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

Experiments at the National Ignition Facility have probed laser-plasma interactions and hot-electron production at plasma conditions relevant to direct-drive ignition, with predicted density scale lengths of L_n ~500-700 μm, electron temperatures of T_e ~4-5 keV, and overlapped laser intensities of I ~6-15 × 10¹⁴ W/cm². The fraction of laser energy converted to hot electrons is 1-3%, while the hot-electron temperature is 45-60 keV. Only a sharp red-shifted feature is observed around ω/2, along with significant stimulated Raman scattering (SRS), including sidescattering, at lower densities, suggesting that SRS dominates hot-electron production, unlike in shorter-scale-length plasmas on OMEGA that are dominated by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. Subsequent measurements of 3ω/2 emission have revealed evidence of TPD; upcoming experiments will identify dominant plasma waves. The coupling of hot electrons to an implosion will be measured in new experiments, which will determine the need for preheat mitigation strategies for direct-drive ignition.