Observation and modelling of Stimulated Raman Scattering driven by an optically smoothed laser beam in experimental conditions relevant for Shock Ignition

TL;DR

This study investigates stimulated Raman scattering in laser-plasma interactions relevant for Shock Ignition, showing significant backscattering at low densities and explaining the absence of TPD/SRS at higher densities through pump depletion and filamentation effects.

Contribution

It provides experimental measurements and a validated analytical model for SRS behavior in conditions pertinent to inertial confinement fusion, highlighting the role of speckle saturation and plasma smoothing.

Findings

Significant SRS backscattering (4-20%) at low plasma densities.

No TPD/SRS signatures at quarter critical density due to pump depletion.

Modeling accurately reproduces SRS onset and plasma conditions.

Abstract

We report results and modelling of an experiment performed at the TAW Vulcan laser facility, aimed at investigating laser-plasma interaction in conditions which are of interest for the Shock Ignition scheme to Inertial Confinement Fusion, i.e. laser intensity higher than 10^16 W/cm2 impinging on a hot (T > 1 keV), inhomogeneous and long scalelength preformed plasma. Measurements show a significant SRS backscattering (4 - 20% of laser energy) driven at low plasma densities and no signatures of TPD/SRS driven at the quarter critical density region. Results are satisfactorily reproduced by an analytical model accounting for the convective SRS growth in independent laser speckles, in conditions where the reflectivity is dominated by the contribution from the most intense speckles, where SRS gets saturated. Analytical and kinetic simulations well reproduce the onset of SRS at low plasma densities in a regime strongly affected by non linear Landau damping and by filamentation of the most intense laser speckles. The absence of TPD/SRS at higher densities is explained by pump depletion and plasma smoothing driven by filamentation. The prevalence of laser coupling in the low density profile justifies the low temperature measured for hot electrons (7 - 12 keV), well reproduced by numerical simulations.