Experimental studies of wave-wave interactions in laser-produced plasmas

Stimulated Raman scattering and the two plasmon decay instability in a neodymium laser produced plasma were studied. Laser light of frequency ω0 was incident onto a variety of targets. Using time resolved spectroscopy with an image convertor camera backscattered emission at 3ω0/2 was observed for a range of experimental conditions. The effect of focal spot size at the target (which also determined the plasma scale length) was investigated. For small focal spots the spectra were double peaked about the nominal frequency. For large focal spots the spectra were single peaked and shifted to higher frequencies. It is suggested that this is due to a transition from the two plasmon decay instability for short density scale lengths, to Raman scattering for a longer scale length of the dominant mechanism for the generation of large amplitude electron plasma waves. For short scale lengths the effect of varying the mean ion charge, Z, in the plasma was studied. It was found that the plasma temperature increases with plasma Z. The results for backscattered emission at ω0/2 suggest this emission involves electron plasma waves produced by the two plasmon decay instability. An attempt to study the instabilities using Thomson scattering gave ambiguous results. (U.K.)