Bremsstrahlung and line spectroscopy of warm dense aluminum plasma heated by xuv free-electron-laser radiation

Abstract

We report the creation of solid-density aluminum plasma using free-electron laser (FEL) radiation at $13.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength. Ultrashort pulses were focused on a bulk Al target, yielding an intensity of $2\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. The radiation emitted from the plasma was measured using an xuv spectrometer. Bremsstrahlung and line intensity ratios yield consistent electron temperatures of about $38\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, supported by radiation hydrodynamics simulations. This shows that xuv FELs heat up plasmas volumetrically and homogeneously at warm-dense-matter conditions, which are accurately characterized by xuv spectroscopy.