Vlasov simulations of kinetic enhancement of Raman backscatter in laser fusion plasmas

Abstract

Stimulated Raman scattering (SRS) is studied in plasmas relevant to inertial confinement fusion (ICF). The Eulerian Vlasov-Maxwell code ELVIS was developed and run for this purpose. Plasma waves are heavily Landau damped in the regimes of interest, and coupled-mode theory predicts back-scattered SRS is a convective instability. Simulations in a finite length, homogeneous plasma show electron trapping drastically elevates the reflected light over convective gain values ("kinetic enhancement"). Average reflectivities are [approx.] 10%, while the instantaneous reflectivity is chaotic and does not reach a steady state. Trapping reduces the plasma-wave Landau damping and downshifts the observed frequencies from their linear values. Two longitudinal acoustic (? ? k) features and light from possible stimulated electron acoustic scattering (SEAS) are present. The phase-matched SEAS plasmon lies on the observed acoustic mode with phase velocity 1.3(Te/me)1/2. As the pump laser intensity is increased or the electron temperature is decreased, SRS transitions sharply from the coupled-mode steady state to kinetically enhanced levels. Enhancement happens for different back SRS seed levels and monochromatic or broadband seeds. Simulations with a Krook relaxation operator to mimic speckle sideloss display enhancement when resonant electrons complete a bounce orbit before escaping, with a sharp onset as the relaxation rate varies. The sudden development of kinetic enhancement as parameters change suggests trapping makes SRS absolutely unstable.

(cont.) Simulations with mobile ions give kinetic enhancement until a burst of activity occurs near the laser entrance, after which back SRS is low. The burst contains several Brillouin and Raman re-scatters and subsequent Langmuir decay instability (LDI), although no LDI of back SRS is seen. SRS runs in a density gradient show kinetic enhancement for long scale lengths and coupledmode convective levels for shorter ones. The reflectivity is higher when the pump propagates toward higher, rather than lower, density. The amplitude of externally-driven plasma waves in a density gradient is also enhanced over linear levels and displays a similar directional asymmetry. These results imply kinetic enhancement of SRS may be a concern in hohlraum plasmas for ICF experiments such as the National Ignition Facility.