Impact of ambient conditions on laser induced damage precursors generation

Precursors generated by fused silica during high-energy laser exposure are known to increase damage initiations on nearby fused silica surfaces, on which they are thought to adsorb. These precursors are of concern to increasing the laser energy in the final optics of the Lawrence Livermore National Laboratory (LLNL) National Ignition Facility (NIF). In this work, precursors were generated by exposing a fused silica sample with a single 3ω, 5 ns, 35 mm-diameter laser shot at approximately 12 J/cm² average fluence in the presence of identical, witness samples not exposed to laser light in a precision cleaned test chamber. Five tests were conducted, one for each of five pressures: 760, 350, 10, 2.5, and 10^-5 torr, using the LLNL Optical Sciences Laser Laboratory. The witness samples were then damage tested with a single laser shot: 3ω, 5 ns, 10 mm-diameter, and 26 J/cm² average fluence to evaluate the effect of the different environmental pressures. The results of this experiment show that laser exposures in ambient pressure above 350 torr resulted in less observed laser-induced damage presumably due to suppression of precursor generation or inhibition of precursor transport. A detailed analysis of the fluence, damage and correlation to environmental pressure will be shown in this work. These results inform potential damage-reduction solutions for NIF final optics for future operation at higher power and energy and may be relevant to other high energy UV fused silica-based laser optical systems.