Modeling Laser--Plasma Interactions at Direct-Drive Ignition-Relevant Plasma Conditions at the National Ignition Facility

Laser--plasma interaction instabilities, such as two-plasmon decay (TPD) and stimulated Raman scattering (SRS), can be detrimental for direct-drive inertial confinement fusion because of target preheat by generated high-energy electrons. The radiation--hydrodynamics code \textit{DRACO} has been used to design planar-target experiments that generate plasma and interaction conditions relevant to direct-drive--ignition designs ($I_{\mbox{L}} \sim 10^{15}{\mbox{W}} \mathord{\left/ {\vphantom {{\mbox{W}} {\mbox{cm}}}} \right. \kern-\nulldelimiterspace} {\mbox{cm}}^{2},\mbox{\thinspace }T_{\mbox{e}} >3\mbox{\thinspace keV},$ density gradient scale lengths of $L_{\mbox{n}} \sim 600\,\mu \mbox{m})$. The hot-electron temperature of $\sim 40\mbox{\thinspace to\thinspace 50\thinspace keV}$ and the fraction of laser energy converted to hot electrons of $\sim 0.5\mbox{\thinspace to\thinspace 2.3\% }$ were inferred based on comparing the simulated and experimentally observed x-ray emission when the laser intensity at the quarter-critical surface increased from $\sim 6\mbox{\thinspace to\thinspace }15\mbox{\thinspace }\times \mbox{\thinspace }10^{14}{\mbox{W}} \mathord{\left/ {\vphantom {{\mbox{W}} {\mbox{cm}^{2}}}} \right. \kern-\nulldelimiterspace} {\mbox{cm}^{2}}.$ The measured SRS energy was sufficient to explain the observed total energy in hot electrons. Implications for ignition-scale direct-drive experiments and hot-electron preheat mitigation using mid-$Z$ ablators will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.