Recovery of krypton from implosion of glass microballoons has been studied in the development of a radiochemical diagnostic for determination of  /sub fuel/. Collection onto metal surfaces following implosions performed on the OMEGA laser with 1-3 TW (1-2 kJ) of 0.35 ..mu..m light is consistent with an ion implantation mechanism. The dependence of the intrinsic collection efficiency on the energy fluence to the collector surface and its variation in implosions carried out under the same nominal conditions indicate ion energies extending to at least 0.1 MeV and energy distribution functions that are sensitive to the details of the implosion dynamics. Intrinsic sticking efficiencies approaching 0.5 can be obtained in the limit of low total energy fluence to the collector surface (less than or equal to 0.1 J cm/sup -2/)

Lane, S.M.

Noyes, S.G.

Prussin, S.G.

Richardson, M.C.

UNT Digital Library

£D^tt?-<f-3l DCRL—93964 DE87 008728 Debris Co l l ec t i on from Implosion o f Hicrobal loons S. G. Prussin and S. H. Lane Un ivers i t y o f C a l i f o r n i a Lawrence Livermore Nat ional Laboratory Liver-more, C a l i f o r n i a 94550 H. C. Richardson and S. G. Noyes Un ive rs i t y o f Rochester Laboratory f o r Laser Energet ics Rochester, New York 14627 This paper was prepared f o r the 6th Topical Conference on High Temperature Plasma Diagnost ics H i l t o n Head I s l a n d , South Carol ina 3/9-13/86 3/28/86 -.'*!. This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is mvle available with the understanding that it will not be cited or reproduced without 'he permission of the author. DISTRSB'JTiGN Gr TBiS GOeiiiilENT IS UKUKITEO DISCLAIMER Tht* dacaawai was areaarvd as aa icc iwl af *ark saianred b> a* ageacy af the failed Stales (»a»eraitt . NriaWr the I ailed States C w w i a w l aar the I'anersity af CaWaraia aar aay af their cawlajm. atafces aay narraaty. ex­press ar imftiti. ar a i i a m aay kfa l liaaWly «r rcaaaasibilily far the ac­curacy, caaiplnnnw. ar an M a i n af aay iafar—liia. aaaaratas. aradaxt. ar •races* dhctascd, ar natewats that its ase waaW aat iafriafi aroafcty a*acd rithts- RcCrreacc hereia ta aay specific caaMwrcial aradacts, arace*, ar sertice by trade aaate. Iradfarfc. NMaafactorer. ar idmnise. daes aat accessarity caastilate ar iatatt its eadarseawat, recaaHacadatiaa. ar filarial! h> the I ailed State* CfateraaKM ar the I'aitmity af Oalifaraia. The *ie*s aad aaiaiaas af aathars expressed hercia da aat atcessarily state ar reflect tbase af the laited Stain Ca»eraawat thereaf. aad shall aat ae used far adterttsiaft at arodact ea-darseawM paraasev MASTER DiSTRIBUTiBN Or Ti!iS DOCUMENT IS UKUHITEU DEBRIS COLLECTION FROM IMPLOSION OF HICROBALLOONS* S.S. Prussiit* and S.M. Lane, University of California, Lawrence Livermore National Laboratory, Livermore, California 94550 and t l . C Richardson and S.G. Noyes, Univers i ty of Rochester, Laboratory fo r Laser Energet ics, Rochester, New York 146E7 *This work was performed under the auspices of the U.S. Department of Energy by Lawrence Liver-more National Laboratory under contract No. W-7405-Eng-48. "Permanent address: Department of Nuclear Engineering, University of Cal i fornia, Berkeley, California 94720. ABSTRACT Recovery of krypton from implosion of glass microballoons has been studied in the development of a radiochemical diagnostic for determination of <frV„.,. Collection onto metal surfaces following implosions performed on the OMEGA laser with 1-3 TW (1-S kj> of 0.35 Hm light is consistent with an ion implantation mechanism. The dependence of the intrinsic collection efficiency on the energy fluence to the collector surface and its variation in implosions carried out under the same nominal conditions indicate ion energies extending to at least 0.1 MeV and energy distribution functions that are sensitive to the details of the implosion dynamics. Intrinsic sticking efficiencies approaching 0.5 can be obtained in the limit of low total energy fluence to the collector surface (< 0.1 J c» _ =>. INTRODUCTION The direct measurement of <*r> r u», in ICF implosion experiments has been accomplished by detection of the high-energy deuterons and tritons (knock—ons) that arise from elastic scattering of l4* MeV neutrons in the D-T fuel" 1'. This method is limited to low areal densities of the fuel and ablator/pusher << 0.1 gm cm-«). We have been investigating a number of the parameters governing the collection of krypton debris from implosions of glass microspheres for the purpose of developing a radiochemical diagnostic that can accommodate higher <fr>fu.i and moderate fusion yields < ='. The method is based on the reaction •°Kr«n,2nJ';"'Kr produced by interaction of l1* MeV D-T neutrons with trace quantities of krypton that are added to the fuel. The reaction has a cross section of 0.^15 b < 3> and for <fr>»u.t > 0.05 gm c m - B and a neutron yield greater than lO^t "a concentration of 0.1 atom percent ""̂ Kr in the fuel could suffice for the method if at least X't. of the ""•Kr atoms formed in the implosion are collected and counted. 1 The choice of krypton was dictated by the requirement that the tracer be chemically inert with 'espect to possible shell materials (e.g., glasses, plastics, etc.) and by the desire to have a diagnostic applicable to cryogenic targets. The presence of high-Z material in the fuel at these concentrations is significant but is not expected to greatly affect the implosion physics. The principal difficulty in the development of the diagnostic is the collection of krypton following an implosion. Due to the absence of chemical binding, the only readily applicable methods are cryogenic absorption, implantation into a substrate after acceleration in an applied field or direct implantation using the kinetic energies of the ions in the expanding plasma formed in the implosion. COLLECTION EFFICIENCY We have studied the characteristics of direct collection of krypton onto various metal surfaces following implosions of glass microtfalloons filled with natural krypton using 1-3 TW of 0.35Hm light from the OMEGA laser of the University of Rochester's Laboratory for Laser Energetics (URLLE). The laser pulse was nominally Gaussian in shape with a width of 600 ps. The total energy on target Cl-E kJ) was delivered in IE or 2<» beams. The targets were a J typically 1-2 Hm thick shells 300-<t00 Hm in diameter filled to a pressure of 3-7 atm. The targets had been irradiated with thermal neutrons to produce the 't.'tB hr • = mKr which served as radioactive tracer for determination of collection efficiencies. The materials tested as collectors were either (l-5)xl0 - 3 cm thick foils of aluminum, titanium, beryllium and zirconium or 1—5 Mm thick layers of aluminum and titanium deposited onto foil backings by sputtering. The collectors were located at 10—SO cm from the target and the angle between the surface normal and the radius vector from the target to the collector midpoint was varied between 0" and 80° to affect a large range in total energy fluence on the col lector. The intrinsic collection efficiency for krypton,€, -defined as the fraction of ions incident on a surface that are collected — was estimated by comparison of the B = m K r activity on a collector with the initial activity in the microballoon. The surfaces showing the highest collection efficiencies were titanium and aluminum and the data obtained with these materials are shown in figure 1 as a function of the total energy fluence to the collector surface. The latter includes all.energy sources radiating from the target (i.e., particle kinetic energies, scattered laser light, x—rays, etc.). To obtain E it has been assumed that all radiations are emitted isotropically following an 3 implosion. The scatter in the data is large. With the possible exception of the energy balance of the beams, no correlations with experimental parameters could be found to account for the variation in £ among seemingly comparable implosions. There was a tendency for £ to increase with greater imbalance in beam energies. Nevertheless! the data indicate a rough inverse variation of £ with increasing energy fluence and indicate that intrinsic collection efficiencies in the range 0.1-0.5 can be achieved with low total energy fluence to the collector surface (i 0.1 J cm=). SURFACE EROSION The dependence of £ on energy fluence can be expected with an ion implantation mechanism if erosion of the collector surface is'significant and if the target debris must penetrate the eroded material to reach an underlying stable surface. The large scatter in the collection efficiency is also consistent with an ion implantation mechanism if ion kinetic Energies depend sensitively on the details of the implosion dynamics. The expected ranges of low-energy ' B < ,Kr ions in titanium and aluminum are small. For references we show in figure 2 mean ranges for e**Kr in titanium and carbon calculated in the low-temperature limit according to the formulation of electron and nuclear 4 stopping summarized in ref «». (Ranges in aluminum lie between those shown in the figure.) For the mean energies expected from these implosions <Ti„„ < 1 keV), the depth of penetration at normal incidence is on the order of 100 A. Thus, relatively small variations in ion energies and in the extent of surface erosion can produce large variations in €. To examine the effect of surface erosion, the thickness of titanium removed from a mirrored surface was measured. The mirrors were prepared by vacuum evaporation of Ti onto silicon wafers and one half of the surface was covered by a thick titanium foil to prevent exposure to radiation and target debris. The assemblies were located at a mean distance of 11 cm, from the microballoons with the surface normal at an angle of 8H° with respect to the target—col lector radius vector. Following the implosions, the protective foil was removed and the step height between the original and eroded surfaces was measured with a profilometer, The results from two such experiments are shown in figure 3. For total irradiance in the range 0.8-1.2 GW cm-=, corresponding to total energy fluences of 0.5-0.7 J cm-=, surface thicknesses of 100-300 A were removed. Tfiese results suggest that surface erosion may be the dominant factor limiting the collection efficiency for Kr. Reference to figure 2 indicates that ion energies Dn the order of 100 keV would be required for implantation 5 into a stable surface if the eroded material were present at the time of arrival of target debris. Crude estimates of maximum expansion velocities indicate that this is likely for ions with sufficient kinetic energy to imbed with high probability <~ O.a-0.5 keV). Although the range-energy correlations shown in figure 2 neglect ionization of the stopping medium, the maximum temperature of the eroded titanium is ~ 10 eV and therefore the conclusions above are expected to be conservative '"•>. ION ENERGIES Some evidence that a fraction of the Kr ions from implosions has energies as high as 0.1 MeV was obtained in a series of experiments in which Ti foils covered with thin layers of formvar or"carbon were placed adjacent to uncovered foils at target—to—collector distances sufficiently large that erosion of the exposed Ti surface was expected to be small (I < 0.2 J cm-"). The relative collection fractions of trie two parts of the collectors were determined. Under the assumption that any of the formvar and carbon lost from the surface was penetrated by the arriving ions, the fraction of Kr ions with energies exceeding that required to penetrate the cover foil was calculated. These data are shown in figure 4. Although 6 quite crude, the results are consistent with the assumption that a few percent of the ions from the implosions have kinetic energies in excess of 100 keV. These results are also consistent with the efficiency data in figure 1. CONCLUSIONS Implosion experiments with Ki—filled microballoons indicate that a scheme for direct collection of activated Kr from ICF implosions by ion implantation can be r:ade viable if surface erosion can be minimized and if sufficiently high mean ion energies are produced. Additional experiments are being planned to obtain more conclusive information on ion energy distributions at URLLS, to demonstrate the direct measurement of <#r>fu»i with this method and"to investigate the problems associated with Kr collection in implosions at the NOVA laser facility. r The authors gratefully acknowledge the support of the entire staff at URLLE and the target fabrication groups at LLNL and KMS Fusion, inc. One of us (S.G.P.) wishes to acknowledge the hospitality of Y-Division at LLNL and the support afforded by E.M. Campbell. 7 References 1. S. Kacenjar, S. Skupsky, A. Entenberg, L. Boldman and M. Ri-hardson, Phys. Rev. Let. f»9 (1983) 463. 2. see, for example, E.M. Campbell, S.M. Lane, Y.L. Pan, J.T. Larsen, R.J. Wahl and R.H. Price, J. Appl. Phys. 51 <1980) 6062. 3. E. Kondiah, N. Ranakumar and R.W. Fink, Nucl. Phys. AE10 (1968) 337 4. T.A. Mehlhorn, J. Appl. Phys. 52 C19B1) 652S 8 Figure Captions Figure 1. Intrinsic collection efficiencies for Kr ions versus total energy fluence to the collector surface. It has been assumed that all matter and radiation is emitted isotropically from the target. Statistical errors in the data are negligible compared to the scatter shown in the data. Figure £. Range of B*Kr ions in titanium and carbon in the low temperature limit. Ranges in aluminum lie between those shown. Figure 3. Thickness of titanium eroded from a mirrored ' f surface versus total irradiance. Representative error bars are shown. Figure '*. Estimated fraction of *"*Kr ions with energies exceeding abscissa values. The errors indicated are representative of total statistical errors in the measurements. 9 Collection efficiency o o * © I CO o I o O N) H o . OJ_ o E CD 3 ; CD -• I I 1 I I l l l l l | 1 I I I l l l l | 1 | I M i l CO •< t 5 °= 3 ° o CD O 11 mi « W O F i i i i mil i i i i mil L I I I IH I I I I I lllR c o u CO D) c CO c o Ion energy in keV V) c 03 •a m • a o 03 if* Q) C ouu 1 1 1 1 1 1 1 1 • 600 1 1 400 1 • 1 • • t 1 • • • • -200 n • 1 • • • . 1 1 , 1 , 1 1 ' 0.6 0.8 1.0 1.2 1.4 Total irradiance (GW/cm2) 1.6 0.14 i 1 • 1 ' 1 ' 0.12 — — 0.10 — — c - -2 0.08 — . I I — o CO ~ i -c 0 0 6 — — _0 - 1 -0.04 T — 0.02 1 i * • i — 0.00 • ,t 1 1 , 1 , 1 1 , 100 200 300 400 5G Minimum Kr ion energy (keV) 

Debris collection from implosion of microballoons

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Debris collection from implosion of microballoons

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