Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session UO3: Hohlraum Physics I |
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Chair: Nathan Meezan, Lawrence Livermore National Laboratory Room: Rosen Centre Hotel Salon 9/10 |
Thursday, November 15, 2007 2:00PM - 2:12PM |
UO3.00001: Optimizing the NIF Ignition Hohlraum Debra Callahan, Denise Hinkel, Laurent Divol, Steve Haan, Ogden Jones, Bruce Langdon, Pierre Michel, Larry Suter, Richard Town, Ed Williams In order to optimize the hohlraum for ignition on the National Ignition Facility laser, we need to consider a variety of aspects of the system: laser plasma interactions (LPI), hohlraum energetics, capsule performance, hohlraum asymmetry, laser performance, and target fabrication. In preparation for the first ignition campaign, we have designed a suite of ignition hohlraums that span the range of temperatures consistent with initial NIF operations: 300 eV, 285 eV, and 270 eV. Experiments with 96 beams that are designed to emulate these ignition designs, coupled with flexible target fabrication and laser phase plate production, will allow us to use experiments to guide our choice of the optimal hohlraum for ignition. In this talk, we will describe the suite of ignition designs under consideration, and discuss the trade-offs between the different parts of the system for each design. [Preview Abstract] |
Thursday, November 15, 2007 2:12PM - 2:24PM |
UO3.00002: Scaling and modeling of x-ray preheat in laser heated hohlraums L. Suter, O. Landen, E. Dewald, J. Schein, M. Rosen X-rays with photon energy $>$1.5keV can preheat the inside of the ablator of an indirect drive ignition capsule. Since the amount of inner ablator preheat affects the hydrodynamic stability of the ablator-DT ice interface, ignition capsules include mid-Z dopants such as Cu or Ge in their ablators to control the inner surface preheat. The amount of dopant depends on the level of x-ray preheat in the hohlraum. In this talk we present measurements of x-ray preheat levels from hohlraum experiments performed with a single quad of NIF (NEL). We show the scaling of x-ray preheat with intensity and with radiation temperature and compare that to predictions made with Lasnex. Finally, we discuss how this affects our expectations for preheat in upcoming NIF ignition hohlraums. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48. [Preview Abstract] |
Thursday, November 15, 2007 2:24PM - 2:36PM |
UO3.00003: X-ray induced preheat in indirect drive candidate ablators for the NIF Shon T. Prisbrey, David K. Bradley, David G. Braun, Otto L. Landen, Jon H. Eggert We have developed a hohlraum platform to experimentally measure preheat in materials during the first 1-2 nanoseconds of the current Haan pulse for ignition on the National Ignition Facility. The platform design approximates the radiation environment of the pole of the capsule by matching the laser spot intensity and illuminated hohlraum wall fraction. VISAR reflecting off the back of the sample was used to measure sample motion prior to shock breakout. We will present our experimental results and simulations for candidate ablator samples. Both experiment and simulation results indicate that the ablator materials remain far from melt or the coexistence region which satisfies the NIF ignition requirement. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. UCRL-ABS-232885. [Preview Abstract] |
Thursday, November 15, 2007 2:36PM - 2:48PM |
UO3.00004: Using view factor analysis to understand symmetry for NIF Indirect Drive Cliff Thomas, Michael Edwards To achieve ignition, a NIF capsule requires a high degree of drive symmetry. In practice, this is obtained with the careful balance of several parameters (such as the number of laser cones, and/or the distribution of laser power between separate laser cones). Since the available parameter space is large, it can be difficult to optimize the symmetry using full-physics models. To motivate the further investigation of the available parameter space, and to provide greater insight on symmetry, this study considers a view factor description of radiation transport for indirect drive. Using this approach, the flux on the capsule can be understood as a function of laser pointing, laser spot size, cone balance, and hohlraum geometry (hohlraum length, capsule radius, and LEH size). As a result, avenues for tuning symmetry are explained, and suggestions are made to improve symmetry through the full laser drive. [Preview Abstract] |
Thursday, November 15, 2007 2:48PM - 3:00PM |
UO3.00005: Simulation studies of the re-emit technique for foot tuning of the NIF ignition pulse Jose Milovich, Edward Dewald, John Edwards, Don Meeker The re-emit technique has been proposed to tune the drive flux asymmetry within the first 2 ns of the ignition pulse. This technique measures the soft x-ray emission of a high-Z sphere (in place of a NIF capsule) using several frequency bands. We present numerical results designed to validate this method. Capsule-only simulations show a linear relationship between imposed and re-emitted flux asymmetry. 2D hohlraum simulations show similar plasma conditions for both ignition and re-emit capsules up to $\sim$4 ns. However, simulations predict that an inner-beam-driven heat conduction wave impacts the re-emit sphere limiting the applicability of this technique beyond 2 ns. The effect of diagnostic holes and beam removal is assessed by performing 3D simulations. They show a 4\% offset in the measured asymmetry, largely time independent and consistent with view-factor calculations. These results combined with the experimental accuracy estimates, show that the re-emit is effective for tuning the first $\sim$2ns of the ignition pulse. [Preview Abstract] |
Thursday, November 15, 2007 3:00PM - 3:12PM |
UO3.00006: Development of the re-emit technique for ICF foot symmetry tuning for indirect drive ignition on NIF Eduard Dewald, Jose Milovich, John Edwards, Cliff Thomas, Dan Kalantar, Don Meeker, Ogden Jones Tuning of the the symmetry of the hohlraum radiation drive for the first 2 ns of the ICF pulse on NIF will be assessed by the re-emit technique [1] which measures the instantaneous x-ray drive asymmetry based on soft x-ray imaging of the re-emission of a high-Z sphere surrogate capsule. We will discuss the design of re-emit foot symmetry tuning measurements planned on NIF and their surrogacy for ignition experiments, including assessing the residual radiation asymmetry of the patches required for soft x-ray imaging. We will present the tuning strategy and expected accuracies based on calculations, analytical estimates and first results from scaled experiments performed at the Omega laser facility. \newline [1] N. Delamater, G. Magelssen, A. Hauer, Phys. Rev. E 53, 5241 (1996.) [Preview Abstract] |
Thursday, November 15, 2007 3:12PM - 3:24PM |
UO3.00007: Using backlit thin shell capsules to tune drive symmetry during ignition implosions. John Edwards, R. Kirkwood, E. Dewald, D. Meeker, J. Milovich, D. Kalantar, O. Landen, R. Goldman, M. Schmitt, B. Afeyan In order to attain ignition in cryogenic implosions on the NIF it is necessary for the assembling hot spot to be highly symmetric. To achieve this the drive must be carefully managed to integrate out to less than $\sim$ 1{\%}, avoiding symmetry swings larger than $\sim$ few {\%}. Drive symmetry swings typically occur during rapid changes in the laser pulse due to albedo changes but also as a result of laser spot motion. The swings can be controlled by the relative powers in the inner and outer laser cones as a function of time. Thin, light shells respond rapidly to drive asymmetries, which can then be detected in radiographs of the shells at some later times. Here we describe how this technique can be used to tune drive symmetry in the early parts of an ignition laser pulse. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. UCRL-ABS- 232769 [Preview Abstract] |
Thursday, November 15, 2007 3:24PM - 3:36PM |
UO3.00008: Implosion of Large Scale, Be, Thin Shell Capsules on Omega, Under NIC Conditions R.K. Kirkwood, E. Dewald, D. Meeker, J. Milovich, D.H. Kalantar, O.L. Landen, S.R. Goldman, M. Schmitt, B.B. Afeyan One technique to obtain symmetric ignition implosions on NIF is to measure the symmetry of the hohlraum radiation drive as a function of time by monitoring the shape of the imploded capsules by x-ray backlit imaging. To enhance the sensitivity of this measurement to time-dependent asymmetries, different thickness surrogate capsules will be used. Backlit images of thin (13 to 27 $\mu $m) Cu doped Be shells are planned during the foot of the ignition pulse between 2.0 and 10.2 ns. Recent experiments at the Omega laser facility provided images of 0.7-scale Be capsules doped with 2{\%} Cu under NIC foot conditions. Images of the imploding shell were recorded with a 4.7 keV (Ti), folded foil backlighter between 6.0 and 7.7 ns. These images showed an absorption limb shape, with minimum transmission and radius that closely match the design values, indicating the integrity and response of the imploding shell up to the measurements times. Capsule and drive symmetry, and the quality of the images and comparison with predicted signal and noise models will be discussed. [Preview Abstract] |
Thursday, November 15, 2007 3:36PM - 3:48PM |
UO3.00009: Design of Symmetry Capsules for the National Ignition Facility S.V. Weber, N. Izumi, M.J. Edwards, D.H. Kalantar, N.M. Hoffman, D.C. Wilson Symmetry capsules (SymCaps) will be used to tune the symmetry of the hohlraum x-ray drive for the National Ignition Facility (NIF). Stringent symmetry requirements are specified for the drive from each of four steps of the laser pulse powering the NIF indirect drive ignition capsule. SymCaps are gas-filled surrogates used to tune the high-power 4th step of the pulse. The first three pulse steps will be tuned in earlier experiments. X-ray emission from the capsule core near peak compression will be observed with a gated x-ray imager. Round images, indicating symmetric drive, may be achieved by adjusting the hohlraum length, beam pointing, and power ratio of the laser beam cones. The full thickness SymCap design replaces the frozen DT layer of the ignition capsule with equivalent $\rho \Delta $R of Be. This SymCap is predictive of the core shape at ignition of the cryogenic capsule. Time-dependent drive symmetry will be optimized using thinner SymCaps having temporal sensitivity weighted earlier. We will present a simulated tuning campaign and demonstrate that SymCaps facilitate achieving adequate symmetry to drive the cryogenic capsule to ignition. [Preview Abstract] |
Thursday, November 15, 2007 3:48PM - 4:00PM |
UO3.00010: Symergy Designs for 96 Beam Full-pulse Pre-Ignition Hohlraums S. Robert Goldman, P.A. Bradley, E.S. Dodd, N.M. Hoffman, D.C. Wilson A major thrust in the development of 192 beam, 4 cone-angle hohlraums for NIF is the measurement, prediction, and tuning of drive symmetry with 96 beam, 2 cone-angle hohlraums within the next year, as well as the diagnosis of laser plasma interaction (LPI) conditions. Hohlraum designs to emulate relevant LPI behavior are currently available\footnote{ NB Meezan (private communication)} and are being extended for scaled versions of 270 to 300 eV radiation temperature drives. We will present simulations with variable cone pointing and beam phasing for symmetry capsules in these configurations. The experiments we are designing will be crucial for demonstrating that we can measure, predict, and tune hohlraum drive symmetry in the presence of LPI under expected NIF conditions. The results should be useful for correlating the symmetry and timing of capsule implosions with laser beam propagation in the two cones. [Preview Abstract] |
Thursday, November 15, 2007 4:00PM - 4:12PM |
UO3.00011: Calculations for Omega symmetry capsule implosion experiments in $\sim $0.2 NIF scale high temperature hohlraums N.D. Delamater, D.C. Wilson, G.A. Kyrala, E.S. Dodd, A. Seifter, N.M. Hoffman, D.W. Schmidt, V. Glebov, C. Stoeckl, C.K. Li, J.A. Frenje Symmetry capsules are planned to be used as a diagnostic of implosion symmetry at varying times during the NIF drive. A suitably designed symmetry capsule samples the drive symmetry up to the implosion ``commit'' time of the capsule, which varies for symmetry capsules of different shell thickness. Our capsules use Ge-doped plastic shells with shell thickness varying from 25 $\mu $m to 55 $\mu $m. We present calculations for Omega experiments using symmetry capsule implosions in gold hohlraums 1900 $\mu $m x 1200 $\mu $m, and 70 {\%} laser entrance hole, which is approximately a 0.2 NIF scale ignition hohlraum and reaches temperatures of 265-275 eV similar to those during the NIF drive. These capsules may be used as a diagnostic of shell $\rho $r, since the gas fill is d-He3 at 36 atm. The protons produced in the implosion escape through the shell and produce a proton spectrum, which is measured using wedge range filters. The neutron, proton yield and spectra change with capsule shell thickness as the un-ablated mass or remaining capsule $\rho $r changes. This technique to measure capsule un-ablated mass will be applied to future NIF experiments with ignition scale capsules. Support by US DOE/NNSA, LANS LLC, Contract DE-AC52-06NA253. [Preview Abstract] |
Thursday, November 15, 2007 4:12PM - 4:24PM |
UO3.00012: Using beam phasing and pointing to control indirect drive implosion symmetry G. Kyrala, A. Seifter, N. Hoffman, D. Wilson, S.R. Goldman, N. Delamater, F.J. Marshall, V. Yu Glebov, C. Stoeckl, J. Frenje, C. Li Implosions using inertial confinement fusion must be symmetric to achieve ignition on the NIF. This requires precise control of the drive symmetry incident on the ignition capsule. We performed two studies, using either beam pointing, or power imbalance [phasing] of three cones from the OMEGA laser to affect the symmetry of an imploded capsule. For pointing we used a NIF 0.7 scale vacuum-hohlraum and D$_{2}$-filled 1400 $\mu $m CH capsules to verify the technique. We captured images at different times for different pointings of the inner and middle laser cones, verifying the technique and demonstrating symmetry tuning. For phasing, a 1/4 scale NIF vacuum-hohlraum was used to drive a 475 $\mu $m diameter D$_{2}-^{3}$He-filled capsule. Imaging of the imploded core was used to measure the implosion symmetry and to verify its control. We also show that propagation of the inner beam cone is important, even in a vacuum hohlraum, and has the largest effect on the hohlraum energetics. [Preview Abstract] |
Thursday, November 15, 2007 4:24PM - 4:36PM |
UO3.00013: Demonstrating Control of Symmetry Capsule Implosions in Omega Experiments using NIF 0.7-Scale Hohlraums Nelson Hoffman, Robert Goldman, George Kyrala, Achim Seifter We have demonstrated the ability to detect the weak x-ray emission from, and control the imploded core shape of, large weakly-driven plastic capsules in NIF 0.7-scale hohlraums at the Omega laser. The capsules are similar to those that can be used to diagnose hohlraum radiation drive symmetry during the early stages (``foot'') of the drive in eventual NIF ignition experiments. Because the foot drive temperature is so low ($\sim $90 eV), it had been doubtful that capsule x-ray emission would be detectable. Because the capsule shells must be thin, making them hydrodynamically unstable and subject to complete ionization (``burnthrough'') before peak compression, it had been doubtful that their shape would respond to variations in laser beam pointing. We have now put those doubts to rest. These experiments used 1400-$\mu $m diameter CH shells, of thickness 15-20 $\mu $m, filled with 1 atm D$_{2}$, in 6.38 mm x 3.56 mm gold hohlraums. No higher-Z dopant was necessary to make the x-ray emission detectable. [Preview Abstract] |
Thursday, November 15, 2007 4:36PM - 4:48PM |
UO3.00014: Using Symcaps as Ignition Capsule Replicas for Diagnosing Symmetry at NIF I.L. Tregillis, N.M. Hoffman, N.D. Delamater Achieving ignition at NIF will require a high degree of symmetry in the imploded capsule cores. Ignition capsule symmetry specifications are stated in terms of the hotspot shape. ``Symcaps'' (symmetry capsules), which can be used to replicate the ignition capsule during different stages of implosion, show promise in revealing the hotspot shape. Here we present an analysis of the correlation between ignition capsule and symcap core shapes under a wide variety of beam phasing conditions. Using image galleries and a metric developed for quantifying the correspondence between capsule shapes, we find that a well-designed symcap can mimic the response of an ignition capsule to beam phasing changes in highly symmetric and highly asymmetric situations. Symcaps are a viable method of predictiing the level of $P_{2}$ and $P_{4}$ asymmetry in an ignition capsule implosion. We will also describe an effort to optimize the correspondence by refining the symcap design. [Preview Abstract] |
Thursday, November 15, 2007 4:48PM - 5:00PM |
UO3.00015: Experimental demonstration of increased radiation temperature using foam-walled hohlraums A.B. Reighard, P.E. Young, M.D. Rosen, J.H. Hammer, W.S. Hsing, S.G. Glendinning, R.E. Turner, R. Kirkwood, J. Schein, C. Sorce, J. Satcher, A. Hamza, G. Nyce, O. Landen, S. McAlpin, M. Stevenson, B. Thomas Hohlraums are used in ICF applications to produce a nearly uniform x-ray radiation drive for imploding capsules. Primary x-rays produced by laser beams focused onto the inner wall are absorbed and re-radiated by the hohlraum interior. Analytic analysis and simulations have shown that there is an optimum hohlraum wall density which maximizes the temperature in the radiation heat wave and minimizes energy loss from hydrodynamics [1]. This has been demonstrated in experiments using cylindrical hohlraums, with either 100 mg/cc or 4 g/cc Ta$_2$O$_5$ inner walls. The low-density hohlraums had a maximum of 15\% higher peak x-ray flux, and 5\% higher radiation temperatures than 4 g/cc Ta$_2$O$_5$ targets in time-resolved Dante measurements. This work was performed under the auspices of the U.S. DOE by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. \newline \newline [1] M. Rosen and J. Hammer, Phys. Rev. E., \textbf{72}, 056403 (2005). [Preview Abstract] |
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