerated grain growth is known to occur in one of the

following two ways: a) through a critical strain-anneal treatment, or b) through a process of

secondary recrystallization. A critical strain-anneal treatment is done by making a critical

deformation of a few percent strain to a fully recrystallized sample. Then the sample is subjected

to a high temperature annealing in a thermal gradient along the direction of the deformation.

This situation is hardly applicable to the present situation. We believe that the process of

secondary recrystallization is more likely to occur here, provided that the primary

recrystallization treatment was done at a lower temperature than 800oC. This needs to be

confirmed. For SNSw2 samples, grain growth proceeded steadily for samples under heat

treatments higher than 600oC. However, the difference in grain size between regions close to the

surface and the interior still existed even after the heat treatment at 800oC for three hours (see

Fig.47), but is much less significant than that of the as-prepared sample. We know that the final

grain size of a metal after recrystallization depends on the following factors98

: a) the degree of

prior strain hardening, b) the annealing temperature (above or equal to the temperature that is

required for a complete recrystallization), c) the duration at the annealing temperature, and d) the

rate of heating to reach the annealing temperature. The observation of steady grain growth

seemed to indicate that a substantial amount of strain energy was stored in the as-received SNSw2

sample or the primary recrystallization was done at a temperature lower than 600oC. The later

possibility is highly unlikely since the as-received sample already had some crystals. For SNSt

samples, some grains started to grow and became quite big for all samples under the heat

34

treatment at 800 oC and even for the heat treatment at 600

oC for only three hours. However, they

were not fully recrystallized until heat treatment at 800oC for three hours.

From Fig.50, we can see that the secondary recrystallization tends to change the

dependence dramatically for SNSw1 samples. The two data points of SNSw1 samples fall into the

line for the SNSw1 samples. This may implies that the primary reason responsible for the grain

growth for SNSw2 samples may also be the process of secondary recrystallization. Alternatively,

it can also means that the average grain sizes obtained from these two SNSw1 samples and from

five SNSw2 samples that their data points form a straight line represent their real microstructures

nicely in the same way in terms of their mechanical properties. The one anomalous data point

for SNSw1 samples may be due to the fact that the difference in grain size between the regions

close to the surface and the interior is too much so that an average grain size cannot be obtained

to represent the mechanical properties of the sample. Since there is only one data point for SNSt

sample, it is not meaningful to say anything here except that one should not compare it with data

from SNSw1 and SNSw2. This is because the mechanisms of grain growth can be different

between SNSt and SNSw2/SNSw1. Consequentially, the final state after the heat treatment may

not be the same. In STSt samples, the grain growth started from polygonization, primary

recrystallization, and then grain growth. Besides, the RRR value is also different between STSt

and SNSw2/SNSw1.

A careful mechanical study was also done on Nb with RRR=~300 to study the

relationship between grain size, elongation curve, and annealing temperature as reported in

Ref.99. They found nice variation of the grain size as a function of annealing temperature as

shown in Fig.52. in Fig.53 the corresponding elongation curves are shown. All the samples

were annealed for duration of 4 hours at the different temperatures shown in Fig.53. Similar to

what shown in Fig.50, the yield strength also decreases significantly with the increase in grain

size as shown in Figs 52 and 53.

It is expected that the variation of mechanical properties of Nb at 4.2 K to grain size and

RRR can be different from those at room temperature, since there is a transition of the

workability of Nb from ductile to brittle at ~126 K (see Table 1).

Recently SRF cavities made100,101

from large/single grain Nb have attracted quite some

attentions. This is a very attractive approach since one can skip several steps in Nb production

procedure (see Fig.43) for SRF community. Nb manufacturers can deliver Nb ingots instead of

35

sheets, provided that the ingots are large enough for making SRF cavities. In fact, progress in

Nb production technology in industry makes it possible to make Nb with ingot sizes ranging

from 300 to 500 mm. This is larger enough for making most shapes of high frequency SRF

cavities. For instance, a TESLA shape cavity needs only a disk of 265 mm diameter.

Furthermore, it has been demonstrated100

that an accelerating gradient as high as 45

MV/m is possible for a 2.3 GHz single grain Nb single cell cavity. The mechanical properties of

large/single grain Nb are different from those of regular Nb with grain size of ASTM 4 or 5.

Fig.54 shows typical elongation curves for single grain and polycrystalline RRR Nb, showing

excellent ductility for the Nb single crystal. The stress-strain curve for Nb single crystal even

has a yield plateau similar to what unusually found on fine grain metals. It is rather surprising to

see this since normally the elongation curves of high purity single crystal metals show a

continuous yield since there are not grain boundaries to hinder and not enough pinning sizes to

pin the movements of dislocations generated by the plastic deformation as discussed in Section

4.3.

The effect of RRR and Ta concentration on the mechanical properties of as-received and

high temperature treated Nb single crystals was reported in Ref.101 as reproduced in Fig.55.

Here we can see typical elongation curves for single crystal metals with some impurities serving

as pinning sites for dislocations. It is not clear that witch curve corresponds to which Ta

concentration from Fig.55. I would assume that a higher yield strength come from a higher Ta

concentration.

More controlled and characterized studies of the relationship between mechanical

properties and grain orientations for Nb single crystals can be found in Refs.96 and 102.

Section 5: Summary and Perspective

This chapter gives an overview on the use of Nb for fabricating SRF cavities to be used

in particle accelerators. It also review several examples of significant progresses made in this

field in the past 10 years from my personal viewpoints. SRF technique based on Nb has been

advancing steadily. One example to illustrate this is the Fig.56 where it shows a plot of achieved

accelerating gradient as a function of year103

. As one can see, improved RF performance also

comes with new ideas and innovations. A significant step jump in gradient can be seen when

HPWR and EP were introduced and widely accepted starting in 1995.

36

Efforts have been made mainly in the following two battle grounds: 1) Try to push the

performance limit of Nb. 2) Try to reduce the fabrication cost of Nb SRF cavities. Currently, it

is possible to obtain Nb SRF cavities with performance close to the limit. For instance, an

accelerating gradient of 59 MV/m has been reported104

on a Nb SRF cavity. However, the

production yield for cavities of gradient higher than 35 MV/m with a decent Qo of 1010

is low

and often cavity performance is limited by field emission. An example to see about this is a

statistic summary of the performance of 9 cells cavities given in Fig.57 where one can see that

the data points scatter a lot. One possible way to improve this among others is to improve the

surface smoothness. One major difference between BCP and EP is the surface roughness (see,

for instance, Fig.58) and so100

is between cavities made from large/single grain Nb and from the

regular grain Nb. Therefore, the ability to produce very smooth Nb surfaces routinely appears to

be highly desirable.

New development in Nb quality control and characterization techniques will enable us to

avoid bad starting materials and can help us understand better the art of cavity fabrication and the

mechanism behind each fabrication step. This can lead to a significantly improved cavity

production yield too and therefore is also highly desirable.

Better and more effective ways to remove the chemical residuals and other contaminants

on the surfaces of Nb cavities also deserve great attention. These help us pass by the field

emission barrier and it is one of the necessary steps that you have to go through in order to

achieve routinely the performance limit of Nb SRF cavities.

Reduction of cavity production cost can also be possible if we really understand better the

metallurgical and mechanical properties of Nb. I feel that efforts in this respect are not enough.

Dislocations, interstitial atoms, stacking faults, secondary phases, embedment of foreign

materials, and other defects can be introduced to Nb SRF cavities during cavity fabrication.

They have to be better understood and controlled. For instance, deep drawing in the cavity

fabrication process cause huge plastic deformation and can introduce many edge or screw

dislocations. Although some of them are removed by annealing at high temperatures during

either hydrogen outgasing process or purification process, many of them still stay (see, for

instance, Fig.49). Those defects and imperfections can degrade RF performance seriously.

Again, I would recommend here that MOM cross-section observation should be used as a quality

control tool for both Nb manufacturers and end users.

37

Looking ahead for the use of Nb in SRF application, one has to turn to Nb alloys that

have higher Tc and/or higher Hc such as NbN, NbTiN, and Nb3Sn in order to achieve higher RF

performance limit. One relatively easier way to reach this is perhaps by thin film coating

technique to create the SIS structure as suggested by Gurevich105

or by generating NbN, NbTiN,

and Nb3Sn on Nb or Cu surfaces. Regarding the way to reduce Nb cavity fabrication cost, Nb on

Cu seems to be promising.

*: andywu@jlab.org. Notice: Authored by The Southeastern Universities Research Association,

Inc. under U.S. DOE Contract No. DE-AC05-84ER40150 . The U.S. Government retains a non-

exclusive, paid-up, irrevocable, world-wide license to publish or reproduce this manuscript for

U.S. Government purposes.

38

References:

1. H.A. Schwettman, P.B. Wilson, J.M. Pierce, and W.M. Fairbank, International Advances in

Cryogenic Engineering, Plenum Press, New York, (1965) P88

2. J. Preble et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P17

3. K. Hosoyama et al, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) MO405

4. R.C. York, Proceedings of the 12th

SRF Workshop, Ithaca, USA, (2005) P626

5. http://lcdev.kek.jp/Conf/ICFA2002/brinkmann.pdf

6. J. Mammosser, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P62

7. T. Junquera, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P67

8. H. Weise, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P7

9. R.E. Laxdal et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P81

10. http://www.linearcollider.org/

11. http://minerals.usgs.gov/minerals/pubs/commodity/niobium/mcs-2009-niobi.pdf

12. J. Bardeen, L.N. Cooper, and J.R. Shrieffer, Phys. Rev., 108 (1957) P1175

13. K. Schulze, Proceedings of the international Symposium on Niobium, The Metallurgical

Society of AIME, (1981) P163

14. A. Gladun et al, Journal of Low Temperature Physics, 27 (1977) P873

15. A.M. Valente-Feliciano, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) Tu6B

16. The Fourth International Workshop on Thin Films and New Ideas for Pushing the Limits of RF

Superconductivity, Padua, Italy, (2010)

17. W. Singer et al, Proceedings of the 10th

SRF Workshop, Tsukuba, Japan, (2001) P170

18. V. Palmieri, Proceedings of the 11th

SRF Workshop, Travemunde, Germany (2003) TuP26

19. A.T. Wu, in the book Neural Computation and Particle Accelerators: Research, Technology, and

Application, Nova Science Publishers Inc., New York, USA, (2010) Chapter 9

20. X.Y. Lu, Seminar at JLab, (2010)

21. W. Singer, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) Tutorial 4A2

22. W. Singer et al, Proceedings of the 8th

SRF Workshop, Padova, Italy, (1998) P850

23. W. Singer et al, Proceedings of the 12th

SRF Workshop, Ithaca, USA, (2005) TuP49

24. A. Farr et al, Proceedings of the 11th

SRF Workshop, Travemunde, Germany (2003) ThP40

25. Y. Iwashita et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P297

39

26. Y. Iwashita et al, Phys. Rev. ST Accel. Beams 11, 093501 (2008)

27. W.D. Moller, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P109

28. S. Berry et al, Proceedings of the 11th

SRF Workshop, Travemunde, Germany, (2003) ThP04

29. S. Berry et al, Proceedings of EPAC, Lucerne, Switzerland, (2004) P1000

30. M. Ge et al, , Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P384

31. G. Wu, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P491

32. A.T. Wu, http://indico.fnal.gov/conferenceDisplay.py?confId=3118 and seminar at JLab (2010).

33. K. Saito et al, Proceedings of the 4th

SRF Workshop, Tsukuba, Japan, (1989) P635

34. A.T. Wu et al, Applied Surface Science, 253 (2007) P3041

35. V. Palmieri et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P463

36. M. Raskovic et al, J. Vac. Sci. Technol. A, 27 (2009) P301

37. A. Lozano-Morales et al, http://www.nasf.org/staticcontent/A.MoralesNobium.pdf

38. J. Delayen et al, Proceedings of the 10th

SRF Workshop, Tsukuba, Japan, (2001) P499

39. S. Jin et al, Phys. Rev. ST Accel. Beams 13, 061001 (2010)

40. S. Jin et al, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) WEP86

41. E. Wang et al, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) WEP65

42. F. Eozonou et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P772

43. A.T. Wu et al, Proceedings of PAC, Vancouver, Canada, (2009) WE5PFP060

44. X.Y. Lu, Seminar at JLab, (2009)

45. J. Beveridge and A.T. Wu, Report of SURI project, JLab, (2010)

46. F. Eozonou et al, Phys. Rev. ST Accel. Beams 13, 083501 (2010)

47. A.T. Wu, Proceedings of the 11th

SRF Workshop, Travemunde, Germany (2003) ThP13

48. A.T. Wu et al, Proceedings of the 1st IPAC, Kyoto, Japan (2010) WEPEC080

49. A.T. Wu et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P755

50. M. Ceccato et al, http://conferences.jlab.org/tfsrf/Thursday/Th2_3-V_Palmieri-TFSRF.pdf

51. L. Vuskovic et al,

http://www.hep.uiuc.edu/LCRD/LCRD_UCLC_proposal_FY06/2_52_Vuskovic_ProgressReport

.pdf

52. E.J. Taylor, Private communication, (2009)

53. A.T. Wu et al, in the book Neural Computation and Particle Accelerators: Research, Technology,

and Application, Nova Science Publishers Inc., New York, USA, (2010) Chapter 4

40

54. A. Brinkmann et al, Proceedings of EPAC 2006, Edinburgh, Scotland, (2006) P418

55. D.R. Swenson et al, Nuclear Instruments and Methods in Physics Research, Section B: Beam

Interactions with Materials and Atoms, Vol.241 (2005) P641

56. D.R. Swenson et al, Nuclear Instruments and Methods in Physics Research, Section B: Beam

Interactions with Materials and Atoms, Vol.261 (2007) P630

57. A.T. Wu and D.R. Swenson, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009)

P760

58. Z. Insepov et al, Vacuum, 82 (2008) P872

59. A.T. Wu et al, Phys. Rev. ST Accel. Beams 13, 093504 (2010)

60. A.T. Wu et al, Proceedings of the 1st IPAC, Kyoto, Japan (2010) WEPEC081

61. D.R. Swenson et al, AIP Conference Proceedings, 877 (2006) P370

62. D. Proch et al, Proceedings of the 10th

SRF Workshop, Tsukuba, Japan, (2001) P463

63. D. Reschke et al, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) TUP48.

64. A. Dangwal et al, Journal of Applied Physics, 102, 044903 (2007)

65. https://srf.jlab.org/SurfaceScienceLab.htm

66. A. Dacca et al, Particle Accelerators, 60 (1989) P103

67. Q. Ma and R.A. Rosenberg, Applied Surface Science, 206 (2003) P209

68. Hui Tian et al, Applied Surface Science, 253 (2006) P1236

69. Q. Ma et al, Journal of Applied Physics, 96 (2004) P7576

70. Hui Tian et al, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) TUP18

71. Q. Ma and R.A. Rosenberg, Proceedings of PAC, Chicago, USA (2001) P1050

72. Q. Ma and R.A. Rosenberg, Proceedings of the 10th

SRF Workshop, Tsukuba, Japan, (2001)

P368

73. K. Kowalski et al, Proceedings of the 11th

SRF Workshop, Travemunde, Germany (2003)

ThP09

74. A. Chincarini et al, Proceedings of the 10th

SRF Workshop, Tsukuba, Japan, (2001) P382

75. A.T. Wu et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P300

76. B. Visentin, Proceedings of the 11th

SRF Workshop, Travemunde, Germany (2003) P199

(TUO01)

77. G. Ciovati et al, Phys. Rev. ST Accel. Beams 13, 022002 (2010)

41

78. P. Bauer et al, Review of Q-slope in SRF Cavities, Fermi Lab Report, TD-05-56, (2006). Can be

viewed from Google Search.

79. A. Romanenko, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P95

80. B. Visentin et al, Proceedings of the 9th

SRF Workshop, Santa Fe, USA, (1999) P198

81. B. Visentin et al, Proceedings of the 12th

SRF Workshop, Ithaca, USA, (2005) TuP05

82. H. safa, Proceedings of the 10th

SRF Workshop, Tsukuba, Japan, (2001) P279

83. J. Halbritter, Proceedings of the 11th

SRF Workshop, Travemunde, Germany (2003) MOP44

84. G. Ciovati, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) TU102

85. A. Romanenko, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) TU103

86. J. Knobloch et al, Proceedings of the 9th

SRF Workshop, Santa Fe, USA, (1999) P77

87. K. Saito, Proceedings of the 11th

SRF Workshop, Travemunde, Germany (2003) THP15

88. B. Bonin and R. Roth, Proceedings of the 5th

SRF Workshop, Hamburg, Germany (1991) P210

89. H. Padamsee et al, RF Superconductivity for Accelerators, John Wiley & Sons, (1998) P209

90. F. Koechlin et al, Superconductors Science and Technology 9 (1996) P453

91. Reference 89, P206

92. T. Carneiro et al, Proceedings of the 10th

SRF Workshop, Tsukuba, Japan, (2001) P417

93. K.K. Schulze, Journal of Metals, 33 (1981) P33

94. P. Kneisel et al, Proceedings of PAC, Knoxville, USA (2005) P3955

95. A.T. Wu and G. Myneni, JLab-TN-02-027 (2002)

96. T.R. Bieler et al, Phys. Rev. ST Accel. Beams 13, 031002 (2010)

97. G. Myneni and H. Umezawa, Meteriaux and Techniques, 7-8-9 (2003) P19

98. A.M. Shrager, Elementary metallurgy and metallography, Dover Publications, Inc., New York,

(1969) P22

99. W. Singer, SRF materials workshop, Fermi Lab, (2007)

100. P. Kneisel et al, Proceedings of the 12th

SRF Workshop, Ithaca, USA, (2005) MOP09

101. G. Myneni, Proceedings of the 12th

SRF Workshop, Ithaca, USA, (2005) MOP08

102. D. Baars et al, Proceedings of the 14th

SRF Conference, Berlin, Germany, (2009) P144

103. K. Saito, Proceedings of the 13th

SRF Workshop, Beijing, China, (2007) TU202

104. R.L. Geng et al, Proceedings of PAC2007, Albuquerque, USA (2007) P2337

105. A. Gurevish, Applied Physics Letters, 88 012511 (2006)

42

Figure Captions

Fig.1: Photo of a typical Nb SRF cavity and its schematic side view.

Fig.2: A typical measured surface resistance as a function of temperature for a 9-cell Nb SRF

cavity.

Fig.3: Photo of a spoke cavity.

Fig.4: Flow chart for a typical Nb SRF cavity fabrication steps.

Fig.5: Schematic illustrated the working principle of hydroforming20

.

Fig.6: First double cell seamless Nb SRF cavity fabricated from hydroforming17

.

Fig.7: Excitation curves measured on a single cell seamless cavity after buffered chemical

polishing (bcp) and electropolishing (e-pol)17

.

Fig.8: Photos of various steps during the fabrication of a n1.5 GHz copper cavity by spinning.

Seamless Nb SRF cavities are fabricated in the same way18

.

Fig.9: Typical excitation curves for a single cell seamless Nb SRF cavity measured at 1.6 K.

Before the measurements, this cavity was mechanically grinded for 100 µm, then barrel polished

for 84 hours, vacuum annealed at 750 oC for 3 hours followed by electropolishing for 50 µm and

then high pressure water rinsed18

.

Fig.10: Photo of the first 9 cell Nb SRF cavity fabricated by spinning18

.

Fig.11: Shaped Al cathode fabricated at JLab a) top view, b) side view.

Fig.12: Nb dumbbells a) before BEP treatment, b) after BEP treatment.

Fig.13: Schematic illustration of the working principle of an eddy current scanning device for

SRF applications21

.

Fig.14: Schematic illustration of the working principle of a SQUID scanning device for SRF

applications24

.

Fig.15: Schematic diagram of Kyoto Camera a) and its main components b). The inset shown in

the upper right corner of b) shows the mirror and the pulse motor that drives it. The camera

moves for focal adjustment. The electro-luminescence sheets for illumination are shown in the

lower part of b). (all from Ref. 26)

Fig.16: Overview of Kyoto Camera26

.

43

Fig.17: Example of a quantitative measurement on the defect structure observed at one of the

equator region of a 9 cell Nb SRF cavity. a) shows the photo of the defect taken by Kyoto

Camera. b) shows the profile of the defect after the calculations based on the movement of the

bright point across the defect activated by controlled illumination26

.

Fig.18: A typical example28

of the inner surface of a Nb SRF cavity at the quench site replicated

by the technique developed by S. Berry et al. Top is 3D view. Bottom is corresponding contour

plot.

Fig.19: A typical example of the inner surface of a Nb SRF cavity near the welding zone

replicated by the JLab technique32

. The replica was coated by palladium for 5 nm. The lower

image was taken by an optical microscope on the replica from the heat affected zone. The upper

left is taken on the heat unaffected zone. The upper right is an optical image taken on the surface

of Nb for comparison.

Fig.20: Typical high resolution 3D profilometer scans on the surfaces of: (a) buffered

electropolished Nb, (b) electropolished Nb, and (c) buffered chemical polished Nb. The scans are

plotted with the same parameters for comparison34

.

Fig.21: Nb half cell a) before BEP treatment, b) after BEP treatment for 1800 seconds34

.

Fig.22: Polishing currents42

obtained at 5V for different rotation speeds at 30 oC.

Fig.23: Modern vertical BEP system at JLab. The small photo shows the old vertical BEP

system47

used at JLab in 2002.

Fig.24: Typical excitation curves49

for single cell Nb SRF cavity (regular grain size) of CEBAF

shape treated by BEP.

Fig.25: Typical microscopic photos of Nb a) before polishing, b) after polishing by an ionic

liquid50

(see the text for more details) at 190 oC.

Fig.26: Schematic illustration of a microwave glow discharge system for Nb sample exposure36

.

Fig.27: Photo (right) and schematic drawing of a low flow channel cell used to electropolish

25.4 mm x 25.4 mm x 3 mm Nb coupons via Faradayic Electropolishing37

.

Fig.28: Typical microscopic photo of the surface of a Nb coupon treated by Faradayic

Electropolishing in a H2SO4 electrolyte37

.

Fig.29: Schematic of working principal of GCIB.

44

Fig.30: Typical results of measurements on GCIB treated Nb flat samples done by a SFEM. a) A

Nb sample that was treated by O2 GCIB on one half while the other half was not. b) A sample

that was treated by NF3+O2 on one half while the other half was not. Both Nb samples were

polished by BCP prior to GCIB the treatments.

Fig.31: Mesoscale modeling of a Nb surface irradiated by O2 GCIB treatments where it shows

that the features shown in a) are removed by the treatments as shown in b).

Fig.32: Measured etch rates of Nb treated by NF3+O2, Ar, and O2 GCIB as a function of

acceleration voltage61

.

Fig.33: Photo of the nozzle and CO2/N2 jet during a DIC cleaning of a Nb SRF cavity54

.

Fig.34: The DIC system at DESY: a) Photo of the setup54

. b) Schematic diagram of the setup63

.

Fig.35: Filed emission maps of polycrystalline Nb measured by a SFEM over the same 7.5x7.5

mm2 before (left) and after (right) DIC up to a DC field gradient of 120 MV/m.

Fig.36: Typical oxygen depth profiles from SIMS measured on a BCP treated sample (reference

sample), BCP-treated + baked in air at 120 oC for 12 hours (baking 1), and BCP-treated + baked

in air at 120 oC for 48 hours (baking 2)

Fig.37: Typical deconvolution components of Nb 3d spectrum from XPS measurements68

.

Fig.38: Atomically resolved TEM cross-section image75

of an BCP treated Nb sample obtained

using JEM-2100F.

Fig.39: Schematic of the oxide layer structure on Nb surfaces.

Fig.40: Schematic of the high field Q-slope (see the text for more details).

Fig.41: Schematic illustration of an oxygen diffusion model78

.

Fig.42: Lattice misorientation maps85

for a hot spot (right) and a normal area (left). Please see

the text for more details.

Fig.43: Flow chart of Nb production process (taken from internet).

45

Fig.44: Performance94

of Nb SRF single cell cavity made from high Ta content Nb (1300

wtppm). Shown is Q0 vs. Eacc before heat treatment, after heat treatment and “in situ” baking at

120 oC for 48 hours.

Fig.45: Typical MOM cross-section photos of as-received Nb, a) showing that the sample was

not crystallized, b) showing a variation in microstructure from the surface to the interior,

implying that both samples were not fully annealed after rolling.

Fig.46: An amorphous area was still seen from MOM on one sample cut from a Nb sheet treated

at 800 oC for three hours.

Fig.47: Variation of grain size from surface to the interior observed on one sample cut from a

Nb sheet treated at 800 oC for three hours. a) On the surface. b) On the interior adjacent to the

surface shown in a). The arrow indicates the area with a smaller grain size as compared with that

on the surface.

Fig.48: MOM photo on a sample cut from a Nb sheet that was treated at 800 oC for three hours.

The arrow indicates that locally the stresses in this region were not fully released. There were

still many dislocations shown as pits on the surface.

Fig.49: Typical MOM photo of preferential crystal growth along the rolling direction after heat

treatment at 800 oC for three hours.

Fig.50: Measured yield stress plotted as a function of inverse square root of diameter of grain

size form samples cut from Nb sheets supplied by two Nb manufacturers. (see text for more

details)

Fig.51: Typical example of huge grains observed by MOM on a SNSw1 sample after annealing

at 800 oC for three hours (see text for more details)

Fig.52: Grain size measured99

as a function of annealing temperatures for Nb RRR ~300.

Fig.53: Stress-strain curves99

for the Nb RRR~300 treated at various temperatures (see the

figures) for 4 hours.

Fig.54: Typical elongation curves100

measured on polycrystalline and single crystal Nb.

46

Fig.55: Elongation curves for as-received and high temperature treated Nb single crystals of

different RRR and Ta concentration (see the figure).

Fig.56: History103

of high accelerating gradient of Nb SRF cavities.

Fig.57: Statistics of the performance of 9-cell Nb SRF cavities at DESY.

Fig.58: Typical BCP and EP treated Nb surfaces as seen from a SEM.

47

Figures

Fig.1

48

Fig.2

49

Fig.3

50

Fig.4

51

Fig.5

52

Fig.6

53

Fig.7

54

Fig.8

Fig.9

55

Fig.10

Fig.11

56

Fig.12

57

Fig.13

58

Fig.14

59

Fig.15

60

Fig.16

61

Fig.17

62

Fig.18

63

Fig.19

64

Fig.20

65

Fig.21

66

Fig.22

I=f(t) for Different Rotation Speeds BEP 4-5-11 Mixture.

5V Vs AgAgCl. 30°C

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 100 200 300 400 500 600

Time (s)

I (A

)

200 rpm

400 rpm

800

rpm

1200 rpm

2000 rpm

3000 rpm4000 rpm 4994 rpm

0 rpm

0 rpm

67

Fig.23

68

Fig.24

69

Fig.25

70

Fig.26

Fig.27

71

Fig.28

72

Fig.29

73

Fig.30

74

Fig.31

75

Fig.32

76

Fig.33

77

Fig.34

78

Fig.35

79

Fig.36

80

Fig.37

81

Fig.38

82

Fig.39

83

Fig.40

84

Fig.41

85

Fig.42

86

Fig.43

87

Fig.44

88

Fig.45

89

Fig.46

90

Fig.47

91

Fig.48

92

Fig.49

93

Fig.50

94

Fig.51

95

Fig.52

96

Fig.53

97

Fig.54

98

Fig.55

99

Fig.56

100

Fig.57

101

Fig.58

102

Table 1: Some useful information and constants of Nb

Nb, Niobium

(Columbium)

Atomic

Number:

41

Atomic

Weight:

92.9064

g.mol-1

Color at room

temperature:

steel grey

Crystal

structure

: body-

centered

cubic

(bcc)

Element

category:

transition

metal

Superconducting

transition

temperature:

9.25

Electron

configuration:

[Kr]4d45S

1

Electrons

per shell:

2,8,18,12,1

Phase:

solid

Density: 8.57

g/cm3

First

ionizatio

n energy:

6.88 eV

Lattice

constant

at 20 oC:

3.294Å

Electronegativity

: 1.6

Covalent

radius:

1.64±6Å

Melting

Temperature

: 2468 oC

Boiling

Temperature:

4927 oC

Poisson’s

ratio: 0.38

Specific heat at

15oC: 0.268

(J/g); at 1227oC:

0.320

Workability:

ductile to brittle

transition: -150oC

Recrystallization

temperature:

reactor grade 900-

1300 oC; RRR

grade 750-850 oC

Stress relieving

temperature:

reactor grade

800oC; RRR

grade 649-663 oC

Hardness

(VHN): 60-100

Atomic volume:

10.8 (CC/mol)

Refractive Index:

899-1204

Heat of melting:

26.4 kJ/mol

Modulus of

elasticity: 10600

kg/mm2

Coefficient of thermal

expansion: oC

-1:

@500oC 7.47;

@900oC 77.94

Thermal

conductivity J/(m-

sec-deg): @ 0oC

52.3; room

temperature 53.7;

@302oC 53.6; @

799oC 57.1;

@1600oC 69.1

Tensile strength,

typical: MPa

@20oC 172;

@799oC 103;

@1199oC 34

Thermionic

work

function:

4.01eV

Yield strength,

typical MPa:

@20oC 103;

@799oC 69;

@1199oC 14

Young’s

Modulus GPa:

@20oC 98.5;

@799oC 82.7;

@1199oC 14;

@1788oC 51.7

Dielectric constant

of surface oxide:

41

Oxidation

states:

5,4,3,2,-1

Atomic

radius:

1.46Å

Magnetic

ordering:

paramagnetic

Electrical

conductivity

typical @0oC 152

nΩ.m

Shear

Modulus:

38GPa

Bulk

modulus:

170GPa

Debye

Temperature:

250K

Heat of

vaporization: 694

kJ/mol

Magnetic susceptibility: @

room temperature +2.20x10-6

gauss.cm2/g

Bulk Nb is type II

superconductor

Lower critical

magnetic field:

0.17 tesla

Thermodynamic

critical magnetic

field: 0.2 tesla

Higher critical

magnetic field: 0.24

tesla

Superheating field: 0.23 tesla Mole entropy: 35 J/mol.K

103

Table 2: Popular Instruments Used in the Studies of the Physical Properties of Nb for SRF

Applications: Properties They Probe and Their Characteristics

Instrument SEM TEM SIMS AFM

Properties Detected

imaging

imaging, atomic structure, defects, dislocations,

interstitial atoms,

secondary phases

composition, depth

profile, elemental mapping

imaging, surface

topography and morphology

Characteristics lateral resolution of 1-5

nm lateral resolution down to

0.05 nm

~ppm down to ppb, can

be very surface sensitive

(static SIMS) 5% of the surface top layer, can

measure all elements

including H

lateral resolution down to 1nm, limited scan size

~50 µm2 at room

temperature and smaller at low temperatures

Instrument STM EDX XPS Profilometer

Properties Detected

Imaging, surface topography and

morphology and electron

density of state

elemental analysis, composition, elemental

mapping

composition, electronic structure or chemical

state, depth profile,

elemental mapping

imagine, surface

topography and morphology

Characteristics

lateral resolution down to

0.1 nm, limited scan size

~50 µm2 at room temperature and smaller

at low temperatures, only

works on metallic or semiconducting surfaces

not surface sensitive,

typical probing depth ~

µm, cannot detect elements with atomic

number smaller than Be,

typical sensitivity to impurity ~ 1% (atm)

very surface sensitive,

typical probing depth

<10nm, typical sensitivity to impurity 200ppm

(atm), cannot detect H

lateral resolution down to

5 nm, extremely larger

scan size up to 15x15 cm2 or larger

Instrument MOM SFEM SAM EBSD

Properties Detected

imaging detecting field emitters and imaging, mapping

composition, element,

imaging, depth profile, elemental mapping

crystal orientation,

texture, defects, phase

identification, morphology, orientation

mapping

Characteristics

relatively shallow depth of field (compared with

SEM), lateral resolution >

200 nm

gradient up to 250 MV/m

with lateral resolution ~2.5 µm

very surface sensitive, typical probing depth

<10nm, typical sensitivity

to impurity 0.1% (atm), cannot detect elements

with atomic number

smaller than Be

surface sensitive, probing

depth 10 nm-50 nm,

lateral resolution for mapping can be down to

10 nm

104

Table 3: Requirements for the impurity contain of Nb ingots

Element Ta W Ti C O N H Fe Max Parts per Million weight

(ppm) 1000 100 40 30 40 30 10 50

Element Si Mo Max Parts per Million weight

(ppm) 50 50

Table 4: Requirements for dissolved interstitial elements of Nb sheets

Element O N C H Max Parts per Million

weight (ppm) 40 30 30 10

Table 5: Relative influence of different impurities on RRR of niobium (from Ref.93)

Element N C O H Ti Hf Zr W Mo Ta

Percentage 100 83 87 15 27 27 12 to 27 8 to 21 4 5

Table 6: Specifications on mechanical properties of Nb for SNS project

Yield Strength

(0.2% offset)

Tensile Strength

Elongation (1 inch gauge

length)

Hardness, HV 10

Recrystallization Grain Size (d)

7000 psi (48.2

N/mm2) minimum

14000 psi (96.4

N/mm2) minimum

40% minimum

longitudinal, 35%

minimum transverse

50 maximum

90% ASTM4

(90µm)>d>ASTM5(64µm)