A shadowing technique to arrest laser-induced damage growth on exit surface silica

An effective damage mitigation strategy is necessary to operate laser systems at energy densities above the damage growth threshold of their optical components. On the National Ignition Facility, growth of laser-induced damage has conventionally been arrested in situ by employing spatially registered cm-scale “spot blockers” in the laser beam to shadow mm-scale damage sites. Spot blockers come at a cost, however, as they obscure a portion of the laser light delivered to the target and thus require an increase in beam energy to compensate for this loss. This increase adds incremental stress to all optics in the beamline. Most spot blockers assigned to an optic are eliminated as part of the repair process when the optic is removed from NIF. However, defects too wide or deep to repair travel with the optic, along with the need for the blocker, throughout its life. Due to obscuration budgetary constraints, these permanent blockers reduce the optic’s usable lifetime. In this work, we propose an alternative method for mitigating a growing damage site by placing a scattering structure of comparable size to the site upstream to shadow the site. This solution obscures much less of the laser light and increases the lifetime of the optic compared to current mitigation strategies.