Energy Balance and Temperature in a Carbon Dioxide Laser Produced Plasma.

The absorption of intense (LESSTHEQ)10('14) W/cm('2) CO(,2) laser radiation by an underdense plasma is investigated experimentally. The plasma is produced by the CO(,2) laser beam focussed onto a stabilized laminar gas jet emanating from a Laval nozzle. The electron temperature of the laser produced plasma is measured by soft x-ray diagnostics which yield a 300 eV thermal and a 2000 eV suprathermal temperature. Time resolved studies are made of the radial expansion of the plasma to determine the absorbed energy and temperature using a modified blast wave analysis that involves the identification of the Chapman-Jouguet detonation point in the expansion. The absorbed energy is also determined from Ulbricht sphere photometry. Information on the plasma dimensions and electron density is obtained by time resolved ruby laser interferometry. The electron temperature and absorbed energy measurements are used to corroborate one another and to provide a self -consistent picture of the laser plasma coupling. Although vacuum laser intensities are sufficiently high where saturation of collisional (inverse bremsstrahlung), absorption is expected to occur, the experimental evidence indicates that linear inverse bremsstrahlung accounts for the observed thermal electron temperature and measured level of absorption. The energy balance shows that there is a sufficient level of absorption to accommodate the measured thermal electron temperature as well as <15% of the electrons at a 2 keV suprathermal temperature.