Laser Damage performance of fused silica optical components measured on the Beamlet Laser at 351 nm

A statistics-based model is being developed to predict the laser-damage-limited lifetime of UV optical components on the NIF laser. In order to provide data for the model, laser damage experiments were performed on the Beamlet laser system at LLNL (aperture: 34 cm x 34 cm). Three prototype NIF focus lenses were exposed to 351 nm pulses (1.5ns or 3ns) during four experimental campaigns, each consisting of 23 to 38 pulses at NIF relevant fluences. Each lens was sol-gel AR coated and all laser exposures were performed in a vacuum environment. Through inspections of the lens before, during and after the campaigns, pulse-to-pulse damage growth rates were measured for damage initiating both on the surfaces and at bulk inclusions. Radial growth rates measured for rear surface damage was typically 10x higher than that measured in the bulk or at the front surface. No significant correlation of growth rate to precursor type was indicated. For 5 J/cm(2), 3 us pulses the typical radial growth rate was nominally 20 mu m/pulse. Average growth rates measured on three lenses made by two manufacturers were in good agreement. While the growth rate clearly increased with fluence, the data obtained was insufficient to quantify the dependence. The growth rates reported here were 20x -50x higher than values predicted from offline studies of bare surfaces in air.