Study of Half Harmonic Plasma Waves in Carbon Dioxide Laser-Plasma Interactions.

Half harmonic plasma waves, which can be generated by the two plasmon decay (TPD) and stimulated Raman scattering (SRS) instabilities in the regions with plasma density near n_{c}/4, are studied experimentally in the interactions of an intense (1 <=q 10^{14}W/cm ^2) CO_2 laser beam with an underdense plasma. The plasma is produced by focusing the CO_2 laser pulse (2 ns FWHM) onto a stabilized laminar nitrogen gas jet flowing from a Laval nozzle into low pressure helium. The plasma waves were investigated with ruby laser Thomson scattering. The absolute TPD and SRS instabilities were observed at pump intensities greater than 1.3 times 10^{13}W/cm ^2 for a plasma with a density scale length of 2400 mu{rm M} and an electron temperature of 300 eV. The difference predicted by the theory between plasma waves generated by the two plasma decay instability and those due to the stimulated Raman scattering deduced from theory are experimentally confirmed. Two plasmon decay is found to dominate the generation of half harmonic plasma waves. The TPD plasma waves appear in series of up to 8 bursts. The intensity distribution of the unsaturated TPD plasma waves is determined to be well described by linear theory. The intensity distribution of the saturated TPD plasma waves is shown to be governed by the saturation and quenching mechanisms. The angular distribution of (3/2)omega_{o } emission deduced from the intensity distribution of the saturated TPD plasma agrees well with the experimental results. The experimental evidence shows that the coupling of plasma waves to ion acoustic waves and the steepening of the density profile are the dominant saturation and quenching mechanism for the TPD instability.