Measurement of two plasmon decay instability development inkspace of a laser produced plasma and its relation to 3/2-harmonic generation

The two plasmon decay instability in a uniform ${\mathrm{CO}}_2$-laser produced plasma is studied by Thomson scattering of ruby laser radiation. Initially the instability grows as predicted by linear theory as decay plasma wave pair of expected wave vectors. During saturation unstable plasma wave activity spreads over wide regions in k space extending in places beyond the Landau cutoff contour. Comparing the saturated plasma wave intensity distribution in k space and measurements of the angular variation of 3/2-harmonic emission confirms the mechanism in which the radiation is generated by scattering of pump photons off two plasmon decay waves.