Laser-accelerated proton conversion efficiency thickness scaling

The conversion efficiency from laser energy into proton kinetic energy is measured with the 0.6ps, 9×1019W∕cm2 Titan laser at the Jupiter Laser Facility as a function of target thickness in Au foils. For targets thicker than 20μm, the conversion efficiency scales approximately as 1∕L, where L is the target thickness. This is explained by the domination of hot electron collisional losses over adiabatic cooling. In thinner targets, the two effects become comparable, causing the conversion efficiency to scale weaker than 1∕L; the measured conversion efficiency is constant within the scatter in the data for targets between 5 and 15μm, with a peak conversion efficiency of 4% into protons with energy greater than 3MeV. Depletion of the hydrocarbon contaminant layer is eliminated as an explanation for this plateau by using targets coated with 200nm of ErH3 on the rear surface. The proton acceleration is modeled with the hybrid-particle in cell code LSP, which reproduced the conversion efficiency scaling observed...