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 JO4: Direct- and Polar Direct-Drive |
Hide Abstracts |
Chair: Brian Spears, Lawrence Livermore National Laboratory Room: Salon E |
Tuesday, October 28, 2014 2:00PM - 2:12PM |
JO4.00001: Design of a Polar-Drive, Alpha-Heating Platform for the National Ignition Facility T.J.B. Collins, J.A. Marozas, J.A. Delettrez, P.W. McKenty, S. Skupsky, D. Cao, J. Chenhall, G. Moses Polar drive (PD)\footnote{ S. Skupsky \textit{et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} allows one to conduct direct-drive--ignition experiments at the National Ignition Facility (NIF) while the facility is configured for x-ray drive. A PD-ignition design has previously been developed.\footnote{ T. J. B. Collins \textit{et al.}, Phys. Plasmas \textbf{19}, 056308 (2012).} A new, robust PD design has been developed with the goal of achieving alpha-heating and deuterium--tritium yields in excess of 10$^{16}$ neutrons at the NIF with the final optics and direct-drive cryogenic target positioner intended for subsequent PD-ignition experiments. This design uses a higher fuel adiabat, which precludes scaling to ignition but results in greater stability and experimental control, minimizing fuel--shell mix during the deceleration phase of the implosion. The new design also incorporates the effects of cross-beam energy transfer and nonlocal electron transport. This platform will make it possible to test radiation--hydrodynamic codes in preparation for PD-ignition experiments. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 28, 2014 2:12PM - 2:24PM |
JO4.00002: Achieving Symmetry with Polar Direct Drive N. Krasheninnikova, T. Murphy, J. Cobble, I. Tregillis, P. Bradley, P. Hakel, S. Hsu, G. Kyrala, K. Obrey, M. Schmitt, R. Kanzleiter, J. Baumgaertel, S. Batha Direct Drive, widely used on Omega, provides high coupling energy and core temperatures per drive. NIF's much higher power offers a prospect for attaining hotter, larger cores enabling higher fidelity burn experiments. To use Omega's knowledge on NIF involves understanding the differences between PDD and SDD. Achieving symmetric implosions in PDD is essential for attaining high temperatures and neutron yields. LANL team tested laser cone-power tuning designs done with rad-hydro code HYDRA utilizing a flux-limited heat conduction (FLHC) model on NIF and Omega. Both campaigns produced symmetric implosions in PDD configuration. Omega campaign confirmed P$_{2}$ tunability that was in agreement with the simulations, while in experiments on NIF \textbar P$_{2}$\textbar \textless 3{\%} was sustained for 100's ps at NBT (similar to indirect drive and SDD). However, we need to recognize the role of LPI effects which are often left out in simulations. We found that when I\textgreater 10$^{15}$W/cm$^{2}$ on NIF, FLHC model in HYDRA was insufficient to accurately predict symmetry, bright equatorial self-emission band, and enhanced hot electron population. We were able to account for these effects by including CBET and non-local heat transfer models. Here we present out analysis of PDD symmetry data. We report on hot electron and CBET effects and assess our ability to model them with rad-hydro codes. We will also discuss laser intensity limits in PDD. [Preview Abstract] |
Tuesday, October 28, 2014 2:24PM - 2:36PM |
JO4.00003: Determining Acceptable Limits of Fast-Electron Preheat in Polar-Drive--Ignition Designs J.A. Delettrez, T.J.B. Collins, C. Ye In direct-drive--ignition designs, preheat by fast electrons created by the two-plasmon--decay instability at the quarter-critical density surface can increase the adiabat in the fuel layer and prevent ignition. Since eliminating the preheat entirely is not possible, it is necessary to understand the levels of preheat our targets can withstand before ignition is precluded. The current polar-drive point design is used as the basis for examining the effects of increasing the levels of fast electrons using the one-dimensional, radiation--hydrodynamics code \textit{LILAC}. Once ignition failure is obtained, the design is then reoptimized using \textit{Telios}, a downhill simplex method program, to recover ignition. This cycle is repeated until the design can no longer be reoptimized to produce ignition. Mappings of these final results provide insight into ignition failure caused by preheat and what specific target parameters serve to best stave off the effects of the preheat. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 28, 2014 2:36PM - 2:48PM |
JO4.00004: Using the outgoing shock wave to characterize low mode ablator symmetry Arthur Pak, Laurent Divol, Tilo Doeppner, John Field, Andrea Kritcher, Tammy Ma, Laura Benedetti, Debbie Callahan, Denise Hinkel, Laura Berzak Hopkins, Omar Hurricane, Shahab Khan, Nobuhiko Izumi, Andrew Mackinnon, Nathan Meezan, Brian Spears, Richard Town, David Bradley At the National Ignition Facility, experiments are being conducted to optimize the performance of indirectly driven inertial confinement fusion implosions. To ignite the fuel in this scheme, a cascade of nuclear reactions must first be triggered by achieving a central hot spot pressure of several hundred Gbar over a duration $\sim$ 100 ps. Low mode asymmetries in the shape of the assembled fuel and ablator are indicative of momentum asymmetries that reduce the transfer of kinetic energy to hot spot internal energy, thus reducing the hot spot yield and overall implosion performance. Here details of a new method that utilizes the x-ray emission created by the outgoing shock to probe the low mode asymmetry of the ablator at radius of 100 $\mu$m and time of $\sim$ 150 ps after stagnation will be presented. This signal can provide information on the shape of the ablator at convergence ratios $\sim$ 2X higher than current area-backlight radiographs and can be made in-situ on layered cryogenic implosions that require all the laser beams. Experimental results of the inferred ablator shape from implosions performed with cryogenic thermonuclear fuel will be compared to radiation hydrodynamic calculations. [Preview Abstract] |
Tuesday, October 28, 2014 2:48PM - 3:00PM |
JO4.00005: Inferring Low-Mode Asymmetries from the Elastically Scattered Neutron Spectrum in Layered Cryogenic DT Implosions on OMEGA C.J. Forrest, V.Yu. Glebov, V.N. Goncharov, T.C. Sangster, C. Stoeckl, J.A. Frenje, M. Gatu Johnson High-resolution neutron spectroscopy is used to probe the areal density of layered cryogenic DT direct-drive implosions in inertial confinement fusion experiments on OMEGA. Advanced scintillation detectors record the neutron spectrum using time-of-flight techniques. The shape of the energy spectrum is fully determined by the neutron elastic scattering cross-section for spherically symmetric target configurations. Significant differences from the expected shape have been measured for some recent implosions, which indicate a deviation from a spherically symmetric fuel assembly. Neutron scattering with low-mode perturbations in the DT fuel assembly have been simulated in the Monte Carlo n-particle transport code. The experimental data shows good agreement with the model when the mass distribution of the compressed DT shell is highly asymmetric with one side having a factor-of-2 higher areal density. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 28, 2014 3:00PM - 3:12PM |
JO4.00006: Understanding the Performance of Low-Adiabat Cryogenic Implosions on OMEGA V.N. Goncharov, T.C. Sangster, R. Epstein, S.X. Hu, I.V. Igumenshchev, C.J. Forrest, D.H. Froula, F.J. Marshall, D.T. Michel, P.B. Radha, W. Seka, C. Stoeckl, J.A. Frenje, M. Gatu Johnson While the moderate-adiabat ($\alpha $ \textgreater 3.5) cryogenic implosions on OMEGA are well understood using multidimensional hydrocode simulations, the performance of lower-adiabat implosions is degraded relative to code predictions. The potential degradation mechanisms (not fully accounted for in simulations) include target-nonuniformity sources (excessive laser imprint, target debris, beam-overlap nonuniformity) and inaccuracies in laser-coupling modeling, especially during the pulse rise. To address the target-stability issues, target designs with thicker ice layers and smaller implosion velocities are considered. These targets have smaller in-flight aspect ratios, making them less susceptible to hydrodynamic instability growth. To address inaccuracies in laser coupling, a design with a slower main pulse rise is considered. This talk will summarize progress made on these issues. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 28, 2014 3:12PM - 3:24PM |
JO4.00007: Fuel--Shell Mix and Pressure Measurements Based on X-Ray Continuum Emission from Isobaric Implosion Cores on OMEGA R. Epstein, F.J. Marshall, V.N. Goncharov, R. Betti, R. Nora, A.R. Christopherson At a spectral energy matched to the anticipated hot-spot temperature range, the x-ray emissivity of an imploded target hot spot is dependent almost entirely on pressure. In this way, the hot-spot pressure at the time of peak emission can be inferred from the spatially resolved core emission. The pressure and temperature dependences of the x-ray emissivity and the neutron production rate explain a simple scaling of the total filtered x-ray emission as a power of the total neutron yield for target implosions of similar design over a broad range of shell implosion adiabats. Excess emission from less-stable, low-adiabat implosions (above the level expected from this neutron-yield scaling) attributed to the higher emissivity of shell carbon mixed into the hot spot, indicates ``fuel--shell'' mix fractions in the 2{\%} to 5{\%} range. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944 and the Office of Fusion Energy Sciences Number DE-FG02-04ER54786. [Preview Abstract] |
Tuesday, October 28, 2014 3:24PM - 3:36PM |
JO4.00008: Analysis of a High-Adiabat Cryogenic Implosion on OMEGA A.R. Christopherson, R. Betti, R. Nora, R. Epstein, F.J. Marshall, C.J. Forrest, C. Stoeckl, J.A. Delettrez, P.B. Radha, J. Howard The performance of high-adiabat implosions $\agt$ 10 is marginally affected by nonuniformities because of the strong ablative stabilization. To test the validity of the one-dimensional (1-D) physics included in existing hydrocodes, a study of high-adiabat cryogenic DT implosions is carried out by comparing the results of 1-D simulations with several measured quantities. It is found that after including nonlocal transport, cross-beam energy transfer, and hot electrons, 1-D simulations reproduce most of the observables with reasonable accuracy. Since the analysis is applied to the only high-adiabat DT implosion fielded on OMEGA, these results do not fully validate the 1-D physics of current hydrocodes. However, this work shows the framework for establishing a validation capability of the 1-D physics of inertial confinement fusion implosions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944 and the Office of Fusion Energy Sciences Number DE-FG02-04ER54786. [Preview Abstract] |
Tuesday, October 28, 2014 3:36PM - 3:48PM |
JO4.00009: Constraining the Rocket Efficiency in Hydrodynamic Simulations of Direct-Drive Cryogenic Implosions by Simultaneous Measurements of the CD Burnthrough and the Shell Trajectory D.T. Michel, A.K. Davis, R. Epstein, V.N. Goncharov, S.X. Hu, I.V. Igumenshchev, D.D. Meyerhofer, T.C. Sangster, D.H. Froula Time-resolved imaging of the soft x rays emitted by the coronal plasma of a directly driven imploding cryogenic target on the OMEGA Laser System is used to measure the shell trajectory and the time to ablate the outer CD layer. These simultaneous measurements constrain both the shell velocity and the mass ablation rate. Two simulations have been performed and compared to the measurements: (1) including cross-beam energy transfer (CBET) and nonlocal thermal transport models and (2) using a flux limiter adapted to match the measured shell trajectory. Good agreement with both the trajectory and mass ablation rate is found with CBET and nonlocal models. While the modified flux limiter matches the trajectory (by construction), the CD burnthrough occurs $\sim 200$ ps later than in experiments. This demonstrates that by adapting a flux limiter, both the shell velocity and the mass ablation rate cannot be reproduced simultaneously. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 28, 2014 3:48PM - 4:00PM |
JO4.00010: Simulations of laser-driven targets with thin high-Z coatings Andrew J. Schmitt, Max Karasik, Jason Bates, Steve Obenschain Previous theoretical and experimental work at NRL\footnote{S.P. Obenschain \textit{et al.}, Phys. Plasmas \textbf{9}, 2234 (2002).} has shown that very thin (100's of {\AA}) of high-Z --- e.g., Au or Pd --- layers coated onto targets can be used to suppress early-time laser imprint and RM growth of hydrodynamic instabilities during the low-intensity foot of directly-driven targets. This work has been extended recently\footnote{M. Karasik, \textit{et al.}, to be published} to include the use of higher intensity laser spikes that are used for adiabat-tailoring of the target. In these studies, it was shown that a minimum layer thickness (dependent upon the material) was needed before the suppression was observed. Additionally, it was observed that the condition of the layer prior to the drive laser pulse can be crucial to the accurate simulation of the physics. We will address here the physics behind the imprint suppression effects and explore the limitations and sensitivities of modeling these systems. The implications and limits of using even thicker layers to extend the effect further into the laser drive will also be discussed. [Preview Abstract] |
Tuesday, October 28, 2014 4:00PM - 4:12PM |
JO4.00011: Benefits of Moderate-\textit{Z} Ablators for Direct-Drive Inertial Confinement Fusion M. Lafon, R. Betti, K.S. Anderson, T.J.B. Collins, S. Skupsky, P.W. McKenty Control of hydrodynamic instabilities and DT-fuel preheating by hot electrons produced by laser--plasma interaction is crucial in inertial confinement fusion. Moderate-$Z$ ablators have been shown to reduce the laser imprinting on target and suppress the generation of hot electrons from the two-plasmon--decay instability. These results have motivated the use of ablators of higher-$Z$ than pure plastic in direct-drive--ignition target designs for the National Ignition Facility (NIF). Two-dimensional radiation--hydrodynamic simulations assess the robustness of these ignition designs to laser imprint and capsule nonuniformities. The complex behavior of the hydrodynamic stability of mid-$Z$ ablators is investigated through single and multimode simulations. A polar-drive configuration is developed within the NIF Laser System specifications for each ablator material. The use of multilayer ablators is also investigated to enhance the hydrodynamic stability. Results indicate that ignition target designs using mid-$Z$ ablators exhibit good hydrodynamic properties, leading to high target gain for direct-drive implosions on the NIF. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944 and the Office of Fusion Energy Sciences Number DE-FG02-04ER54786. [Preview Abstract] |
Tuesday, October 28, 2014 4:12PM - 4:24PM |
JO4.00012: Imprinting of Pre-Imposed Laser Perturbations on Targets With a High-Z Overcoat Max Karasik, J.L. Weaver, Y. Aglitskiy, J. Oh, A.J. Schmitt, J.W. Bates, V. Serlin, S.P. Obenschain In direct drive ICF, most of the laser imprint is expected to occur during the initial part of the laser pulse, which generates the first shocks necessary to compress the target to achieve high gain. Previous experiments found that a thin (400--800{\AA}) high-{Z} (Au or Pd) overcoat on the laser side of the target is effective in suppressing broadband imprint\footnote{Obenschain et al., Phys. Plasmas 9, 2234 (2002)}$^,$\footnote{Karasik et al., submitted for publication.} The overcoat initially absorbs the laser and emits soft x-rays that ablate the target, forming a large stand-off distance between laser absorption and ablation and smoothing the drive perturbations. We investigate the effectiveness of imprint suppression for different spatial wavelengths via perturbations imposed on top of the beams smoothed by Induced Spatial Incoherence (ISI). Measurements of areal mass non-uniformity on planar targets driven by the Nike KrF laser are made by curved crystal x-ray radiography. Simultaneous side-on radiography allows observation of the layer dynamics and monitoring of the laser absorption - target ablation stand-off. X-ray flux from the high-Z layer is monitored using absolutely calibrated time-resolved x-ray spectrometers. Work supported by the Department of Energy/NNSA. [Preview Abstract] |
Tuesday, October 28, 2014 4:24PM - 4:36PM |
JO4.00013: Target Performance in Pd-Overcoated Spherical OMEGA Implosions P.B. Radha, C. Stoeckl, G. Fiksel, V.N. Goncharov, S.X. Hu, J.P. Knauer, D.T. Michel, T.C. Sangster, W. Seka Improved yields in implosions of plastic (CH) shell targets overcoated with a thin (approximately a few hundred angstroms) of Pd have been measured in OMEGA implosions.\footnote{A. N. Mostovych \textit{et al}., Phys. Rev. Lett. \textbf{100}, 075002 (2008).} Implosions with triple-picket pulses and room-temperature, Pd-overcoated CH shells, where the in-flight aspect ratio (IFAR) has been varied between 19 and 28, are studied on the OMEGA laser. Marginal improvement in yield is found for the lower-IFAR implosions, whereas the higher-IFAR, ignition-relevant implosions show no improvement. Simulations of scattered light, trajectories, bang-time, areal densities, and time-resolved x-ray spectra are compared to experiments. Progress in understanding the role of imprint in target performance in OMEGA implosions is presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 28, 2014 4:36PM - 4:48PM |
JO4.00014: Measurement of the Si Mass Ablation Rate in Direct-Drive Implosions on the OMEGA Laser System A.K. Davis, D.T. Michel, I.V. Igumenshchev, R.S. Craxton, R. Epstein, V.N. Goncharov, S.X. Hu, T.C. Sangster, D.H. Froula The Si mass ablation rate in direct-drive inertial confinement fusion implosions was measured using a pinhole x-ray framing camera on the OMEGA Laser System. In targets consisting of a Si layer over a CH layer, two x-ray self-emission peaks from the coronal plasma were measured once the laser burned through the higher-$Z$ outer layer. The location of the inner peak is related to the position of the ablation front and the location of the outer peak corresponds to the position of the interface of the two layers. The emergence of the interface peak was used to measure the burnthrough time of the outer layer, giving its average mass ablation rate. By repeating this experiment for different outer-layer thicknesses, time-resolved measurements of the mass ablation rate were obtained. Simulations validated the methods and verified that the measurement techniques are not sensitive to perturbation growth at the ablation surface. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944. [Preview Abstract] |
Tuesday, October 28, 2014 4:48PM - 5:00PM |
JO4.00015: ABSTRACT WITHDRAWN |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700