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High Energy Density Physics xx (2007) 1e6www.elsevier.com/locate/hedp

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Effect of higher z dopants on implosion dynamics:X-ray spectroscopy

George A. Kyrala a,*, Douglas C. Wilson a, John F. Benage a, Mark Gunderson a,Ken Klare a, Johan Frenje b, Richard Petrasso b, Warren Garbett c, Steven James c,

Vladimir Glebov d, Barukh Yaakobi d

a Los Alamos National Laboratory, Los Alamos, NM 87545, USAb M.I.T., Boston, MA, USAc AWE, Aldermaston, UK

d Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623-1299, USA

Abstract

Low-z dopants such as argon are used to characterize the plasma properties in imploding inertial confinement fusion (ICF) capsules. Higher zdopants will be used as the temperature of the capsules is increased especially as we approach ignition on the National Ignition Facility (NIF).The presence of the higher z dopants also affects the dynamics of the implosion as it increases the electron density, which increases the radiationlosses from the plasma allowing the plasma to compress to a smaller volume. In the present study, we examine the effects at higher dopantdensities. While a normal glass capsule loses energy to radiation during compression, the high-z shells will confine the radiation even as equi-librium burn is approached. We have fielded thin 1-mm diameter glass shells filled with varying amounts of xenon and krypton gas to study theprogression from non-equilibrium to equilibrium burn as the dopant gas concentration is increased. The shells used for these experiments alsocontained 3He to measure the proton spectrum from the D3He reaction providing information about the target temperature and the density-radiusproduct, rR. Here we present results using the X-ray measurements, as well as discuss some of the issues and the progress we made.� 2007 Elsevier B.V. All rights reserved.

Keywords: Inertial fusion; Implosions; X-ray spectroscopy

1. Introduction

The burn of DT fuel within a high-z shell was one of theearliest approaches to ignition [1] because a high-z pusherled to minimum energy requirements for volume ignition,where the whole volume ignites, to be distinguished from cur-rent designs where a small volume at the center of the capsuleignites and propagates outward. Wilson et al. [2] designeda high-z single shell capsule with pulse shaping that was cal-culated to ignite at the same low implosion velocity. However,if material mixing occurs due to hydrodynamic instabilities inthese capsules, the yield would also be affected. The mix

* Corresponding author.

E-mail address: gak@lanl.gov (G.A. Kyrala).

1574-1818/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.hedp.2007.02.018

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(2007), doi:10.1016/j.hedp.2007.02.018

reduces the yield and the radiation from the high-z dopantfrom the shell becomes a good diagnostic for amount and spa-tial distribution of the mix.

For a fusion capsule, high-z materials are used to diagnosethe electron temperatures of the plasma from the measure-ments of the spectral line shapes and the relative intensitiesamong the spectral lines. However, when a high-z material isadded to an imploded capsule, the reaction rate is affected.The yield, temperature and target column density, rR, mayalso be changed, especially for high dopant density. The pres-ence of the high-z dopants increases the radiation losses fromthe plasma, allowing the plasma to compress to a smaller vol-ume. Furthermore, we expect that as we add dopant to theplasma the electron and the ion temperatures are lowered,but get closer together.

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In the present set of experiments we attempted to vary theamount of dopant to study the plasma conditions; such as theprogression from non-equilibrium between the electron andion temperatures, to equilibrium burn, as a function of the dop-ant gas concentration. However, the maximum dopant densi-ties are limited due to the optical thickness for the emissionfrom the He-like resonance lines and H-like Lyman a linesof the ionized Krypton at the highest pressures we used. Inthis experiment, we used the emission from capsules witha lower density of Kr ions as a spectroscopic diagnostics ofthe electron temperature. This estimate was derived fromboth the slope of the continuum emission, and the ratio ofthe amplitudes within the of He-b (1s2-1s3p) line complexwhich was optically thin under our conditions. Xe gas fillpressure was used to vary the electron and ion temperaturesin the imploding plasma. In addition, we introduced 3He gasinto the target as a measure of target ion temperature byanalyzing the recorded secondary proton spectrum. We alsomeasured the target rR distribution using neutron diagnostictechniques developed at MIT [3,4].

2. Experiment

High-z capsules are difficult to fabricate and which limitedtheir use, hence in the present experiment we used SiO2 shellsfilled with a mixture of gases, including high-z gases, to over-come such difficulties. The shells were made by the siliconglow discharge polymer (Si-GDP) -to-glass technique devel-oped at General Atomics [5]. In this process, Poly AlphaMethyl Styrene (PAMS) shells were coated with silicon-dopedGDP and then the PAMS mandrel was pyrolyzed away. Theresulting Si-GDP shells were then converted in a series ofpyrolysis steps to SiO2 glass in an open-air oven such thatthe glass wall was not at full density. To densify the shellsand entrap the desired krypton and/or xenon gases in the shell,they were pyrolyzed in their controlled atmosphere oven withthe desired gases present at a quantity calculated to result inthe appropriate gas pressure at room temperature.

Each target had an outside diameter of 925� 15 mm anda thickness that varied from 4 to 6 mm. The targets were filledwith 6.7 atm of deuterium and 3.3 atm of 3He. In addition,some of the targets were filled with various fills of Kr up to0.7 atm, and Xe up to 0.7 atm, thus allowing the dopantmass fraction to vary up to 75% of the total mass of the cap-sule fill.

The targets were illuminated with 60 laser beams at theOmega system at the Laboratory for Laser Energetics [6].The lasers were pointed and focused at the center of the cap-sules. The laser pulse shape had a 1-ns flat top pulse. Each la-ser beam used the maximum available 2D smoothing byspectral dispersion to smooth any hot spots in the beam, usingthe maximum available bandwidth 1.05 THz, which is 1.38 Ain the X direction, 10.80 A in the Y direction, w4.0� 0.15 AFWHM at a central wavelength of 3510.1 A. Furthermore,each beam used a distributed polarization rotation and a phaseplate for further smoothing. The SG-4 phase plates produceda super-Gaussian beam irradiance spot with a distribution:

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IðrÞ ¼ I0e�ðr=wÞn

where w¼ 380 mm is the 1/e radius, and n¼ 3.70 is the super-Gaussian exponent. The sum of the energy in all the beamswas w23 kJ.

Several diagnostics were used to observe the implosion.Neutron time of flight diagnostics were used to diagnose theyield and ion temperatures of the plasma, while proton yieldfrom the D3He reaction was used to measure the total capsulerR and ion temperatures. Filtered and time gated X-ray imagesprovided the distribution of the emitted radiation above 9 keV,while streaked and non-streaked spectrometers produced thespectrum of the X-rays integrated over the emission volumeof the capsule. We will describe the X-ray spectra measure-ments and its analysis.

2.1. Time integrated X-ray measurements

A time integrated flat-crystal X-ray spectrometer observedthe implosion [7]. The spectrometer used a LIF(200) crystalto observe the spectrum in the X-ray region 12e22 keV onDEF film. Since we expected the implosion to generate highplasma temperatures, the Kr gas was expected to be ionizedto the He-like Kr34þ and/or the H-like Kr35þ charge states,thus allowing us to use the He-a lines (1s2-1s2p) spectroscopyto determine an electron temperature, Te, in the range fewkiloelectron volt [8]. Furthermore, the continuum emissioncomes from the freeefree and freeebound emissions in theplasma, which depend exponentially on the electron tempera-ture, kTe, and photon energy, hy. The spectral emission energydensity of the continuum, in units of energy per energy band-width, is d(I(y,Te))/d(hy) w exp(�hy/kTe). Fitting the expo-nential to the measured emitted energy spectrum yields ameasure of Te. An example of such a fit is shown in Fig. 1.In this example, we fit the data over the range 17e26 keV,however, the presence of a feature at 20 keV has a small effecton the fit. The accuracy of the measurement is estimated to be0.5 keV, due largely from the error in the calibration of thefilm, and the noise in the data.

The effect of the xenon dopants is shown in Fig. 2. Here,we do not show the absolute values of the spectral yield butthe raw recorded film optical density, in order illustrate thepoint. When dopants were absent, we did not observe any ofthe Kr H- and He-like lines, and we found the lowest outputradiant energy. When a small amount (0.012 atm) of kryptonwas added, we observed the He-like Krypton lines clearly,and the He-a line was almost saturated on the film. The He-b lines and their satellites are clearly observed in the spectrumand the continuum emission increased slightly. When we fur-ther added a small amount of xenon (0.010 atm), the totalemission increased over the full spectrum, caused by the sig-nificant increase in the number and density of free electrons.However, the He-b lines of Kr emitted less, indicative of lowerTe, confirmed by the disappearance of the He-g and higherlines. The He-a line seemed to increase, but the increase couldbe due to the underlying continuum contribution. When

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a significant amount of Xe was added (0.150 atm), the He-likeemission from Kr were extinguished and the total emission de-creased as well, indicative of even lower temperature, electrondensity and/or ionization of the dopants.

Fig. 1. The measured time integrated X-ray emission spectrum for a target

with 6.9 atm D2, 3.3 atm 3He, 0.048 atm Krypton and no Xenon. The mea-

sured electron temperature was 4.74� 0.3 keV over the range 17e26 keV,

where most of the uncertainty comes from the calibration of the film, the sub-

traction of the background, and the existence of some un-identified features at

20 keV.

500

1000

1500

2000

2500

3000

12.5 13 13.5 14 14.5 15 15.5 16

44471- 0.00 Kr 0.00 Xe44470- 0.012 Kr 0.01 Xe44467- 0.012 Kr 0.00 Xe44472- 0.014 Kr 0.150 Xe

PD

S C

ou

nts

Energy (keV)

Fig. 2. The optical density on DEF film caused by the X-ray emission spec-

trum from different capsules [every 800 PDS units is equivalent to one OD].

The open circles are for no dopants, open squares are for 0.012 atm Kr dopant,

the black circles are for an added 0.01 atm of Xe to the 0.01 atm of Kr dopant,

while the filled triangles are for an added 0.15 atm of Xe to the 0.014 atm of

Kr dopant densities.

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3. Time resolved spectra

Time resolved spectra of the implosion were recorded withstreaked X-ray spectrometers. One spectrometer used a rela-tively slow speed to observe the beginning of the laser pulseas well as the spectrum at peak implosion time. This enabledus to time the implosion relative to the laser, while the secondstreak camera ran at a higher speed to resolve any temporalstructure in the X-ray spectrum. The streak camera spectrom-eters used a grazing angle PET crystal to measure the He- andH-like Kr spectrum between 13.11 and 15.7 keV. A typical setof streak records are shown in Fig. 3 for two cases, wheresome Xe was absent and when 0.01 atm of Xe was added toa 0.012 atm of Kr. The spectrum and the total emissionchanged and a lineout through both spectra is shown inFig. 4. The lineout shows that the addition of Xe reducedthe He-b line, and almost eliminated the He-g line. The back-ground below the He-a line also increased in agreement withthe time integrated spectra shown in Fig. 2. Also, notice thatthe He-like lines started emitting about 50 ps before thepeak in the continuum, but ended about the same time.

If we examine the satellite to the He-b line, we can usethe ratio of the satellite emission to the resonant line todetermine a Te. However, we had to be sure that the ratiodid not depend significantly on the electron density of theimplosion. To understand the results, spectral post processedcalculations were performed using three types of input. Pop-ulations, oscillator strengths etc. were generated for the Krmodel using the collisional radiative equilibrium transportatomic physics code CRETIN [9]. Electron temperature,electron density and number of Kr atoms in each zonewere generated from hydrodynamic calculations [10]. Starkbroadened line shapes, including electron and ion broaden-ing, ion dynamic shifts and line shifts, were created usingthe multi-electron radiator line shape code MERL [11,12].Using all these inputs a simulated time-resolved spectral out-put was generated.

As was expected, we found that the ratio of the He-b to then¼ 2 Li-like satellites, using the CRETIN populations, wasstrongly dependent on Te but weakly dependent on the electrondensity, as can be seen in Fig. 5. When we examined the twoshots, one without Xe and one with 0.01 atm of Xe, we foundthat the ratio changes from 1.84 to 1.32, indicating that the Te

cooled from 3.0� 0.05 keV to 2.6� 0.05 keV [see Fig. 6 forthe lineouts], whereas the continuum slope in the time inte-grated data gave Te of 3.67 and 2.73 keV, respectively.

The lineouts also show an interesting He-b line feature thathad three peaks, indicative of high electron densities. The line-width should be an indicator of electron density. When wecalculated the expected line shapes, see Fig. 7, at electron den-sities between 1 and 8� 1024 cm�3, the line shape changed asexpected from a two-component line to a three-componentline, and its width increased with electron density. The relativeline shape gave an indication that the electron density was be-tween 5 and 8� 1024 cm�3. But, the increase in the calculatedline width was only 75% of the experimental width at thoseexpected electron densities.

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20 25 30 35 40Position (mm)

0

10

20

30P

ositio

n (P

ixel)

44467SSCA

0.00

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re

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sitio

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m)

44470SSCA

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1.0

Ex

po

su

re

Fig. 3. The raw streak camera recorded spectra showing the effect of adding Xenon to the Kr doped capsule. On the left, (a), shot 44,467 which only had 0.012 atm

of Kr, while on the right (b), shot 44,470, had 0.01 atm of Xe hi-z dopant added to the 0.012 atm of Kr dopant. The intensity scale is not the same for both but was

equalized to show the details. The temporal dispersion is 55 ps/mm.

Further, on one shot having very high Kr density (0.75 atm)the He-a line appeared at the beginning of the emission, thenextinguished before quickly emerging in the continuum, whichis unexpected, see the data in Fig. 8. We do not have an expla-nation of this behavior, as it had not been predicted in the

0

0.5

1

1.5

2

13 14 15

He-β

16 17

4446744470

Exp

osu

re (E

rg

/cm

2)

Energy (keV)

Fig. 4. Lineouts of the spectra in Fig. 3 measured at the time of the peak of the

continuum emission. The dotted line is for shot 44,470 [0.012 atm

Krþ 0.010 atm Xe], and the continuous line is for shot 44,467 [0.012 atm

Kr and no Xe]. The spectra clearly show the disappearance of the He-g line

with the added Xe, the decrease in the He-b line integrated intensity, and de-

crease in the He-b line width.

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current calculations, and is not explained well by an increasedoptical thickness for the line.

4. Summary

We have used high-z dopants (Kr and Xe) and filled withdeuterium and 3He to diagnose an imploding ICF capsulewith SiO2 walls. Xe was used as high-z dopant to affect theconditions in the implosion, while Kr, e.g. He-like line emis-sion, was used measure the electron temperature and electrondensity in the implosion. Spectroscopy allowed us to observe

0

1

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3

4

5

6

7

8

9

1000 2000 3000 4000 5000 6000 7000 8000Temperature (eV)

1.00E+243.00E+245.00E+248.00E+24

Ratio

H

e-

to

n

=2 S

attelite

Fig. 5. A calculation of the ratio of the He-b to the n¼ 2 satellites as a function

of electron temperature, demonstrating the low sensitivity of the ratio to the

different electron densities between 1 and 8� 1024 e/cm3. The different lines:

squares for 1.0� 1024 e/cm3, triangles for 3.0� 1024 e/cm3, continuous line

for 5.0� 1024 e/cm3, and grey squares for 8.0� 1024 e/cm3, all lie close to

each other as a function of electron temperature.

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0.00

0.01

0.02

0.03

0.04

0.05

15.35 15.4 15.45 15.5 15.55Photon Energy (keV)

In

ten

sity

1.00E+243.00E+245.00E+248.00E+24

Fig. 7. Simulated line shapes for the Helium-b line at different expected elec-

tron densities and at an electron temperature of 3 keV. The continuous line is

for 1� 1024 e/cm3, the open squares for 3� 1024 e/cm3, the closed diamonds

for 5� 1024 e/cm3, and the grey squares for 8� 1024 e/cm3. The calculated

widths are w75% of the experimental widths at the expected electron densi-

ties. The measured line shape, however, indicates an electron density between

5 and 8� 1024 e/cm3.

Shot 44467 He- & n=2 Satellite

Shot 44470 He- & n=2 Satellite

0

200

400

600

800

1000

1200

Photon Energy (keV)

Co

un

ts

(a)

(b)

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450

15.2 15.25 15.3 15.35 15.4 15.45 15.5 15.55 15.6

15.2 15.25 15.3 15.35 15.4 15.45 15.5 15.55 15.6

Photon Energy (keV)

Co

un

ts

Fig. 6. (a) Measured spectrum for shot 44,467 with 0.012 atm Kr and 0.0 atm

Xe showing a measured He-b/n¼ 2 Satellite ratio¼ 1.84. (b) Measured spec-

trum for shot 44,470 with 0.012 atm Kr and 0.01 atm Xe, showing a measured

He-b/n¼ 2 satellite ratio¼ 1.32.

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a decrease in the electron temperature of the core, and anincrease in the radiated X-ray flux with increasing dopantdensity, in agreement with initial expectation and code calcu-lations. In this paper, we did not discuss the various otherneutron and charged particle diagnostics which supplementedthe X-ray spectroscopic measurements, and which indicatedthat the ion temperatures and neutron yield dropped more pre-cipitously than the electron temperature as well. The presentinitial effort, confirms the utility of using high-z dopants tomeasure and affect the properties of the implosions, andrequires further work to completely understand and moreaccurately calculate the effects.

Acknowledgments

We acknowledge the help of many people in this work,Scott Evans, Tom Sedillo and Joe Cowan who helped withdata acquisition and operation, Don Haynes for discussions,Hanna E. Makaruk for some data analysis. Ray Bahr andChuck Sorces with the spectrometer setups, and Julie Fooksfor filling the targets. John Kline for making useful sugges-tions to the manuscript. The shots would not have been fea-sible without the Help from General Atomics for fabricatingthe targets, and for the staff at LLE who operate and run thelaser. Los Alamos National Laboratory is operated by LosAlamos National Security, LLC., under Contract No. DE-AC52-06NA25396 with the United States Department ofEnergy.

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ants on implosion dynamics: X-ray spectroscopy, High Energy Density Phys.