Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session KI3: ICF Simulations |
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Chair: George Kyrala, Los Alamos National Laboratory Room: Ballroom AC |
Tuesday, November 15, 2011 3:00PM - 3:30PM |
KI3.00001: A High-Resolution Integrated Model of the NIC Cryogenic Layered Experiments Invited Speaker: We have developed a capability to do very high spatial resolution 2D integrated hohlraum-capsule simulations using the Hydra code. Surface perturbations for all ablator layer surfaces and the DT ice layer are typically calculated explicitly through mode 60, and for some calculations up to mode 100. Separate calculations have shown that mode 60 has the highest growth rate at the ablator-fuel interface. The higher angular resolution also leads to finer zoning in the hohlraum where laser absorption and x-ray production are occurring. The effects of the fill tube, grooves in the ice layer, and surface defects on the ablator are included via models extracted from higher resolution calculations. High wave number mix can be included through a mix model that has been extracted from capsule-only calculations that include up to mode 2000. Measured backscatter and a model for crossbeam energy transfer are included to enable a best estimate of the drive asymmetry for each shot. We have applied this model to National Ignition Campaign (NIC) symmetry capsule and cryogenic layered tritium-hydrogen-deuterium (THD) experiments. We have also included some adjustments to our standard physics models to bring the calculations into better agreement with the experimental measurements from several NIC experimental campaigns. Radiation drive multipliers for the first three shocks were derived to match the experimental shock timing data. The opacity of the Ge-doped plastic was increased, and the opacity of the undoped plastic ablator was decreased in order to match the measured peak shell velocity. We compare the simulated diagnostic signatures extracted from the integrated high-resolution calculations to the measured x-ray and neutron diagnostic signatures from a number of THD experiments in order to assess the fidelity of this model and gain insight into the implosion performance. [Preview Abstract] |
Tuesday, November 15, 2011 3:30PM - 4:00PM |
KI3.00002: Preparing for Polar Drive at the National Ignition Facility Invited Speaker: Polar drive (PD)\footnote{S. Skupsky\textit{ et al.}, Phys. Plasmas \textbf{11}, 2763 (2004).} will make it possible 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 been developed that achieves high gain in simulations including single- and multiple-beam nonuniformities, and ice and outer-surface roughness. Target robustness has been studied by examining the sensitivity of this target to target and drive nonuniformities. The choice of laser pointing, spot shape, and ablator material will also be discussed. This design requires both single-beam UV polarization smoothing and one-dimensional Multi-FM single-beam smoothing to achieve the required laser uniformity. The Multi-FM smoothing is employed only during the pickets, allowing use of sufficient smoothing-by-spectral-dispersion bandwidth while maintaining safe laser operations. Finally, recent experiments on both OMEGA and the NIF using PD will also be presented. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302. \\[4pt] In collaboration with J. A. Marozas, S. Skupsky, P. W. McKenty, V. N. Goncharov, A. Shvydky, F. J. Marshall, T. C. Sangster, R. Epstein (Laboratory for Laser Energetics, Univ. of Rochester). [Preview Abstract] |
Tuesday, November 15, 2011 4:00PM - 4:30PM |
KI3.00003: Trapping induced nonlinear behavior of backward stimulated Raman scattering in multi-speckled laser beams Invited Speaker: In inertial confinement fusion experiments, Stimulated Raman Scattering (SRS) occurs when electron density fluctuations amplify resonantly by the incident laser beams. These beams comprise several thousands of individual laser speckles. We have found in single-speckle studies that electron trapping lowers the threshold intensity for SRS onset to a value below that from linear theory and enhances scattering. The trapping-induced plasma-wave frequency shift leads to wave-front bowing and self-focusing, processes that saturate SRS and limit scattering within a speckle. With Petaflop-class supercomputers, we have now examined how laser speckles interact with one another through 3D particle-in-cell (PIC) simulations of two interacting speckles and 2D PIC simulations of ensembles of laser speckles (100s of speckles). Our work shows that an intense speckle can destabilize its neighbors, resulting in enhanced emission of particles and waves back to the original speckle, thus affecting its behavior in a manner that is nonlinear and nonlocal. Ensembles of speckles are thus found to collectively lower the SRS onset threshold. Simulations of the hohlraum interior where laser beams overlap show that multi-speckled laser beams at low average intensity (a few times $10^{14}$ W/cm$^2$) have correspondingly lower thresholds for enhanced SRS and that SRS saturates through trapping induced nonlinearities (other potential saturation mechanisms are also examined). Results from VPIC simulations and comparison with pF3d results will be discussed that employ inhomogeneous plasma profile taken from rad-hydro modeling of NIF ignition experiments. Implications for experiments at higher laser power will also be discussed. [Preview Abstract] |
Tuesday, November 15, 2011 4:30PM - 5:00PM |
KI3.00004: Nonlinear kinetic modeling of stimulated Raman scattering Invited Speaker: Despite its importance for many applications, such as or Raman amplification or inertial confinement fusion, deriving a nonlinear estimate of Raman reflectivity in a plasma has remained quite a challenge for decades. This is mainly due to the nonlinear modification of the electron distribution function induced by the plasma wave (EPW), which, in turn, modifies the propagation of this wave. In this paper is derived an envelope equation for the EPW valid in 3D and which accounts for the nonlinear change of its collisionless (Landau-like) damping rate, group velocity, coupling to the electromagnetic drive, frequency and wave number. Our theoretical predictions for each of these terms are carefully compared against results from Vlasov simulations of stimulated Raman scattering (SRS), as well as with other theories [1]. Moreover, our envelope model shows to be as accurate as a Vlasov code in predicting Raman threshold in 1D. Making comparisons with experimental results nevertheless requires including transverse dimensions and letting Raman start from noise. To this end, we performed a completely new derivation of the electrostatic fluctuations in a plasma, which accounts nonlinear effects. Moreover, based on our Multi-D simulations of Raman scattering with our envelope code BRAMA [2], we discuss the effect on SRS of wave front bowing [3], transverse detrapping [4] and of a completely new defocussing effect due to the local change in the direction of the EPW group velocity induced by the nonlinear decrease of Landau damping.\\[4pt] [1] N.A. Yampolsky and N.J. Fisch, Phys. Plasmas 16, 072104 (2009).\\[0pt] [2] D. Benisti et al, Phys. Plasmas 17, 102311 (2010).\\[0pt] [3] L. Yin et al., Phys. Plasmas 15, 013109 (2008).\\[0pt] [4] H.A. Rose and D.A. Russel, Phys. Plasmas 11, 4784 (2001). [Preview Abstract] |
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