Impact of supra thermonuclear neutrons on inertial fusion reactor technology

Fusion Engineeringand Design 37 (1997)185-195

Fusion Engineen.'ng and Design

ELSEVIER

Impact of supra thermonuclear neutrons on inertial fusion reactor technology I Anil Kumar School o[" Engineering and Applied Science, University of Cal![brnia at Los Angeles (UCLA), Los Angeles, CA 90 095, USA

Abstract It is estimated that one could anticipate as many as 10% supra thermonuclear neutrons, of energy as high as 20 MeV, for each thermonuclear neutron, from an optically thick d - t core of a typical inertial fusion energy (IFE) reactor target (pR > 3 g cm-2). This high energy neutron tail generated in an IFE target opens up additional threshold reaction channels leading to enhanced activation of the target, the surrounding reactor blanket and the shield. For example, SiC activation increases by as much as six orders of magnitude when supra thermonuclear neutrons are accounted for. It is recommended to validate the conclusions of the present studies experimentally through D - T shots at NOVA and the planned N I F facility at Lawrence Livermore National Laboratory. © 1997 Elsevier Science S.A. Keywords: Inertial fusion energy; Thermonuclear neutrons; NIF

I. Introduction The optical thickness ( p R ) o f a compressed d t core o f a typical inertial fusion energy (IFE) reactor target (of the h o h l r a u m kind) could be as m u c h as 3 g cm - 2 or more. The neutrons p r o d u c e d in a thermonuclear reaction interact with the target materials before escaping to the surrounding blanket and shield [1 17]. The target materials get activated before the target explodes. As for the interaction processes in the target before disintegration, the products

Presented at 2nd International Workshop on Nuclear Data for Fusion Reactor Technology, held in Del Mar, California, USA, December 4 6 1995.

o f a thermonuclear reaction between d and t ions, i.e. a 14.1 M e V neutron and a 3.5 M e V c~-particle, might undergo nuclear elastic and large angle c o u l o m b scattering collisions with the host ions and generate energetic d and t recoils [1,11-14]. These athermal recoils could undergo supra thermonuclear fusion reactions while slowing d o w n and produce supra thermonuclear neutrons and a-particles. Multiple generations o f the recoiling d, t ions could give rise to neutrons o f as high energy as 20 M e V or more. It is estimated that one could have as m a n y as 10% supra thermonuclear neutrons for each thermonuclear neutron. These higher energy neutrons open up additional pathways o f activation o f the target and the surrounding blanket/shield.

0920-3796/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0920-3796(97)00042-2

A. Kumar /Fusion Engineering and Design 37 (1997) 185 195

186

Comparison of Target, NIF First wall, and NIF Dose rates (HIB driven type target of LLNL, 20 MJ yield) ~'-L't ..................................................

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Section 2 describes the calculational model. Section 3 discusses the results of the activation calculations for a conventional, thermonuclear neutron source. Section 4 presents neutron energy spectra of supra thermonuclear neutrons as a function of plasma temperature and their role in enhancing activation of all the components. In Section 5, the possibilities of experimental validation of the enhancement effect of supra thermonuclear neutrons are presented. Finally, the concluding section summarizes the issues raised and the key results.

2. I F E

target and chamber

modeling

The Nova laser facility at Lawrence Livermore National Laboratory (LLNL) is scheduled

to be used for preparatory experiments for a much more powerful, planned facility, i.e. the National Ignition Facility (NIF) [2-6]. The NIF is produced to use a > 1.8 MJ, 0.35 I~m laser, which represents a 50-fold increase over the energy of the Nova laser [3]. As much as 20 MJ target yield per shot is expected at NIF. One of the objectives of the N I F experiments is to resolve critical inertial fusion energy (IFE) issues related to heavy ion beam (HIB) driven targets, using ion-like laser-driven targets [3,7-9]. The target chamber will be a 5-m radius sphere, with a 10 cm thick A1-5083 first wall followed by 40 cm thick 'shotcrete' concrete shield [4]. In addition, the concrete may be impregnated with 10 wt.% lead. In our calculations for the target, we adopted the HIB driven type target design described in Ref. [7]. This hohlraum target consists

A. Kumar /Fusion Engineering and Design 37 (1997) 185-195

187

Relative Target Dose rates for various Hohlraum Wall materials (HIB driven type target of LLNL, 20 MJ yield) ~-.--~-'-"

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Fig. 2. Relative dose rates (R h - ] m - 3), at 1 m distance in air, from alternative h o h l r a u m wall materials, as a function of cooling time following a single shot. The target is H I B type and the target yield is 20 MJ.

of a spherical capsule inside an ellipsoidal leadwalled cavity filled with helium gas. The original capsule contains D - T gas at 0.3 mg cm 3, followed by solid D - T at 0.25 g cm 3 and a beryllium pusher. The pusher contains 0.5 at.% of bromine [7]. A piece of lead-impregnated beryllium converts an incident heavy ion beam to X-ray radiation. It is assumed that, soon after the compression, the D - T core attains a density of 700 g cm 3 [5]--the optical thickness (pR) being 3.6 g cm 2. For neutron transport calculations, a burn-up of 50% was assumed. An appropriate source neutron spectrum was used for calculations with pure thermonuclear and supra thermonuclear admixed situations. M C N P was used with ENDF/B-VI for calculating neutron energy spectra in the target as well as in the first wall and the shield [18]. Subsequently, the neutron spectra was

used to do radioactivity calculations, with REAC3 code [19], at all the locations of interest. The fusion yield per shot was fixed at 20 MJ. Dose rates in air, at a distance of 1 m from each system component, were also calculated. Also, the radioactivity calculations were repeated for alternative hohlraum materials that included Au, W, Ta, Hg, Bi, Pt and SiC, among others.

3. Performance with thermonuclear neutron source

Fig. 1 shows that the dose rate at 1 m in air from 20 MJ target is five orders of magnitude larger than that from the NIF first wall, after a cooling time of ~ 1 day. For a 10 year cooling time, the target activity is at least an order of

188

A. Kumar /Fusion Engineering and Design 37 (1997) 185 195

magnitude larger than that from the first wall. Impurities play a very important role in enhancing radioactivity. For example, the presence of even minute quantities of impurities in lead (Pb, 99.92 at.%; Ti, 0.0043 at.%; Cu, 0.016 at.%; Ag, 0.0096 at.%; Sn, 0.028 at%; Bi, 0.023 at.%) leads to more than two orders increase in activity for a 10-year cooling time. The major culprits are 6°Co (5.3 year half life), l°8mAg (127 years), 121mSn (55 years). As for alternative hohlraum materials, with 100% pure composition, the following sequence of observed dose rates from the hohlraum components of the target is found (from low to high) for a 10 year cooling time: SiC, Hg, Au, Pb, Pt, W, Ta and Bi. Fig. 2 graphically shows the dose rates (R h-~ cm 3) from the alternative hohlraum materials as a

Table 1 Fractional contributions to dose rates at a 10 year cooling time following a 20 MJ shot of a LLNL heavy ion beam type target at NIF Material

Leading isotopic contributors

6°Co (5.3 years, 92.4%), 54Mn (312 days, 3.1'7,,), 26A1 (0.72 x 106 years, 3.5%), 65Zn (244 days, 0.9'7,,) 22Na (2.6 years, 98.6%), 54Mn (312 days, Shield with 10 1.4%), 26A1 (0.72 x 106 years, 0.024%) wt.% Pb Pb as Hohlraum 2°4T1 (3.8 years, 98.6%), 2°Spb (14 x 106 wall years, 1.4%) Ta as Hohlraum 179Ta (1.8 years, 99.9%), 182Ta (0.3 years, 8.5e-2%), t78m2Hf (31 years, 3.4ewall 3%) 26A1 (0.72 x 106 years, 100%) SiC as Hohlraum wall 195Au (0.5 years, 99.2%), 194AH (1.6 Hg as days, 0.46%), 193pt (50 years, 0.36%) Hohlraum wall W as Hohlraum tV9Ta (1.8 years, 99.87%), ~7sm2Hf (31 years, 0.12%) wall 194mlr (0.5 years, 99.97%), 1921r (0.2 Au as Hohlraum years, 0.0048%) wall Bi as Hohlraum 2°8Bi (0.37 x 106years, 99.38%), 2°7Bi (38 years, 0.62%), 21°mBi (3 x 106 years, wall 0.0013%) Pt as Hohlraum 193pt (50 years, 96.42%), 192Ir (74 days, 2.6%), 194Ir (19 h, 0.97%), (1941r comes wall from 194mlr (0.47 years)) AI-5083 first wall

function of cooling time. After approximately 1 week of cooling, SiC dose rate settles to a value that is nine orders below that from bismuth. Table 1 lists the leading isotopic contributors from each of these hohlraum materials. Other important observations are as follows. The activated target debris are much more radioactive than the NIF first wall. As the number of neutron producing shots increases, the cumulative decay rate due to the debris could rise to unacceptable levels. In addition, the activated shrapnel from the target positioner and the target manipulation tubes will make a large contribution to the dose rates inside the NIF chamber. If these debris were allowed to pile up on the inside surface of the NIF chamber, they could be a floating source of radioactivity. Possibly, a part of this activity may eventually reach low activation areas outside the NIF chamber. Thus, it might be needed to remove these radioactive debris from time to time. Also, these activated debris are likely to be qualified as higher level of radioactive waste due to such a large differential with respect to the NIF first wall. As for target positioner and manipulator tubes, the best bet lies in going for low activation structural materials. For IFE machines, it is simply not permissible to allow a significant amount of the target debris to get deposited on the surface of the first wall, as it could become even more difficult to protect the latter against X-rays and target debris from subsequent target implosions. The material properties of the deposited layer will be very different from that of the original first wall--with all consequences one could think of. For IFE reactor conditions, even if activated debris were to be transported out of the IFE chamber, one would have to provide a large amount of 7-ray shielding at a storage or reprocessing facility. Even a 20 MJ single shot at NIF is likely to yield a fluence as high as approximately 10 ~8 n cm -2 at the hohlraum wall. This opens up an important possibility of measuring/testing activation cross-section data for long-lived radioactive products, including those from sequential reactions [20], that will be generated in first wall of future fusion power reactors, including D E M O reactors

189

A. Kumar /Fusion Engineering and Design 37 (1997) 185 195

Normalized Supra thermonuclear Neutron Energy Spectrum density

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that would demonstrate engineering performance of fusion power reactors. Such high fluences are very, very difficult to achieve otherwise. In a hohlraum target, for example, one could encapsulate the target inside a thin shell of this D E M O material. Our calculations show that it will be possible to produce a countable quantity of long-lived activity in practically all materials of interest for fusion.

4.

Enhancement

effect

of

supra

thermonuclear

neutrons

As mentioned earlier, in a compressed D - T core (in IFE plasmas), both thermonuclear fusion neutrons ('14.1 MeV') and alphas ('3.5 MeV') could undergo nuclear elastic and largeangle coulomb scattering collisions with the host d/t ions and impart a relatively significant

amount of their energy to the latter. These recoiling d/t ions have a possibility of undergoing supra thermonuclear fusion before merging with the Maxwellian, thermal background. This process could repeat itself many times over [11-14]. The contribution of neutrons to supra thermonuclear fusion (through d/t recoils) has potential of even raising largest fusion neutron energy, due to kinematics, above '14.1 MeV'. Theoretically, one could have neutrons of as high energy as 78 MeV. We have calculated the source neutron energy spectra, including the supra thermonuclear neutrons, as a function of plasma temperature for an infinite medium. We have ignored the nuclear scattering of the d/t recoils for simplification. Fig. 3 shows normalized neutron spectra for plasma temperature varying from 10 to 100 KeV. It is to be noted that, the fraction of neutrons above '14.1 MeV' grows steadily as the

A. Kumar /Fusion Engineering and Design 37 (1997) 185-195

190

Impact of Suprathermal Neutrons on Target, NIF First Wall and Shield Dose Rates after

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Fig. 4. Enhancement factor, due to supra thermonuclear neutrons, for dose rates for the N I F target, the first wall, and the shield, as a function of cooling time. Ten percent supra thermonuclear neutrons are added with a uniform energy distribution between 10 and 20 MeV.

plasma temperature rises. A conservative estimate yields as many as approximately 20% neutrons lying between 14.1 and 20 MeV. Assuming 50% escape probability for a source neutron from the target, we obtain a supra thermonuclear neutron fraction as high as approximately 10%. Calculations of induced radioactivity with REAC-3 were repeated for the target and the surrounding structures with 10% additional neutrons above thermonuclear D - T neutron energy of 14 MeV. The energy distribution of these supra thermonuclear neutrons was assumed uniform for simplicity. A number of important observations follow. Fig. 4 shows enhancement factors for the dose rate, as a function of cooling time, for the target (with lead cavity wall), the first wall and the shield. For the target, just a 10% supra thermonuclear neutron contribution leads to increase in the

dose rate of as much as a factor of 2 for the target, 1.5 for the NIF shield and 4.5 for the NIF first wall. This is due to additional radioactivity generated by proliferation of additional reaction channels due to higher energy of supra thermonuclear neutrons. As for the alternative hohlraum materials (100% purity), as shown in Fig. 5, the largest increases are observed for SiC, Bi, Hg, Ta and Au. In fact, the SiC dose rate even overtakes that of Pb due to six orders raise brought about by opening of the 2sSi(n,t)Z6A1 reaction channel. Fig. 6 compares the energy dependence of the cross-sections for the reactions of direct importance, i.e. 28Si(n,d)ZVA1, 28Si(n,np)27A1, 28Si(n,t)Z6Al and 2ssi(n,nd)Z6A1. Note that, conventionally, a two-step reaction, i.e. 2sSi(n,d/ np)2VAl(n,2n)26Al, is responsible for generation of 26A1 in pure SiC. The new order of dose rates

A. K u m a r / F u s i o n E n g i n e e r i n g a n d Design 3 7 (1997) 185

Impact

191

195

of Su p r at h er mal Neutrons on H ohl raum after a single 20 MJ shot

Dose

rate

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after 10-year cooling time is as follows (from low to high): Pb, SiC, Au, Hg, Pt, W, Ta, Bi. Interestingly, only W and Pt are the least perturbed of all, and incidentally, have the largest number of naturally occurring stable isotopes. The responsible reaction channels are listed in Table 2. Figs. 7 and 8 show the energy dependence of the cross-sections for the important reactions in gold, namely, 197Au(n,3n)X95mAu/195Au and mercury, namely, ]96Hg(n,3n)]94I--Ig,respectively. The cross-sections have been extracted from a 63 group REAC-3 library.

5. E x p e r i m e n t a l

validation possibilities at the IFE

facilities

In view of such a large enhancement brought

about by such a small supra thermonuclear neutron source for most of the investigated materials, it is very important to do experimental validation of this enhancement factor. We propose that the measurements at Nova and N I F be conducted to validate the amounts and types of additional radioactivity produced by the supra thermonuclear neutrons. The experimental validation will be invaluable as so many factors come into the calculation of the radioactivity, including, target dynamics, material-geometric composition of the sub-zones in the compressed target, ion and electron temperature profiles, target burn-up profiles, and the energy dependence of higher threshold reaction cross-sections. Theoretically, the energy distribution of the supra thermonuclear neutrons could extend much beyond 20 MeV, and, as a result, even a minute fraction of the higher energy

A. Kumar /Fusion Engineering and Design 37 (1997) 185-195

192

Si28(n,x)AI26 and Si28(n,x)AI27 Cross-sections

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Fig. 6. 28Si(n,d)27A1, 2+Sn(n,np)27Al, 28Si(n,t)26A1 and 2ssi(n,nd)26A1 cross-sections as function of neutron energy. 26A1 has a half life of 0.72 My. The cross-section data are taken from 63 group library of REAC-3.

neutrons could open up additional threshold reaction channels producing substantial amounts of radioactivity. The validation of the supra thermonuclear neutron effect will provide guidance to the fusion reactor designers as to if they would need to radically modify their current design philosophies. In fact, there is an additional reason to verify the supra thermonuclear neutron energy spectrum. The energy distribution of neutrons escaping from the target has been proposed as one of the key diagnostics to deduce ion temperature profiles of burning D - T plasmas [21-23]. It is evident that the shape of the neutron energy distribution around the thermonuclear peak of 14.1 MeV broadens considerably due to supra thermonuclear neutrons in an IFE/ICF target. A modified treatment will be needed for deducing the plasma ion temperature. Also, the supra ther-

monuclear neutron energy distribution will be a sensitive indicator of the optical thickness of the burning plasma for an IFE/ICF target, and, a good indicator of alpha confinement for a magnetically confined plasma.

6. Summary and conclusions

We have calculated dose rates (R h - ~ m - 3), at 1 m distance in air, from the target, the first wall and the shield of the National Ignition Facility (NIF) following a 20 MJ fusion yield shot. For a 10 year cooling time, the target activity is an order of magnitude larger than that from the first wall. Impurities play an important role in enhancing radioactivity. The inclusion of approximately 10% supra thermonuclear neutrons in the source

A. Kumar / Fusion Engineering and Design 37 (1997) 185 195

193

Table 2 Radioactivity products most impacted by supra thermonuclear neutrons a Material

Ratio of dose rates with and without supra thermonuclear neutrons for 10 year cooling time

A1-5083 First wall

22Na (half life, 1.8 years): infinity (24Mg(n,t)22Na)b: 26A1 (half life, 0.72 x 106 years) b 1.59 (27Al(n,2n)26A1)b Shield with 10 wt.% 26A1 (half life, 0.72 x 106 years): 2.16 (27Al(n,2n)26Al)b: 22Na (half life, 1.8 years): 1.46 (23Na(n,2n)22Na)b Pb 2°2T1 (half life, 12.2 days): infinity (2°4pb(n,3n)2°2pb > 2°2T1, 2°4pb(n,3n)2°2mpb > 2°2T1, Pb as Hohlraum 2°2mpb > 2O2pb> 2O2T1)b wall 179Ta (half life, 1.7 years): 15.5 (181Ta(n,3n)179Ta)b Ta as Hohlraum wall 26A1 (half life, 0.72 x 106 years): 0.94 x 106 (2sSi(n,t)26A1)u SiC as Hohlraum wall 194Au (half life, 1.65 days): 1.4 x 104, 194Hg (half life, 520 years): 1.4 x 104 (196Hg(n,3n)194Hg > 194Au)b Hg as Hohlraum wall 17sm2Hf (half life, 31 years): 1.34, 182Hf (half life, 9 × 106 years): 1.56 W as Hohlraum wall 195Au (half life, 186 days): 2.3 x 104, 194mIr: 1.4, 193pt (half life, 50 years): infinity, 192mlr (half life, Au as Hohlraum 241 years): 1.99, 192Ir (half life, 74 days): 1.99 (197Au(n,3n)195Au) wall 2°7Bi (half life, 32 years): 1.8 x 104 (~°gBi(n,3n)2°TBi)b Bi as Hohlraum wall 192Ir (half life, 74 days): 1.48, 192mlr (half life, 241 years): 1.48 (192mIr> 192Ir) Pt as Hohlraum wall Ten year cooling time following a 20 MJ shot of a LLNL Heavy Ion Beam type Target at NIF, 10% supra thermonuclear neutrons added up to 20 MeV. b Responsible reaction.

s p e c t r u m l e a d s to m u c h l a r g e r i n c r e a s e s in the r a d i o a c t i v i t y o f all t h e c o m p o n e n t s . T h e s e increases are p r i m a r i l y d u e to o p e n i n g u p o f a d d i t i o n a l c h a n n e l s o f a c t i v a t i n g r e a c t i o n s , as the h i g h e s t n e u t r o n e n e r g y rises to as m u c h as 20 M e V . T h e e x p e r i m e n t a l v a l i d a t i o n o f the s u p r a t h e r m o n u c l e a r n e u t r o n effect is r e c o m m e n d e d first at N o v a a n d , later, at N I F , as it is o f v i t a l i m p o r t a n c e to f u s i o n r e a c t o r d e s i g n e r s as well as plasma diagnostic groups.

Acknowledgements T h i s c o n t r i b u t i o n was s u p p o r t e d by the U n i t e d States D e p a r t m e n t o f E n e r g y , Office o f F u s i o n Energy, under contract Nos. DE-FG0386ER52123 and DE-FG03-94ER5487.

References [1] A. Kumar, Activation of various materials of target and enhancement of NIF target-chamber activation by supra thermonuclear neutrons, Presented at a July 27-28 1995 meeting at Lawrence Berkeley Laboratory, and UCLA internal note AK/FNT/UCLA, July 21st, 1995. [2] M. Tobin, A. Anderson, J. Latkowski, M. Singh, C. Marshall, T. Bernat, Physics issues related to the confinement of ICF experiments in the US National Ignition Facility, Presented at 12th International Conference on Laser Interaction and Related Plasma Phenomena, Osaka, Japan, April 24-28, 1995. [3] M. Tabin, G. Logan, T. Diaz Del La Rubia, et al., Contributions of the National Ignition Facility to the development of inertial fusion energy, Fus. Eng. Des. 29 (1995) 3-17. [4] J.F. Latkowski, M.T. Tobin, M.S. Singh, Neutronics and shielding analysis of the National Ignition Facility, Fus. Tech. 26 (1994) 842-846.

A. Kumar /Fusion Engineering and Design 37 (1997) 185-195

194

Au197(n,3n)Au195m and Au197(n,3n)Au195 Cross-sections C 0

2.5

,

,

,

,

i

~ , ~ ,

i

,

~ ,

,

i

,

-Au197(n,3n)Au195m

,

~ ,

i

,

,

,

,

I

,

, ~ ,

i

,

,

,

,

I

i I I

1

.............

I

W 0 ,.&.. ,i

1.5

,,Q ""

1

¢,.

Au197( n,3n)Au195 0.5

OI I -10

,

,,I

,

,,

, I , ,

0

. . . .

10

20

Neutron

I,

,

,

30

I

,

,

,

,

40

,

50

60

energy(MeV)

Fig. 7. '97Au(n,3n)195mAu and 197Au(n,3n)195Au cross-sections as a function of neutron energy. 195mAu has a half life of 30.5 s and decays to t95Au, having a half life of 186 days. The cross-section data are taken from 63 group library of REAC-3.

Hg196(n,3n)Hg194 2.5

~" ¢j)

Cross-section

, , , , i , , , , i , , , , i , , , , i , , , , i , , , ~ 1 , , ,

1.5 L_

&m u~J~ LO. ....~

1

¢.)

0.5

0 -10

, l l t l

.... 0

I , , J 10

Neutron

I .... 20

I .... 30

I , , , , 40

I,,, 50

, 60

energy(MeV)

Fig. 8. I 9 6 Hg(n,3n) 1 9 4 Hg cross-section as a function of neutron energy. 194Hg has a half life of 520 years and it decays to t94Au (half life, 1.65 days). The cross-section data are taken from 63 group library of REAC-3.

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(1973) 403 409. [15] L.A. Booth, Central station power generation by laserdriven fusion, Nucl. Eng. Des. 24 (1973) 263-313. [16] R. Bangerter, D. Meeker, Ion Beam Inertial Fusion Target Designs, UCRL-7874, Lawrence Livermore National Laboratory, 1976. [17] M.E. Sawan, L.A. El-Guebaly, G.A. Moses, W.F. Vogelsang, Nuclear analysis of the heavy-ion beam driven fusion reactor HIBALL, Fus. Tech. 4 (1983) 79-92. [18[ J.F. Breismeister (Ed.), M C N P - - A general Monte Carlo n-particle transport code, version 4A, LA-12625-M, Los Alamos report, 1993. [19] F.M. Mann, REAC*2: Users manual and code description, WHC-EP-0282, Westinghouse Hanford Company report, 1989. [20] A. Kumar, Radioactivity induced by sequential reactions in fusion materials, Proc. 16th IEEE-NPSS Symp. on Fusion Engineering, IEEE, Piscataway, N J, 1996, pp. 1010-1013. [21] T.J. Murphy, R.A. Lerche, C. Bennett, G. Howe, Iontemperature measurement of indirectly driven implosions using a geometry-compensated neutron time-of-flight detector, Rev. Sci. lnstrum. 66 (1995) 930-932. [22] J. Kallne, The role of neutron measurements in the study of burning fusion plasmas, Comments on Plasma Physics of Controlled Fusion 12 (5) (1989) 235-248. [23] G. Gorini, L. Ballabio, Alpha-particle kinetic effects in the neutron emission of burning DT plasmas, Rev. Sci. lnstrum. 66 (1995) 936 938.