Hot-electron preheat and mitigation in polar-direct-drive experiments at the National Ignition Facility
A. A. Solodov, M. J. Rosenberg, Manuel Stoeckl, A. R. Christopherson, R. Betti, P. B. Radha, C. Stöeckl, M. Hohenberger, B. Bachmann, R. Epstein, R. K. Follett, W. Seka, J. F. Myatt, P. Michel, S. P. Regan, J. P. Palastro, D. H. Froula, E. M. Campbell, V. N. Goncharov
Abstract
Target preheat by superthermal electrons from laser-plasma instabilities is a major obstacle to achieving thermonuclear ignition via direct-drive inertial confinement fusion at the National Ignition Facility (NIF). Polar-direct-drive surrogate plastic implosion experiments were performed on the NIF to quantify preheat levels at an ignition-relevant scale and develop mitigation strategies. The experiments were used to infer the hot-electron temperature, energy fraction, and divergence, and to directly measure the spatial hot-electron energy deposition profile inside the imploding shell. Silicon layers buried in the ablator are shown to mitigate the growth of laser-plasma instabilities and reduce preheat, providing a promising path forward for ignition designs at an on-target intensity of about 10^{15}W/cm^{2}.