Measurements of the optical contrast on OMEGA: a 60-beam, 30-kJ UV fusion laser

Summary form only given. For most direct-drive ICF experiments, it is important that the 0.5- to 4-ns laser pulses that irradiate targets on OMEGA have high optical contrast. Using a photodiode that is sensitive to intensities that are 10/sup -8/ of the main OMEGA pulse, the UV output of OMEGA was measured for a duration of /spl sim/20 ns before the arrival of the main pulse. By cross-calibrating this with the main pulse shape, we constructed the entire temporal pulse shape over 20 ns and over approximately eight orders of magnitude. To assess the tolerable level of contrast, we performed experiments that probe the Al barrier layers placed on most OMEGA targets. We also correlate the performance of imploding spherical targets with the measured optical contrast.