Exploration of the Fundamental “Damage Limit” Light Flux for Grazing Incidence Liquid Metal Mirrors

One definition for the “damage limit” of a liquid metal surface used as a final optic for laser fusion power plants is the maximum energy flux that the liquid metal can withstand without any resulting spallation. Some preliminary calculations were performed by Moir to roughly estimate the damage limit by imposing the restriction of a 200°C surface temperature rise. Here, new 1D calculations that account for hydro-motion on the compressible time scales are presented, along with revised estimates of the damage limits for liquid aluminum, sodium, and mercury. Slow compression time scales (~20 ns) produced negative pressures in the liquid film on the order of MPa, and fast ignition time scales (~10 ps) yielded GPa pressures for the laser energy densities set out by Moir. For Na and Al the peak energy densities normal to the beam on the order of 5 to 10 J/cm² were acceptable for fast ignition when 85° grazing incidence is assumed. Some experimental data on the generation and damping of surface waves resulting from surface ablation recoil is also presented, where large waves are seen to damp out after about 50 ms following the laser pulse.