Effect of laser pulse duration on damage to metal mirrors for laser IFE

A Grazing Incidence Metal Mirror (GIMM) is a chief candidate for beam delivery for Inertial Fusion Energy (IFE). The goal for GIMM survival is greater than 3×108 laser pulses with 5 J/cm2 laser fluence normal to the incident beam. Laser-induced damage to metal mirrors is primarily a thermomechanical process. Long-term exposure leads to microstructural evolution analogous to fatigue. We have performed laser-induced damage experiments on high damage threshold aluminum mirrors using commercial KrF excimer (248 nm) lasers. We have studied mirror response to standard, 25 ns long-pulses as well as to IFE prototypic, 5 ns short-pulses achieved using a Pockels Cell. Short-pulse damage fluence was found to be better than predicted using simple thermal diffusion scaling from long-pulse results.