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This report was prepared as an account of work sponsored by an agency of the. United States Government Neither the United States Government nor any agency. thereof nor any of their employm makes any warranty express or implied or. assumes any legal liability or responsibility for the accuracy completeness or use . fulness of any information apparatus product or process disclosed or represents. that its use would not infringe privately owned rights Reference herein to any spe . cific commercial product process or service by trade name trademark manufac . turer or otherwise does not necessarily constitute or imply its endorsement m m . mendktion or favoring by the United States Government or any agency thereof . The views and opinions of authors expressed herein do not necessarily state or. reflect those of the United States Government orany agency thereof . The development and advantages of beryllium capsules for the National Ignition. Douglas C Wilson Paul A Bradley Nelson M Hoffman Fritz J Swenson David P . Smitherman Robert E Chrien Robert W Margevicius D J Thoma Larry R Foreman . James K Hoffer S Robert Goldman Stephen E Caldwell. Los Alamos National Laboratory Los Alamos New Mexico 87545. Thomas R Dittrich Steven W Haan Michael M Marinak Stephen M Pollaine Jorge J . Lawrence Livermore National Laboratory Livermore California 9455 1. Capsules with beryllium ablators have long been considered as alternatives to plastic for. the National Ignition Facility laser now the superior performance of beryllium is. becoming well substantiated Beryllium capsules have the advantages of relative. insensitivity to instability growth low opacity high tensile strength and high thermal. conductivity 3 D calculation s with the HYDRA code NTIS Document No DE 96004569. M M Marinak etal in UCRL LR 105821 95 3 lconfirm 2 D LASNEX G B . Zimmerman and W L h e r Comments Plasmas Phys Controlled Thermonucl Fusion . 2 5 1 2975 l results that particular beryllium capsule designs are several times less. sensitive than the CH point design to instability growth from DT ice roughness These. capsule designs contain more ablator mass and leave some beryllium unablated at ignition . By adjusting the level of copper dopant the unablated mass can increase or decrease with. a corresponding decrease or increase in sensitivity to perturbations A plastic capsule with. the same ablator mass as the beryllium and leaving the same unablated mass also shows. this reduced perturbation sensitivity Beryllium s low opacity permits the creation of 250. eV capsule designs Its high tensile strength allows it to contain DT fuel at room. temperature Its high thermal conductivity simplifies cryogenic fielding . I INTRODUCTION, The use of beryllium to create pressure by X radiation driven ablation has been. explored from the earliest days of the inertial confinement fusion program Capsules with. beryllium ablators have long been considered as alternatives to plastic for the National. Ignition Facility laser now the superior performance of beryllium is becoming well. substantiated Early NIF work considered both beryllium and plastic In hope of using. fabrication techniques similar to those used for Nova capsules design effort was. concentrated on using a bromine and oxygen doped 01 st rene ablator the PT design . placed inside a gold hohlraum as shown in Figure 1F Later Y Y a copper doped beryllium. capsule Be330 which was designed to be similar and placed in the same physical. hohlraum using a different laser pulse shape showed less sensitivity to perturbations on. the DT ice surface3 In this paper we will elaborate on the reasons for this behavior some. of which were due initially to different design choices but highlight intrinsic differences . We then focus on the inherent advantages of beryllium both hydrodynamic and. mechanical These include beryllium s high density low opacity high tensile strength and. high thermal conductivity , Many choices must be made to specify a capsule design Below we discuss the. similarities and differences in these choices between capsules with plastic and beryllium. ablators A plastic ablator has the advantage that it can be diffusion filled with DT as well. as being easily doped with higher Z material to achieve any desired mean opacity The. baseline NIF capsule design has been the PT which uses a bromine 0 25 atom and. oxygen 5 atom doped plastic CH ablator 0 95 1 11 mm radius around a frozen. DT 0 25 g cm shell 0 87 0 95 mm The peak drive temperature for this capsule is. 300 eV Saillard presented a modified design L1000 driven with a peak temperature of. 350 eV Lindl presented a beryllium capsule at 250 eV doped with sodium and bromine A. beryllium capsule3 Be330 was designed using copper dopant 0 9 atom with the same. DT mass and radii and a slightly smaller cjutside radius 1 105 mm as the PT with an. important difference being an 80 higher ablator mass Dittrich has added polyimide. C H N204 and B C designs at 300 eV and beryllium designs at 300 eV Be300 and at. 250 eV with a radially graded copper dopant concentration . A high Z dopant adds extra opacity needed to adjust the penetration of the radiation. front into the ablator separating in the rocket model the payload from the exhaust By. varying the concentration of high 2 dopant in the ablator capsules can be optimized over a. wide range of hohlraum temperatures The choice of atomic dopant is not as critical to a. design as the concentration The original choice of bromine for the plastic dopant has been. changed to germanium because is easier to fabricate Copper was chosen to dope. beryllium because it has the highest solubility of any element in beryllium By creating an. alloy rather than an mixture the copper should be distributed as uniformly as possible. throughout the beryllium avoiding possible concentration at grain boundaries Any. inhomogeneities could seed later instability growth However the greater ablator mass of. the Be330 capsule required a lower ablator opacity than the PT design At 200 eV the. Be330 Rosseland mean opacity is a factor of 3 less than the PT At their peak temperatures. the Be330 opacity at 330 eV is 1 8 times less than the PT at 300 eV . An important factor in the success of an ignition capsule is driving it with the proper. radiation pulse shape The first step of a radiation drive is designed to set a desired entropy. in the fuel Thereafter the pulse is designed to adiabatically compress the capsule until the. final pulse whose time history is governed by limits on the maximum power and energy. available from the laser driver The Be330 discussed here and the PT were designed to. place the fuel on the same adiabat and to drive the fuel to the same final velocities The. NIF laser is designed to have the flexibility and precision to deliver a chosen shape within. the limits set by sensitivity studies performed on the PT design3 The beryllium design has. similar sensitivities , If the proper radiation pulse is provided then the capsule sensitivity to radiation. drive asymmetries becomes critical Radiation drive asymmetries are controlled by 1 the. static placement of cones of laser beams along the axis of cylindrical hohlraums and 2 the. time dependent adjustment of the relative laser power in each cone Integrated calculations . which include both the laser driven hohlraum and the capsule demonstrate that we can. adequately control drive symmetry for the plastic PT2 the plastic LlOOO and the 330 eV. beryllium design Be330 337 An integrated 280 eV beryllium design has been calculated. based upon a capsule scaled from the Be330 design The DT fuel mass was kept constant . but the capsule volume was increased to keep the product of ablation pressure and volume. constant An integrated 250 eV design used only 900 kJ of laser energy to explore the. lower end of expected NIF laser performance Table I summarizes all of the integrated. beryllium NIF target designs and Figure 2 shows their radiation temperature drive. histories , II ADVANTAGES OF BERYLLIUM FOR INSTABILITY GROWTH. A Reduced sensitivity compared to the PT design, Sensitivity to material perturbations surface roughness defects etc will be an.
important issue for NIF capsules because it is one of the most difficult to control to. calculate and to determine experimentally Calculations of such multi mode perturbations. include only the capsule to allow better angular and radial resolution These methods are. described by Hoffman and Marinak lo The multi mode perturbations initialized are based. upon mode spectra derived from measurements of ablator and ice surfaces Random phases. are added for individual modes to obtain one possible realization of a surface roughness . NIF size capsules have not yet been built so we must estimate the expected spectrum of. surface roughness based on measurements from existing capsules of various sizes To. obtain a suitable spectrum we extrapolate the DT ice roughness measurements made on a 1. mm radius beryllium cylinder which typically show a 1 2 pm r m s roughness By. imposing the same perturbation spectra on each capsule we can compare their tolerances to. surface roughness Figure 3 shows the resulting degradation of yield as the DT ice surface. is roughened It combines the 2 D calculations with new 3 D results12 The 2 D. calculations were performed with LASNEX13using modes between 12 and 40 These. show that the Be330 capsule tolerates rougher 4x DT ice surfaces than the PT capsule . The 3 D calculations were performed with HYDRA l4 using modes 15 to 120 on the PT. and Be300 capsules They confirmed the reduced sensitivity of beryllium capsules to ice. perturbations They also showed that higher modes 60 to 100 are the most important for. shell breakup but the lowest modes which penetrate the shell are principally responsible. for degrading the yield , This difference in perturbation growth between the PT and beryllium capsules. appears not to be due to the radiation drive although drive profiles can make a substantial. difference in sensitivity to perturbations For example the Be330 capsule is 50 times. more sensitive to ablator perturbations if the final drive temperature alone is reduced 7 . This may explain why Dittrich found the Be300 design gave 20 the growth from ablator. outside surface perturbations as the PT design while Krauser3 shows the Be330 to have. the same sensitivity as the PT Surprisingly Dittrich was able to use the same radiation. drive pulse on his design as the PT whereas the Be330 has the very different pulse seen in. The instability characteristics of the Be330 and the PT capsules appear to be. affected significantly by one difference in the design optimization The beryllium ablator is. more massive than the plastic 3 9 Vs 2 2 mg and carries more payload inward In fact its. payload includes unablated copper doped beryllium about equal in the mass to the DT fuel . The lower E T ablator mass cannot deliver so much high velocity payload and its dopant. was adjusted so that the radiation completely penetrates the plastic at ignition time The. difference in instability characte istics appears to be due to the extra ablator mass that is. imploded along with the fuel Instability calculations show that after the first shock. transits across the mer DT ice surface a rarefaction carries perturbations and their. Richtmyer Meshkov flow field outward to the ablation front There the perturbations. grow by the ablative Rayleigh Taylor instability move inward with the ablation front and. eventually feed through to disturb the fuel We refer to the whole phenomena as the feed . oudfeed in process The extra ablator mass of the beryllium design attenuates the feed . through of these perturbations both outward and inward At ignition the unablated mass. separates the ablation front from the fuel in the Be330 and Be300 capsules attenuating the. perturbations further when compared to the PT , To confirm this behavior we used the same radiation drive on the Be330 capsule . but adjusted the copper dopant to vary the unablated mass carried in with the fuel This. resulted in only slight increases in implosion velocity with decreasing dopant At 1 2 . copper substantially more mass was carried At 0 6 the radiation front penetrated the. ablator completely leaving no unablated payload Figures 4 and 5 compare the trends in. perturbation growth in these capsules to the 0 9 copper Be330 These figures. schematically show the arbitrarily normalized and angularly weighted r m s perturbation. amplitude at the ablator ice interface S whereS2 j r cosOd0 as a function of. the arbitrarily normalized distance the interface has moved Figure 4 compares capsules. with an initial DT ice roughness and shows that late in the implosion the 0 6 Be330. design gives similar interface perturbations to the PT As the unablated mass. progressively increases in the 0 9 and 1 2 designs the perturbation growth is reduced . Figure 5 based on ablator roughness shows the same progressive decrease of perturbation. growth but the PT capsule now lies between the 0 9 and 1 2 Be330 designs . Since the amount of unablated material affects the instability characteristics of the. implosion it is crucial to measure it experimentally The values of yield weighted rho r. pdr of the ablator in the Be330 capsule with three different dopant concentrations. 0 6 0 9 and 1 2 cu were 0 25 0 39 and 0 51 g cm2 These could . The use of beryllium to create pressure by X radiation driven ablation has been explored from the earliest days of the inertial confinement fusion program Capsules with beryllium ablators have long been considered as alternatives to plastic for the National

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