Hot electron preheat in hydrodynamically scaled direct-drive inertial confinement fusion implosions on the NIF and OMEGA

Abstract

Hot electron preheat has been quantified in warm, directly driven inertial confinement fusion implosions on OMEGA and the National Ignition Facility (NIF), to support hydrodynamic scaling studies. These CH-shell experiments were designed to be hydrodynamically equivalent, spanning a factor of 40 in laser energy and a factor of 3.4 in spatial and temporal scales, while preserving the incident laser intensity of 1015 W/cm2. Experiments with similarly low levels of beam smoothing on OMEGA and NIF show a similar fraction (∼0.2%) of laser energy deposited as hot electron preheat in the unablated shell on both OMEGA and NIF and similar preheat per mass (∼2 kJ/mg), despite the NIF experiments generating a factor of three more hot electrons (∼1.5% of laser energy) than on OMEGA (∼0.5% of laser energy). This is plausibly explained by more absorption of hot electron energy in the ablated CH plasma on NIF due to larger areal density, as well as a smaller solid angle of the imploding shell as viewed from the hot electron generating region due to the hot electrons being produced at a larger standoff distance in lower-density regions by stimulated Raman scattering, in contrast to in higher-density regions by two-plasmon decay on OMEGA. The results indicate that for warm implosions at intensities of around 1015 W/cm2, hydrodynamic equivalence is not violated by hot electron preheat, though for cryogenic implosions, the reduced attenuation of hot electrons in deuterium–tritium plasma will have to be considered.