Experimental investigation of the two-plasmon decay instability in a CO2-laser-produced plasma

The two-plasmon decay instability in a CO2-laser-irradiated gas jet target is investigated by a variety of diagnostic techniques. Growth rates and wavenumber spectrum measured by picosecond resolution Thomson scattering of probe ruby laser light are found to be in agreement with linear theory. The instability is saturated by coupling to ion-acoustic fluctuations of twice the wavenumber of the fastest growing wave and is quenched by radial profile modification caused by decay wave ponderomotive forces. The instability is seen to reappear at periodic intervals. The number of hot electrons measured with an electron spectrometer scales exponentially with the duration of the instability, and the energy distribution is seen to evolve from a one-dimensional (1-D) to a three-dimensional (3-D) Maxwellian, indicating the importance of space-charge potentials. Finally, the observed (3/2)ω0 radiation is shown to be consistent with scattering of CO2-laser radiation of two-plasmon decay fluctuations.