Hot-electron generation and preheat in direct-drive experiments at the National Ignition Facility

Laser–plasma instabilities, such as stimulated Raman scattering, can degrade the performance of direct-drive implosions by generating hot electrons that preheat the target. To assess the extent of hot-electron generation at ignition scales and conditions, planar and spherical target NIF experiments have been designed using the radiation–hydrodynamic codes DRACO and LILAC. Planar-target experiments, with T_e ~5 keV and density gradient scale lengths of L_n ~600 μm, exhibited hot-electron temperatures of ~50 keV and conversion efficiencies ranging from ~0.5 to 3% as the laser intensity at the quarter-critical surface increased from ~6 to 15 ×10¹⁴ W/cm². Spherical target experiments will be fielded in September 2018. The target will consist of an outer plastic ablator and an inner Ge-doped plastic layer. The difference in hard x-ray signals between the mass-equivalent plastic and multilayer implosions and the Ge K_a emission will be used to infer the hot-electron energy deposition in the unablated shell. The experiments will demonstrate how the divergence of hot electrons and the extent to which they slow down in the ablator reduce the preheat.