Planar Laser--Plasma Interaction Experiments at Direct-Drive Ignition-Relevant Scale Lengths at the National Ignition Facility

The first experiments at the National Ignition Facility to probe laser--plasma interactions and the hot electron production at scale lengths relevant to direct-drive ignition are reported. The irradiation on one side of planar CH foils generated a plasma at the quarter-critical surface with predicted density scale lengths of $L_{\mbox{n}} \sim 600\mbox{\thinspace }\mu \mbox{m,}$ measured electron temperatures of $T_{\mbox{e}} \sim 3.5$ to 4.0 keV, and overlapped laser intensities of $I\sim 6$ to $15\times 10^{14}{\mbox{\thinspace W}} \mathord{\left/ {\vphantom {{\mbox{\thinspace W}} {\mbox{cm}^{2}}}} \right. \kern-\nulldelimiterspace} {\mbox{cm}^{2}}.$ Optical emission from stimulated Raman scattering (SRS) and at $\omega $/2 are correlated with the time-dependent hard x-ray signal. The fraction of laser energy converted to hot electrons increased from $\sim 0.5\% $~to $\sim 2.3\% $ as the laser intensity increased from $\sim 6$ to $15\times 10^{14}{\mbox{\thinspace W}} \mathord{\left/ {\vphantom {{\mbox{\thinspace W}} {\mbox{cm}^{2}}}} \right. \kern-\nulldelimiterspace} {\mbox{cm}^{2}},$ while the hot electron temperature was nearly constant around 40 to 50 keV. Only a sharp red-shifted feature is observed around $\omega $/2, and both refracted and sidescattered SRS are detected, suggesting that multibeam SRS contributes to, and may even dominate, hot-electron production. These results imply a diminished presence of two-plasmon decay relative to SRS at these conditions, which has implications for hot-electron preheat mitigation strategies for direct-drive ignition. This work is supported by the DOE NNSA under Award Number DE-NA0001944.