Hot-electron preheat and energy deposition in polar-direct-drive experiments at the National Ignition Facility

Laser--plasma instabilities (LPI's) can degrade the performance of direct-drive inertial confinement fusion implosions by generating hot electrons that preheat the target. To assess the extent of hot-electron preheat, polar-direct-drive experiments have been performed at the National Ignition Facility to study the hot-electron energy deposition in an unablated shell. The experiments employed mass-equivalent plastic targets with Ge-doped layers to measure the radial energy deposition profile of hot electrons in the unablated capsule. Hot-electron properties and energy deposition have been inferred through comparisons of hard x-ray spectra to simulations of electron transport and x-ray generation. Recent experiments used thin silicon layers in the outer portion of the ablators, designed to pass through the quarter-critical region during the period of hot-electron generation in order to suppress LPI. Analysis indicates a reduction in hot-electron generation by a factor of $\sim $2, showing promise as a preheat-mitigation strategy that can expand the ignition-design space to higher intensity.