Reduction of emission in the stimulated Raman scattering frequency band to thermal noise levels by collisional damping in a laser-produced plasma

We report measurements of the noise level of light emissions in the frequency band of the stimulated Raman scattering (SRS) instability. Long-scale-length planar plasmas were produced from thin gold targets using 0.35-{μ}m laser light. At laser intensities up to 5×{}10¹⁴ W/cm², the SRS was completely quenched by collisional damping, and the emissions were thermal, as evidenced by their time dependence and by their spectrum. The onset of SRS was consistent with theory, and the SRS spectral power was five orders of magnitude larger than thermal at a laser intensity of 4×{}10¹⁵ W/cm².