Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session CO4: Hohlraum Physics and Indirect Drive |
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Chair: Forrest Doss, Los Alamos National Laboratory Room: Salon E |
Monday, October 27, 2014 2:00PM - 2:12PM |
CO4.00001: Stimulated Brillouin Scatter Reduction using Borated Gold Hohlraums on the National Ignition Facility Joseph Ralph, David Strozzi, Richard Berger, Pierre Michel, Debra Callahan, Denise Hinkel, Laurent Divol, Brian MacGowan, Felicie Albert, John Moody New target platforms for indirect drive ignition on NIF are being introduced to improve capsule and hohlraum performance. A number of these targets show increased Stimulated Brillioun Backscattering (SBS) late in the laser pulse on the outer cone beams. This scattering reduces the laser power available for x-ray drive in an ignition hohlraum as well as poses a damage risk to the laser optics. We observe a factor of 5 reduction in the SBS power from outer cone beams by doping the Au hohlraum wall with 1.5 $\mu$m layer of 40{\%} Boron in Au. The experiment used a room temperature Neopentane-filled ignition scale hohlraum and a 1 MJ, 370 TW laser pulse. The measured SBS backscatter from the outer cone beams on NIF is quantified temporally and spectrally. Comparing the measurements between a pure Au and a AuB hohlraum show approximately a 5x reduction in SBS power. Simulations show that the reduction is in the hohlraum wall plasma. A continuation of this study will extend the duration of the laser pulse to measure the time-dependence of the outer beam SBS. Experimental results from these experiments and detailed simulation results will be presented. [Preview Abstract] |
Monday, October 27, 2014 2:12PM - 2:24PM |
CO4.00002: Hohlraum fill gas density scaling of x-ray drive, symmetry, and laser coupling backscatter in 6.72-mm NIF hohlraums Ogden Jones, N. Izumi, L.B. Hopkins, D.J. Strozzi, P.A. Amendt, G.N. Hall, D.D. Ho, S.F. Khan, N.B. Meezan, J.D. Moody, S.R. Nagel, J.E. Ralph, R.P.J. Town Most ignition experiments carried out on the NIF to date have used hohlraums with helium gas fill at 1-1.6 mg/cc density in order to prevent excessive hohlraum wall motion and help to control drive symmetry. A unique feature of 2-shock high density carbon (HDC) ignition designs is that they require a much shorter ($\sim$ 7 ns) laser pulse than the $\sim$ 20 ns duration pulses that are typically used for 3-shock or 4-shock CH ablator designs, so there is less time for the wall to move. As a result, it is possible to reduce the hohlraum gas fill density. We have done 2D convergent ablator experiments in a 6.72 mm diameter hohlraum at fill densities of 0.03 and 0.6 mg/cc. These experiments used HDC capsules driven by a 1.5 MJ, 370 TW peak power laser pulse. They demonstrated low backscatter (\textless 4{\%}) and effective drives that are much closer to high flux model predictions than for typical gas-filled hohlraums. The 0.6 mg/cc fill reduced the amount of unabsorbed inner cone power that is reflected out of the hohlraum for the 0.03 mg/cc case. Also, the 0.6 mg/cc has improved symmetry that is in good agreement with modeling. [Preview Abstract] |
Monday, October 27, 2014 2:24PM - 2:36PM |
CO4.00003: Optimizing Near-vacuum NIF hohlraum drives for ICF Sebastien le Pape, Laurent Divol, Laura Berzak Hopkins, Andy Mackinnon, Nathan Meezan, Darwin Ho, Arthur Pak, Joe Ralph, Steven Ross, Steve Haan, Prav Patel, Jack Caggiano, Richard Bionta, Tammy Ma, Ryan Rygg, David Fittinghof, Shahab Khan, Alex Hamza, Peter Celliers, Alex Zylstra, Maria Gatu-Johnson, Hans Rinderknecht, Johan Frenje, Gary Grim, Robert Hatarik Near Vacuum Hohlraum (NVH) is a high coupling platform that might provide a path to ignition using High Density Carbon (HDC) with 10 ns long pulses. We have investigated in a series of experiments on the National Ignition Facility (NIF), our ability to control the symmetry of the implosion in this high efficiency platform. Keeping control of the symmetry as the hohlraum fills in with ablated gold is the main challenge of NVH hohlraum. To help inner beam propagation by increasing the distance from the hohlraum wall to the capsule, the hohlraum diameter was increased from 5.75 mm to 6.72 mm, results from of these experiments will be presented. To reach an ignition relevant design, the adiabat has to be lowered. To lower the adiabat, the 2 shock pulse shape length was increased from 4.5 ns up to 8 ns, results will be presented. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. [1] Le Pape, S., at al., Phys. Rev. Lett. 112, 225002 (2014) - Observation of a Reflected Shock in an Indirectly Driven Spherical Implosion at the National Ignition Facility. [2] Mackinnon, A. J., et al.. High-density carbon ablator experiments on the National Ignition Facilitya). PHYSICS OF PLASMAS 21, 056318 (2014) [Preview Abstract] |
Monday, October 27, 2014 2:36PM - 2:48PM |
CO4.00004: Correspondence between laser coupling and x-ray flux measurements in a NIF hohlraum J.D. Moody, L. Divol, O. Landen, S. LePape, P. Michel, J. Ralph, R.P.J. Town, K. Widmann, A. Moore We describe a simple model relating measurements of the hohlraum x-ray emission (DANTE) to the coupled (incident less backscattered) laser power in NIF indirect drive hohlraum experiments. The model was motivated by observing that the measured x-ray emission showed a lag in rise corresponding to a measured reduction in laser coupling due to backscatter. Two adjustable scalar parameters (a coupling efficiency and a time-scale) in the model are determined for each experiment. Comparing these parameters for different hohlraum gas-fill, ablator, pulse-length, and laser power conditions provides insight into the hohlraum behavior and performance. In some cases, the model can be inverted to estimate the backscatter loss using the measured hohlraum x-ray emission time-history and delivered laser power. We will describe the model and compare the adjustable parameters between different hohlraum platforms. [Preview Abstract] |
Monday, October 27, 2014 2:48PM - 3:00PM |
CO4.00005: Hohlraum $T_{\mathrm{e}}$ Inferred from Au L-Shell Emission S.P. Regan, R. Epstein, D.D. Meyerhofer, T.C. Sangster, M.J. May, M.B. Schneider, M.A. Barrios, J.D. Moody, K.L. Baker, L. Berzak Hopkins, G.V. Brown, D. Callahan, T. Doeppner, K.B. Fournier, D.E. Hinkel, O.S. Jones, R. Kauffman, S. Khan, J.D. Kilkenny, O.L. Landen, D.A. Liedahl, S.R. Nagel, J.S. Ross, V.A. Smalyuk Laser-ablation plasmas created at the inner wall of the hohlraum (Au bubble) and at the laser entrance hole (LEH) radiate L-shell emission from Ne-like to Co-like charge states of Au. A 1-D spatially resolved and time-integrated spectrum in the 6- to 16-keV range with $E$/d$E = $ 100 to 300 is recorded along the axis of the hohlraum. The Au L-shell spectral line shapes of the $2p_{3/2} - 3s$, $2p_{3/2} - 3d_{5/2}$, and $2p_{1/2} - 3d_{3/2}$ transitions are analyzed using an atomic physics code to infer the $T_{\mathrm{e}}$ of the radiating plasma. Preliminary results indicate the Au LEH plasma of a near-vacuum hohlraum has an inferred $T_{\mathrm{e}}$ of 5 to 6 keV, while a gas-filled hohlraum has a significantly lower $T_{\mathrm{e}}$. A comparison of the Au L-shell spectra and the $T_{\mathrm{e}}$ sensitivity will be presented, along with the plan to measure the L-shell emission from the Au bubble. This material is based upon work supported by the Department Of Energy National Nuclear Security Administration under Award Number DE-NA0001944. Part of this work was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, October 27, 2014 3:00PM - 3:12PM |
CO4.00006: The application of quasi-steady approximation in atomic kinetics in simulation of hohlraum radiation drive Guoli Ren, Wenbing Pei, Ke Lan, Xin Li In current routine 2D simulation of hohlraum physics, we adopt the principal-quantum- number(n-level) average atom model(AAM) in NLTE plasma description. The more sophisticated atomic kinetics description is better choice, but the in-line calculation consumes much more resource. By distinguishing the much more fast bound-bound atomic processes from the relative slow bound-free atomic processes, we found a method to built up a bound electron distribution(n-level or nl-level) using in-line n-level calculated plasma condition (such as temperature, density, average ionization degree). We name this method ``quasi-steady approximation.'' Using this method and the plasma condition calculated under n-level, we re-build the nl-level bound electron distribution (Pnl), and acquire a new hohlraum radiative drive by post-processing. Comparison with the n-level post-processed hohlraum drive shows that we get an almost identical radiation flux but with more-detailed frequency-dependant structures. Also we use this method in the benchmark gold sphere experiment, the constructed nl-level radiation drive resembles the experimental results and DCA results, while the n-level raditation does not. [Preview Abstract] |
Monday, October 27, 2014 3:12PM - 3:24PM |
CO4.00007: Integrated P1 Hohlraum/Capsule Simulations for NIF Experiments David Eder, Brian Spears, Richard Town, Oggie Jones, Tammy Ma, Arthur Pak, Robin Benedetti, Steve Hatchett, James Knauer, Andrew Mackinnon, Charles Yeamans, James McNaney, Daniel Casey We discuss integrated hohlraum/capsule post-shot simulations of two full-scale cryogenic NIF experiments that drove a DT symcap capsule downward/upward by having the peak power in the upper laser beams 16{\%} greater/less than the lower beams. This laser asymmetry results in a radiation drive P1/P0 at the capsule ablation surface of $\sim$ 2{\%} and a downward/upward capsule velocity of order 100 microns/ns in agreement with the data. The experimental velocity is determined by comparing measurements at different locations of both the arrival times of DD and DT neutrons at time-of-flight detectors, and by zirconium activation measurements that are a function of neutron energy. We compare these two shots to a control shot for the same target with no specified laser asymmetries. We also discuss simulations of planned sub-scale warm symcap experiments that have a goal of measuring DT and DD ion temperatures and the electron temperature as a function of the imposed P1 to characterize the role of non-thermal velocity on temperature measurements. [Preview Abstract] |
Monday, October 27, 2014 3:24PM - 3:36PM |
CO4.00008: The structure of the Laser Entrance Hole in NIF Ignition gas-filled hohlraums M.B. Schneider, T. Doeppner, C.A. Thomas, K. Widmann, S.A. Maclaren, N.B. Meezan, P.M. Bell, L.R. Benedetti, D.K. Bradley, D.A. Callahan, D. Eder, J.H. Hammer, D.E. Hinkel, O.S. Jones, P. Michel, J.L. Milovich, J.D. Moody, A.J. Moore, H.S. Park, J.E. Ralph, S.E. Regan, D.J. Strozzi, R.P. Town At the National Ignition Facility (NIF), the energy from 192 laser beams is converted to an x-ray drive in a gas-filled hohlraum. The drive heats and implodes a fuel capsule. The laser beams enter the hohlraum via laser entrance holes (LEHs) at each end. The LEH size decreases as heated plasma from the LEH material blows radially inward but this is largely balanced by hot plasma in the laser deposition region pushing radially outward. Compared to models, the LEH size is larger than predicted. In addition, the plasma in the LEH region is hotter than predicted. Instead of being at the radiation temperature of about 300 eV, it is at an electron temperature of 1 to a few keV. The experimental measurements for this conclusion are discussed. Data on the LEH as a function of laser pulse shape, gas fill, and energy transfer are presented. [Preview Abstract] |
Monday, October 27, 2014 3:36PM - 3:48PM |
CO4.00009: Low-foot rugby hohlraum experiments on the NIF: Wall-gas mix and a connection with missing x-ray drive energy? Peter Amendt, J. Steven Ross, Marilyn Schneider, Oggie Jones, Jose Milovich, John Moody Rugby-shaped hohlraums on the NIF have shown strong symmetry anomalies when simulated with the high-flux model [1]. The wall-gas interface is Rayleigh-Taylor unstable and may lead to the formation of a late-time mix layer that impedes inner- cone propagation, resulting in a drive asymmetry on the capsule. Due to the rugby curvature near the laser entrance hole, the effect of mix may be more pronounced than in cylinders. At the same time a persistent pattern of 15-25\% missing energy has been inferred in gas-filled hohlraums ($\rho \geq 0.96$ mg/cc). A possible physical connection between formation of a mix layer and the plasma adiabatic lapse rate [2], where a temperature-gradient reversal is predicted to occur, is explored. Such a profile reversal, in turn, hinders electron conduction to the dense ($\rho>0.2$ g/cc) Au region responsible for $\sim$ 900 eV drive x-ray emission, leading to a hotter coronal plasma and reduced hohlraum efficiency. Remedial measures for recovering the loss in hohlraum efficiency through the use of higher-Z gas fills are explored.\\[4pt] [1] M.D. Rosen et al., High Energy Density Physics 7, 180 (2011).\\[0pt] [2] P. Amendt, C. Bellei and S.C. Wilks, Phys. Rev. Lett.. 109, 075002 (2012). [Preview Abstract] |
Monday, October 27, 2014 3:48PM - 4:00PM |
CO4.00010: The Quartraum platform for measurement of cross-beam energy transfer on NIF L.A. Pickworth, M.B. Schneider, D.E. Hinkel, M.D. Rosen, F. Albert, D.A. Callahan, P.A. Michel, A.E. Pak, E.A. Williams, J.D. Moody, S.S. Wu, L.R. Benedetti, A. Moore NIF routinely utilizes cross-beam energy transfer (CBET) to control the symmetry of the ICF capsule implosion. In the ignition hohlraum, CBET occurs in the laser entrance hole region, transferring power from the outer to the inner beams. The amount of transfer is controlled by the $\Delta\lambda$ between the beam cones and is proportional to laser intensity and $n_e /T_e$. It is most significant at peak power and during the picket of the pulse. Models indicate that energy transfer is not uniform across the beams spots, producing a non-uniform profile in the inner beam, which would introduce a spatio-temporal asymmetry in the drive applied to the ICF capsule. The platform is designed to validate beam intensity variations after CBET in the picket of the laser pulse by observation of the beam x-ray ``foot print'' on a high-Z witness plate using a gated x-ray imaging camera. Results from the first experiments will be discussed. [Preview Abstract] |
Monday, October 27, 2014 4:00PM - 4:12PM |
CO4.00011: The hohlraum radiation temperature and M-band fraction on the SGIII-prototype laser facility Wenyi Huo, Dong Yang, Ke Lan, Sanwei Li, Yongsheng Li The hohlraum radiation temperature and M-band fraction are determined by a shock-wave technique and measured by a broadband soft x-ray spectrometer. The peak radiation temperature T$_{\mathrm{R}}$ and M-band fraction f$_{\mathrm{m}}$ are simultaneously determined by using the observed shock velocities in Al and Ti. For the vacuum Au hohlraum used in the experiments, T$_{\mathrm{R}}$ is about 160 eV and f$_{\mathrm{m}}$ is between 4.3-6.3{\%} under 1ns laser pulse of 2 k. And T$_{\mathrm{R}}$ is about 202 eV and f$_{\mathrm{m}}$ is about 9{\%} with laser energy 6 kJ . The Continuous Phase Plate (CPP) for beam smoothing is applied in the experiment, which increases T$_{\mathrm{R}}$ to 207 eV while has almost no influence on f$_{\mathrm{m}}$. Comparisons between the results from the two kinds of technologies show that T$_{\mathrm{R}}$ from the shock wave technique is lower than that from SXS whether CPP is applied or not. However, f$_{\mathrm{m}}$ from the shock wave technique is consistent with that from SXS without CPP, but obviously lower than the SXS's result with CPP. [Preview Abstract] |
Monday, October 27, 2014 4:12PM - 4:24PM |
CO4.00012: Hohlraum energetics study on Shenguang-III prototype laser facility Dong Yang, Sanwei Li, Zhichao Li, Rongqing Yi, Liang Guo, Xiaohua Jiang, Shenye Liu, Jiamin Yang, Shaoen Jiang, Yongkun Ding, Shiyang Zou, Huasen Zhang, Yiqing Zhao, Wenyi Huo, Xin Li, Yongsheng Li, Ke Lan Comprehensive and accurate characterization of the hohlraum drive needs to use a variety of methods resolving different photon ranges and multiple viewing areas. In recent years, hohlraum physics have been studied extensively on Shenguang-III prototype laser facility. These experiments employed mainly Au hohlraums (vaccum or gas-filled, with capsule or not) heated by smoothing beams where scattering loss is less than 10{\%}. With compact flat-response x-ray detector array and 14-channel soft x-ray spectrometer, the radiation flux from several specific regions inside the hohlraum is measured through the laser entrance hole (LEH) or the diagnostic hole (DH) at different photon ranges and multiple lines of sight. The difference in radiation between the laser spot and the reemitting wall is quantitatively studied to interpret flux onto the capsule. The motion of laser ablated bubble and radiation ablated blow-off plasma is directly measured, and their effects on laser absorption and x-ray escaping LEH are evaluated. In addition, the radiation driven shock propagating in Al and Ti placed on the hohlraum wall, which is more representative of the drive inside the hohlraum, provide a unique information of radiation. [Preview Abstract] |
Monday, October 27, 2014 4:24PM - 4:36PM |
CO4.00013: Scaling formula of ICF ignition targets and study of targets optimized in stability performance Xin Li, Zhensheng Dai, Wudi Zheng LPI and RTI are the two main ingredients affecting the success of ignition. The gas fill near the Au wall along the inner laser cone is the main region which stimulates SRS instabilities. At this region, pressure balance and energy balance between the inside and the outside of inner laser cone path are obtained. A plasma scaling model in ignition hohlraums of ICF has been developed. RTI could be described by IFAR(InFlight Aspect Ratio) according to linear theory. Considering other scaling formula in capsule, a index, SPI(Stability performance Index), has been proposed, which describes the balance between SPI and RTI. Designing of ignition targets is directed by using this index to obtain more margin for LPI and RTI. [Preview Abstract] |
Monday, October 27, 2014 4:36PM - 4:48PM |
CO4.00014: Optimization of the x-ray spectrum and radiation symmetry in the hohlraum Yongkun Ding, Shaoen Jiang, Shenye Liu, Sanwei Li, Tianxuan Huang, Shiyang Zou, Ke Lan, Wenhua Ye, Yongsheng Li, Guoli Ren, Jianfa Gu, Baohan Zhang, Xiaodong Chen, Wenbing Pei, Shaoping Zhu, Weiyan Zhang The ultimate goal of laser-driven hohlraum is to create a radiation environment that ablatively implodes a capsule to ignition and burn. To obtain high fusion yield with minimum laser energy, the hohlraum drive must meet both the limited non-Planckian emission (M-band) and excellent uniformity. On the series of Shenguang laser facilities, several experiments have been done to characterize the x-ray spectrum and radiation symmetry of the laser-heated hohlraum. By optimizing the hohlraum structures and materials, the hohlraum performance was improved in flux intensity, symmetry and spectrum. [Preview Abstract] |
Monday, October 27, 2014 4:48PM - 5:00PM |
CO4.00015: Progress on Octahedral Spherical Hohlraum Study Ke Lan, Jie Liu, Wudi Zheng, Wenyi Huo, Guoli Ren, Dongxian Lai, Xiantu He In this talk, we report our progress on octahedral spherical hohlraum study. First, we propose a spherical hohlraum with 6 Laser Entrance Holes (LEHs) of octahedral symmetry at a specific hohlraum-to- capsule radius ratio of 5.14, which has robust high symmetry during the capsule implosion. In addition, it also has potential superiority on low backscatter without supplementary technology. Second, we study the laser arrangement and constraints of the octahedral hohlraums. As a result, $\theta^{\mathrm{L}}$50$^{\circ}$ to 60$^{\circ}$, the injection angle of laser beams, is proposed as the optimum candidate range for the octahedral hohlraums. Third, we propose a novel octahedral hohlraum with LEH shields and cylindrical LEHs, in order to increase the laser coupling efficiency and improve the capsule symmetry and to mitigate the influence of the wall blowoff on laser transport. Finally, we study the sensitivity of capsule symmetry inside the octahedral hohlraums to laser power balance, pointing accuracy, deviations from the optimal position and target fabrication accuracy. [Preview Abstract] |
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