LLE Review 141

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Highlights

This volume of the LLE Review, covering October–December 2014, features “Direct Observation of the Two-Plasmon–Decay Common Plasma Wave Using Ultraviolet Thomson Scattering.” This article reports the use of a 263‑nm Thomson-scattering beam to directly probe common two-plasmon–decay (TPD) electron plasma waves (EPW’s) driven by between two and five 351-nm laser beams. When probing quarter-critical densities (n_c/4) for 351-nm light, a narrow high-intensity scattering feature was observed at a wavelength consistent with the maximum growth rate given by the linear TPD theory. When not wave-matching common plasma waves, the Thomson-scattering spectrum obtained from n_c/4 shows broad TPD-driven EPW’s, indicative of nonlinear effects associated with TPD saturation. Electron plasma waves corresponding to Langmuir decay of backscattered TPD EPW’s were observed in both spectra, suggesting the Langmuir decay instability as a TPD saturation mechanism. Simulated Thomson-scattering spectra from three-dimensional (3-D) numerical solutions of the extended Zakharov equations of TPD are in excellent agreement with the experimental spectra and verify the presence of the Langmuir decay instability.

Additional highlights of research presented in this issue include the following:

• Measurements of the conduction zone length (110±20 µm at t = 2.8 ns), the averaged mass ablation rate of the CD (7.95±0.3 µg/ns), shell trajectory, and laser absorption were used to quantify the electron thermal transport through the conduction zone in direct-drive cryogenic implosions. Hydrodynamic simulations that use nonlocal thermal transport and cross-beam energy transfer models reproduce these experimental observables. Hydrodynamic simulations that use a time-dependent flux-limited model reproduce the measured shell trajectory and the laser absorption, but they overestimate the mass ablation rate by ~10% and underestimate the length of the conduction zone by nearly a factor of 2.
• The first experiments to study the energetics and preheat in polar direct drive (PDD) implosions at the National Ignition Facility (NIF) are presented. These experiments utilize the NIF in its current configuration, including beam geometry, phase plates, and beam smoothing. Results from these initial experiments are presented, including measurements of shell trajectory, implosion symmetry, and the level of hot-electron preheat in plastic and Si ablators. Experiments are simulated with the 2-D hydrodynamics code DRACO including a full 3-D ray trace to model oblique beams and models for nonlocal electron transport and cross-beam energy transport (CBET). These simulations indicate that CBET affects the shell symmetry and leads to a loss of energy imparted onto the shell, consistent with the experimental data.
• The comprehensive knowledge of the properties of high-energy-density plasmas that is crucial to understanding and designing low-adiabat, inertial confinement fusion (ICF) implosions through hydrodynamic simulations is presented. Warm-dense-matter (WDM) conditions are routinely accessed by low-adiabat ICF implosions, in which strong coupling and electron degeneracy often play an important role in determining the properties of warm dense plasmas. The WDM properties of deuterium–tritium (DT) mixtures and ablator materials, such as the equation of state (EOS), thermal conductivity, opacity, and stopping power, were usually estimated by models in hydrocodes used for ICF simulations. To examine the accuracy of the standard models, we have systematically calculated the static, transport, and optical properties of warm dense DT plasmas, using first-principles (FP) methods over a wide range of densities and temperatures that cover the ICF “path” to ignition. These FP methods include the path-integral Monte Carlo (PIMC) and quantum-molecular dynamics (QMD) simulations, which treat electrons with many-body quantum theory. This article focuses on the combined effects on ICF implosions through hydro-simulations using these FP-based properties of DT in comparison with the usual model simulations. It was found that the predictions of ICF neutron yield could change by up to a factor of ~2.5. This research shows that the lower the adiabat of DT capsules, the more variations in hydro-simulations. The FP-based properties of DT are essential for designing ICF ignition targets.
• Recent experiments on the OMEGA laser that were carried out to produce strong shocks in solid spherical targets with direct laser illumination are presented. The shocks are launched at pressures of several hundred Mbars and reach Gbar pressures upon convergence. The results are relevant to the validation of the shock-ignition scheme and to the development of an OMEGA experimental platform to study material properties at Gbar pressures. It was found that the hot-electron temperature was moderate (<100 keV) and the instantaneous conversion efficiencies of laser power into hot-electron power reached ~15% in the intensity spike.
• The migration of tritium to the surfaces of aluminum 6061, oxygen-free, high-conductivity copper, and stainless-steel 316 from the bulk metal using low-pressure Tonks–Langmuir argon plasma is discussed. The plasma is shown to be effective at removing tritium from metal surfaces in a controlled manner. Tritium is removed in decreasing quantities with successive plasma exposures, which suggests a depletion of the surface and near-surface tritium inventories.