Backscatter and beam refraction in underdense gas breakdown with nanosecond CO2 laser

A 50-J 2-ns FWHM CO2 laser pulse focused to intensities in excess of 1014 W/cm2 has been used to induce breakdown in underdense He gas. The laser radiation scattered from these plasmas has been temporally and spectrally analyzed. It is observed that ion wave scattering can result in reflectivities in excess of 50% of the incident intensity. At filling pressures above 30 Torr the incident laser radiation is explosively refracted around the focal region by plasma formed a few mm upstream from focus thus quenching the high-intensity laser plasma interaction less than 1 ns subsequent to breakdown. A weak but well-defined line at 7.06 μm (ω=3/2 ω0) was observed in the backscatter at filling pressures above 40 Torr.