Evidence that stimulated Raman scattering in laser-produced plasmas is an absolute instability

Abstract

We report a sequence of experiments in which 3-\ensuremath{\mu}m-thick CH foils were irradiated by one arm of the Nova laser with square pulses of 0.35-\ensuremath{\mu}m light. The laser intensity, pulse duration, and spot size were varied to produce plasmas in which the dependence of stimulated Raman scattering (SRS) on laser intensity and on density-gradient scale length could be independently evaluated. The traditional, convective-amplifier model of SRS fails, by many orders of magnitude, to explain either the magnitude or the scaling of these data. We suggest that SRS is absolutely unstable in these plasmas.