High-intensity laser-plasma interactions in the refluxing limit

The interaction of intense laser pulses (5×1016W∕cm2<I<2×1019W∕cm2) with small-volume copper targets (20×20×2μm3<V<500×500×50μm3) is described. Electron refluxing causes Kα yields to be independent of laser intensity at I>1018W∕cm2, where the target is heated to less than 100eV. As target volume is reduced, efficient energy transfer by the refluxing electrons heats the targets to hundreds of eV. This affects L→K and M→K electronic transitions during ion de-excitation that probe the bulk plasma conditions during this rapid isochoric heating phase. Comparison of measured Kα yields in the cold-material limit with a Kα production model, and measured Kβ∕Kα variations with numerical target-heating calculations, provides a robust method for inferring the fast-electron energy content. Bulk electron temperatures of at least 200eV are inferred for the smallest mass targets.