Sub-ablation threshold studies
Rapid heating of surfaces by short-pulse lasers causes near-surface thermal stresses, which can lead to a wide variety of micromechanical responses.
As a part of the High Average Power Laser (HAPL) program, we are developing high-energy grazing-incidence metal mirrors (GIMM's) for laser-IFE (inertial fusion energy). This research includes materials development, fabrication and laser-induced damage testing. The absorbed energy in GIMM's is relatively small - of the order of 10 mJ/cm2, or 106 W/cm2. Although the maximum surface temperature rise is less than 50 K, the thermal stresses can be a significant fraction of the yield strength. Surface evolution occurs slowly over many cycles, and can ultimately degrade the optical properties and lead to rapid onset of laser-induced damage.
We are also exploring the response of IFE chamber armor to high-cycle fatigue induced by repetitive near-surface thermal loading. We use our short-pulse lasers to simulate the thermal and mechanical behavior of unprotected walls subjected to repetitive bursts of x-rays and ions. In this case, the peak absorbed energy is approximately 108 W/cm2, leading to peak temperatures near the melting point of refractory alloys.