EVIDENCE OF STIMULATED RAMAN-SCATTERING OCCURRING IN LASER FILAMENTS IN LONG-SCALE-LENGTH PLASMAS

A time-resolved study is presented of stimulated Raman backscatter (B-SRS) produced in a large-scale-length preformed plasma in an intense laser pulse. The first direct experimental results are presented to (i) show that the B-SRS is generated predominantly in laser filaments and (ii) provide a direct correlation between the spectral and spatial emission profiles of the B-SRS as a function of interaction irradiance. The "anomalous" B-SRS spectral features so commonly reported in time-resolved studies are shown to be the characteristic emission of B-SRS from filaments. The B-SRS fraction was also measured as a function of interaction irradiance. The B-SRS intensity "threshold" and "saturating" behavior recorded can be well described for the present interaction conditions. The onset of B-SRS is shown to be strongly correlated with the intensity threshold of the filamentation instability. The effect of laser beam "smoothing" techniques on the B-SRS emission have also been investigated, using either a random phase plate (RPP) or induced spatial incoherence (ISI). The results suggest that their effectiveness in suppressing B-SRS is strongly associated with their ability to reduce the level of filamentation while still producing a high level of plasma perturbation which can effectively destroy the phase-matching conditions necessary for B-SRS.