Hot-Electron Preheat and Mitigation in Polar-Direct-Drive Inertial Confinement Fusion Implosions at the National Ignition Facility

Target preheat by superthermal electrons from laser–plasma instabilities is a major challenge for direct-drive inertial confinement fusion. Polar-direct-drive surrogate plastic implosion experiments were performed at the National Ignition Facility (NIF) to quantify preheat levels at ignition-relevant scale and develop mitigation strategies. The experiments were used to infer the hot-electron temperature, energy fraction, divergence, and to directly measure the radial hot-electron energy deposition profile inside the imploding shell. This was achieved employing mass-equivalent plastic targets with inner Ge-doped layers and comparing the measured hard x-ray spectra to simulations of the target implosion, electron transport, and x-ray emission. The hot-electron coupling to the unablated shell was found to increase from 0.2% to 0.6% of the laser energy when the incident laser intensity was increased from 0.75 to 1.25x10¹⁵ W/cm², with half of the preheat coupled to the inner 80% of the unablated shell. It is shown that a thin mid-Z Si layer buried in the ablator or a Si-doped outer layer strongly mitigate stimulated Raman scattering, which is responsible for hot-electron generation at direct-drive ignition-relevant conditions. The preheat is effectively suppressed using a Si layer at intensity of 7.5x10¹⁴ W/cm² and reduced by a factor of ~2 at higher intensities. The higher laser absorption efficiency is another beneficial effect of Si. The Si layer should be kept thin to minimize the radiation preheat and utilize the larger ablation efficiency of the innermost lower-Z layer in the ablator. This provides a promising hot-electron preheat-mitigation strategy that can expand the ignition design space to higher intensity. The preheat can be acceptable in future cryogenic DT, ignition-scale implosions on the NIF for on-target intensities close to 10¹⁵ W/cm².