Study of the probability of growth of silica exit surface damage on the National Ignition Facility (NIF) final optics

Abstract

Surface damage of silica optics routinely limits the operation of high energy laser systems. Initiated damage can grow upon additional laser pulses, eventually requiring optic removal/replacement. However, when damage is first initiated, small damage sites grow in a stochastic manner, readily parameterized by the size of the damage site, surface of residence, and the fluence and pulse duration of subsequent laser exposures. The National Ignition Facility (NIF), which exposes ~100 m2 of fused silica optics surface to high-energy-nanosecond-laser light on every full system shot, provides an ideal platform to study the growth behavior of laser-induced damage. High-resolution microscopy of individual damage sites is captured as part of the standard NIF recycling loop. However, not all damage sites are repaired depending on the age and quality of the host optic leaving many thousands sub-50-micron damages sites to resume growth after being imaged. By measuring such sites each time an optic is removed for recycling, high-resolution microscopy becomes available for many thousands of sites before and after exposure to various shot sequences on the NIF laser. Using this observed growth, a multi-shot description was fit to predict the likelihood of exit surface damage site growth under exposure from 3ω, nanosecond regime pulses for shot sequence lengths between 1-100 laser exposures. This provides a basis for accurately predicting when a recycled optic will require additional repair.