Methods for mitigating surface damage growth in NIF final optics

Abstract

We report a summary of the surface damage, growth mitigation effort at 3(omega) for fused silica optics at LLNL. The objective was to experimentally validate selected methods that could be applied to pre-initiated or retrieved-from- service optics, to stop further damage growth. A specific goal was to obtain sufficient data and information of successful methods for fused silica optics to select a single approach for processing NIF optics. This paper includes the test results and the evaluation thereof, for several mitigation methods for fused silica. The mitigation methods tested in this study are wet chemical etching, cold plasma etching, CO<SUB>2</SUB> laser processing, and micro-flame torch processing. We found that CO<SUB>2</SUB> laser processing produces the most significant and consistent results to halt laser-induced surface damage growth on fused silica. We recorded successful mitigation of the growth of laser-induced surface damage sites as large as 0.5-mm diameter, for 1000 shots at fluences in the range of 8 to 13 J/cm<SUP>2</SUP>. We obtained sufficient data for elimination of damage growth using CO<SUB>2</SUB> laser processing on sub-aperture representative optics, to proceed with application to full- scale NIF optics.