Updated LPI Thresholds for the Nike Laser*

Advanced implosion designs for direct drive inertial confinement fusion use high laser intensities (10$^{15}$-10$^{16}$ W/cm$^{2})$ to achieve gain (g$>$100) with a reduction in total laser energy (E$<$1 MJ). Krypton-fluoride lasers such as the Nike laser at NRL are an attractive choice due to their combination of short wavelength (248 nm), large bandwidth (1-2 THz), and beam smoothing by induced spatial incoherence but the potential threat from laser-plasma instabilities (LPI) needs to be assessed. The 2008 LPI campaign at Nike yielded threshold intensities above 10$^{15}$ W/cm$^{2 }$for the two-plasmon instability, a value higher than reported for 351 nm glass lasers. The experiments used a planar geometry, solid polystyrene targets, and a subset of beams (E$<$200 J) with a reduced focal spot (d$<$125 $\mu $m). The 2009 campaign extended the shot parameters to higher laser energies (E$<$1 kJ) and larger spot sizes (d$<$300 $\mu $m). Spectrally-resolved and time-resolved measurements of x-rays and emission near $^{1}$/$_{2}\omega _{o}$ and $^{3}$/$_{2}\omega _{o}$ harmonics of the laser wavelength show threshold intensities consistent with the 2008 results. *Work supported by DoE/NNSA