p.a. amendf, r. e. turner and o.l. landen- hohlraum-driven high- convergence implosion experiments...

8/3/2019 P.A. Amendf, R. E. Turner and O.L. Landen- Hohlraum-Driven High- Convergence Implosion Experiments with Multiple Beam Cones on the Omega Laser Facility

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I US . Department of Energy

PreprintUCRL-JC-147402

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Hohlraum-Driven High-Convergence ImplosionExperiments with MultipleBeam Cones on theOmega Laser FacilityP.A. Amendf, R.E. Turner, and 0.L. Landen

This article w as submit ted toPhysical Review Letter

February 22,2002

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DISCLAIMERThis document was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor the University of California nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately owned rights. Reference herein to any specificcommercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation, or favoring by the United StatesGovernment or the University of California. The views and opinions of authors expressed herein do notnecessarily state or reflect those of the United States Government or the University of California, andshall not be used for advertising or product endorsement purposes.This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may bemade before publication, this preprint is made available with the understanding that it will not be citedor reproduced without the permission of the author.

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Hohlraum-driven high-convergence implosion experimentswith multiple beam cones on the Omega laser facilityPeter Amendt. K.E. Turner and O.L. LandenL.uwc.nce Liverniorr Ntrtiontil Ltrhorctror?: Liverniore. Cdiforniti 94550

High-convergence implosion experiments have been performed on the Omega laserfacility [T.R. Boehly et al.. Opt. Cooiniun. 133, 495 (1W7)]sing cylindrical goldhohlraums with 40 drive beams arranged into multiple cones. These experiments makeuse of improved hohlraum radiation symmetry conditions [T.J. Murphy et 1x1.. Phys.Rev. Lett. 8 I , 10s (199S)I to demonstrate repeatably high, near one-dimensional(spherical), primary neutronic performance of single-shell implosions with measureddeuterium fur l convergences approaching20.

PACS numbers: 52.40.Nk, 52.50.Jm, 5 2 . 5 8 .N~

The goal of inertial confinement fusion (ICF) is to implode a capsule filled with deuterium-tritiumto a sufficient density and temperature for achieving thermonuclear ignition and energy gain [I]. In theindirect-drive option, a capsule is placed at the center of a hollow high-2 radiation enclosure or hohlraumwhich converts abso rkd laser rays into x rays that ablate the outside of the fuel-filled capsule and drive animplosion. A necessary requirement for ignition of the fuel is that th e x-ray flux symmetry havesatisfactory uniformity to achieve a nearly symmetric implosion. The National Ignition Facility (NIF) [2]is planned to demonstrate ignition under hohlraum conditions where the RMS time-integrated flux non-uniformities are


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flux asymmetry mitigation on Nova 1131 or in double-shell experiments on Omega where the fuelconvergenceis generously defined relative to the outer shell radius [141.

Figure 1 shows the Omega hohlraum geometry used for our high-convergence implosion studies.The "scale-1" thinwall gold hohlraums were 2400 p m long with a radius of 80 0 p m and 600 pn i radiiLEHs. The hohlraum wall consisted of 2 pm of Au overcoated with 50 p m of epoxy to accommodatenoninvasive x-ray imaging of the fuel-pusher region at peak x-ray emission. The laser beams werepositioned along the hohlraum symmetry axis to give th e best time-integrated lowest-order f lux symmetryat the capsule. The capsule was a CH plastic) shell of nominal 30 pm thickness and inner radius of 220pm. The shell is doped with 1% Ge to mitigate volumetric x-ray preheat above the n=2 bound-freeabsorption edge of Ge (. 1.2 keV). Deuterium (DJ is used as th e fuel fill and the convergences are varied byadjusting thk room temperature fuel pressure between 5 and 50 atm.

Figure 2 shows the laser power history used to drive the hohlraums. The nominal energy of I4 kJis delivered within 3.5 ns and uses a two-step pulse shape for efficient compression of the fuel. Also shownis the measured and calculated x-ray drive history as seen by Dante (15), an array of x-ray diodes positionedat 37' to the hohlraum axis outside the LEH See Fig. 11. The calculations are based on integrated hohlraumradiation-hydrodynamics simulations [161 and post-processed to simulate the Dante view of the hohlraumwall and laser hot-spm through the LEH [17]. The agreement between the measured and calculated drivetemperature is very closeand suggests weak levels of backscatter losses which have not been included in thesimulations. Indeed. full-aperture backscatter (FABS) measurements on (outer) cones 2 and 3 show totalbackscatter levels into the f/6 lens cones generally less than 200 J; near backscatter outside of themonitoring lenses is estimated to be at a similar level based on near backscatter imaging (NBI) experienceon Nova [18]. Cone 1 FABS monitoring is not ye t available on Omega but comparison of the length ofhohlraum plasma traversed and wall intensity with the geometry of cone 2 argues for a total cone 1backscatter level of d o 0 . Thus, we infer a total backscatter level of only 600 J or 4 8 of th e incidentlaser energy, corresponding to an imperceptible 3 eV deficit in peak drive temperature.

Masured peak x-ray emission times from the imploded core provide a further check on thehohlraum drive. Figure 3 shows a comparison of th e measured and simulated peak x-ray emission times for


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The traditionul figure-of-merit for modeling ICF implosion performance is t h e ratio of obxrvtxlprimary (DD) neutron yield to t h e simulated DD neutron yield. Figure 4(a) shows th e mtmured DD neutronyiclds, normalized to calculated clean DD neutron yields from two-dimensional (2D) integrated bohlraunisimulations [19]. versus the measured fuel convergence. The convergence is inferred from th e ratio ofsecondary (DT) neutron yield. as recorded by a time-resolved neutron sensitive scintillator array - MedusaI: 1J. to the DD neutrcrn yield. Specifically, the meawred DT/DD neutron ratio determines th e fuel arealdensity and hence, by conservation of fuel mass, the Convergence through use of a "hot spot" model[I21 and accounting for triton slowing in a 1 ke V plasma. The hot spot model assumes that all primaryreaction productsare created at the center of a spherical fuel volume with uniform temperature and density.Figure 4(a) sho us that the highest convergence targets at 5 atm D2 il l had a mean YoC2,, performance near30%, including the effects of intrinsic hohlraum (2D) flux asymmetry. Calculations that gauge thedegradation from resolvable long-wavelength radiation asymmetries alone show a 25% yield degradation for50 atm D, fill capsules and over 50% for the 5 atm capsules.

To explain the residual degradation in performance indicated in Fig. 4(a) we have used a Haan-typemix analysis [20] tw assess the role of Rayleigh-Taylor instability at th e fuel-pusher interface. Calculationssuggest that perturbations on this interface are primarily seeded by feedthrough of outer surface perturbationsduring shock transit. Each target was measured for outer surface roughness and the resulting power spectrumwas convolved with a calculated linear growth factor spectrum to generate a timedependent mode spectrumon the inner surface of th e shell. For each type of capsule we calculated the linear growth factor spectrumand found peak values, defmed as the final-to-initial amplitude, in excess of 200. The 5- and 10-atm D,-filled capsules both attain maximum growth near Y,., =20 while for the low convergence capsule,C mx =30, where I IS he Legendre mode number of the surface perturbation. For comparison, the expected

growth factor on the N IF is 400-1000 [ I ] . The Haan prescription for weakly nonlinear saturation is appliedand th e resulting spectrum of modes is summed in quadrature to generate a time-varying mix region. Theyield degradation in a ID simulation with a dynamic mix layer is then multiplicatively applied to a clean2D integrated hohlrauni simulation prediction to estimate th e combined effect of mix and intrinsic radiation


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dcgradadon such as long-wavelengh capsule nonuniformitics and random tlux asymmetry from laser powerimbalances exist but are estimated to contribute less than 10% in total. For th e higher convergence targetsFig. 4(b) shows that only a 20 -308 margin for yield degradation is left. Here, th e effects of a plausible 0.5pm Y= I shell thickness variation alone can contribute a 20%degradation in yield [2 11-

Multiple 4-6 ke V x-ray images of the imploded cores were obtained using an am y of 5 pnipinholes and 70 ps resolution framing camem. Figure 5(a) shows the comparison between the masuredand expected 50% self-emission contour sizes for fuel Bremsstrahlung x rays. This contour correlates wellwith the fuel-pusher interface for th e higher convergence capsules. Le., 5 and 10 arm Dz-fiII, according toth e simulations. For the lowest convergence capsules, i.e.. 50 atm D2-fill, the core conditions are ( 2 . Wless isothermal, giving more localized emission at th e core and a less reliable indicator of fuel size based ona 50%x-rdy self-emission contour. A lower intensity contour could be chosen to better match the locationof th e fuel-pusher interface, but such an exercise is intrinsically modeldependent and illustrates thedisadavantage of inferring a fuel convergence from x-ray core imaging alone [14,221. Figure 5(bI shows themeasured distortions of the 50% emission contour versus i n f e d convergence from secondary neutronmeasurements. The standard metric for core distortion is "&', here ci (b) s th e radius of th e 50%contouralong the hohlraum waist (axis). The shaded region corresponds to an allowed core distortion for a NIF-relevant R M S time-integrated lowest-order flux asymmetry of


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