LASER BEAM SMOOTHING AND BACKSCATTER SATURATION PROCESSES IN PLASMAS RELEVANT TO NATIONAL IGNITION FACILITY HOHLRAUMS*

We have used gas-filled targets irradiated by the Nova laser to simulate National Ignition Facility (NIF) hohlraum plasmas and to study the dependence of Stimulated Raman (SRS) and Brillouin (SBS) Scattering on beam smoothing at a range of laser intensities (3 w, 2- 4 1015Wcm-2) and plasma conditions. We have demonstrated the effectiveness of polarization smoothing as a potential upgrade to the NIF. Experiments with higher intensities and higher densities characteristic of 350eV hohlraum designs indicate that with appropriate beam smoothing the backscatter from such hohlraums may be tolerable. As part of an integrated plan for attainment of ignition on NIF we are now carrying out studies to evaluate the effectiveness of beam smoothing at reducing backscattered light (SRS and SBS) as we depart from the point design hohlraum conditions. These studies are intended to allow us more latitude in choosing the eventual ignition design. We are also exploring new implementations of laser beam smoothing more representative of what will be possible on baseline NIF and potential modifications. This work extends previous studies [1,2] that validated the baseline NIF beam smoothing design through interaction experiments on Nova. The previous work concluded that backscatter from the NIF (300eV) hohlraum would be at tolerable levels an