Results of applying a non-evaporative mitigation technique to laser-initiated surface damage on fused-silica

Abstract

We present results from a study to determine an acceptable CO₂ laser-based non-evaporative mitigation protocol for use on surface damage sites in fused-silica optics. A promising protocol is identified and evaluated on a set of surface damage sites created under ICF-type laser conditions. Mitigation protocol acceptability criteria for damage re-initiation and growth, downstream intensification, and residual stress are discussed. In previous work, we found that a power ramp at the end of the protocol effectively minimizes the residual stress (⪅25 MPa) left in the substrate. However, the biggest difficulty in determining an acceptable protocol was balancing between low re-initiation and problematic downstream intensification. Typical growing surface damage sites mitigated with a candidate CO² laser-based mitigation protocol all survived 351 nm, 5 ns damage testing to fluences ⪆12.5 J/cm². The downstream intensification arising from the mitigated sites is evaluated, and all but one of the sites has 100% passing downstream damage expectation values. We demonstrate, for the first time, a successful non-evaporative 10.6 m CO² laser mitigation protocol applicable to fused-silica optics used on fusion-class lasers like the National Ignition Facility (NIF).