Stimulated Raman scattering mechanisms and scaling behavior in planar direct-drive experiments at the National Ignition Facility

Stimulated Raman scattering (SRS) has been explored comprehensively in planar-geometry experiments at the National Ignition Facility in conditions relevant to the corona of inertial confinement fusion ignition-scale direct-drive targets. These experiments at measured electron temperatures of 4 to 5 keV simulated density scale lengths L n of 400 to 700 μm, and laser intensities at the quarter-critical density of up to 1.5 × 1015 W/cm2 have determined SRS thresholds and the scaling behavior of SRS for various beam geometries. Several SRS mechanisms, including saturated absolute SRS near the quarter-critical density and additional SRS, including near-backscatter or sidescatter at lower densities, have been identified. Correlation of time-dependent SRS at densities ∼0.15 to 0.21 of the critical density with hot-electron signatures as well as the magnitudes of these signatures across different experiments, is observed. Further modeling work is needed to definitively identify the density region in which hot electrons are generated and will guide SRS and hot-electron preheat mitigation strategies for direct-drive-ignition designs.