Laser--Plasma Interaction Experiments at Direct-Drive Ignition-Relevant Plasma Conditions at the National Ignition Facility

Laser--plasma interaction (LPI) instabilities, such as stimulated Raman scattering (SRS) and two-plasmon decay, can be detrimental for direct-drive inertial confinement fusion because of target preheat by the high-energy electrons they generate. The radiation--hydrodynamic code \textit{DRACO} was used to design planar-target experiments at the National Ignition Facility that generated plasma and interaction conditions relevant to ignition direct-drive designs $(I_{\mbox{L}} \sim 10^{15}\mbox{\thinspace W/cm}^{2},$ $T_{\mbox{e}} >3$ keV, density gradient scale lengths of $L_{\mbox{n}} \sim 600\;\mu \mbox{m).}$ Laser-energy conversion efficiency to hot electrons of $\sim $0.5{\%} to 2.5{\%} with temperature of $\sim $45 to 60 keV was inferred from the experiment when the laser intensity at the quarter-critical surface increased from $\sim $6 to $15\times 10^{14}\,\mbox{W/cm}^{2}.$ LPI was dominated by SRS, as indicated by the measured scattered-light spectra. Simulations of SRS using the LPI code \textit{LPSE }have been performed and compared with predictions of theoretical models. Implications for ignition-scale direct-drive experiments will be discussed. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.