Laser-plasma interactions in long-scale-length plasmas under direct-drive National Ignition Facility conditions

Laser-plasma interaction experiments have been carried out on the OMEGA las er system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] under plasma conditions representative of the peak of a 1.5 MJ direct-drive laser pulse proposed for the National Ignition Facility (NIF). Plasmas have been formed by exploding 18-20 mu m thick CH foils and by irradiating solid CH targets from one side, using up to 20 kJ of laser energy with phase plates install ed on all beams. These plasmas and the NIF plasmas are predicted to have el ectron temperatures of 4 keV and density scale lengths close to 0.75 mm at the peak of the laser pulse. The electron temperature and density of the ex ploding-foil plasmas have been diagnosed using time-resolved x-ray spectros copy and stimulated Raman scattering, respectively, and are consistent with predictions of the two-dimensional Eulerian hydrodynamics code SAGE [R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984)]. When the solid- target or exploding-foil plasmas were irradiated with an f/6 interaction be am at 1.5 X 10(15) W/cm(2), well above the NIF f/8 cluster intensity of sim ilar to 2 X 10(14) W/cm(2), stimulated Brillouin backscattering (SBS) was f ound to be completely inhibited. A conservative upper limit of direct-backs cattered SRS was found to be similar to 5% from the solid targets. SRS and SBS are thus unlikely to have a significant impact on target performance at the peak of the NIF direct-drive laser pulse. (C) 1999 American Institute of Physics. [S1070-664X(99)93405-5].