Suppressing stimulated Raman scattering from small scale hohlraums with high power and energy laser beams*

Millimeter-scale, vacuum gold hohlraums illuminated with 200TW of laser power and 400 kJ laser energy have proven useful as high fluence X-ray sources.[1] Remarkably, little stimulated Brillouin (SBS) nor Raman (SRS) scattering and excellent coupling are observed. Both simulations and experiments suggest that laser self-focusing is benign although the hohlraums fill to electron densities above one-fourth of the critical density. This work concentrates on the SRS spectrum and the hot electrons accelerated by the electron plasma waves(EPW) driven by the beat ponderomotive force of the laser and backscattered SRS light. Because of the high electron temperature (Te > 10 keV),  k_epwλ_de is large enough to strongly damp the EPWs except near the quarter-critical surface. Here, k_epw  is the wavenumber of the EPW and λ_de is the electron Debye length. The measured SRS frequency spectrum is consistent with ω ∼ ω₀/2. The Vlasov code, LOKI, is used to obtain both the SRS frequency spectrum and, self-consistently, the electron distribution function modified by the high phase velocity ( v_ph ∼ 0.4c) electron plasma waves where c is the speed of light. The nonuniform electron density and temperature profiles are obtained from the radiation-hydrodynamic simulations of the experiments. For the high phase velocities, relativistic treatments are important and used in LOKI  to obtain the correct EPW frequency and damping rates.[2] The simulated electron distributions are used to explain the measured hard X-rays. M. J. May, et al, Bull Am. Phys. Soc., 2022; J. Gorman, et al, Bull Am. Phys. Soc., 2022;