Fast-electron-relaxation measurement for laser-solid interaction at relativistic laser intensities

Abstract

We present measurements of the fast-electron-relaxation time in short-pulse $(0.5\phantom{\rule{0.3em}{0ex}}\mathrm{ps})$ laser-solid interactions for laser intensities of ${10}^{17}$, ${10}^{18}$, and ${10}^{19}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$, using a picosecond time-resolved x-ray spectrometer and a time-integrated electron spectrometer. We find that the laser coupling to hot electrons increases as the laser intensity becomes relativistic, and that the thermalization of fast electrons occurs over time scales on the order of $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ at all laser intensities. The experimental data are analyzed using a combination of models that include $K\ensuremath{\alpha}$ generation, collisional coupling, and plasma expansion.