Hot-Electron Preheat and Mitigation in Polar-Direct-Drive Experiments at the National Ignition Facility

Superthermal electrons generated by laser–plasma instabilities can degrade the performance of direct-drive inertial confinement fusion implosions by preheating the target. Polar-direct-drive experiments were performed at the National Ignition Facility (NIF) to assess the extent of hot-electron preheat, develop mitigation strategies, and extrapolate preheat levels to ignition designs. In these experiments, hot-electron temperature, fraction, divergence, and radial energy deposition profile in the unablated capsule were inferred by employing mass-equivalent plastic targets with Ge-doped layers and comparing the measured hard x-ray spectra to simulations. Silicon layers buried in the ablator were used to mitigate stimulated Raman scattering and reduce preheat. Experiments determine the parameter regime (e.g. intensity) that produces acceptably low preheat. Extrapolating these results to ignition-scale cryogenic DT implosions on the NIF shows that preheat level should be acceptable at on-target intensity close to 10¹⁵ W/cm².