Fast-electron transport and heating of solid targets in high-intensity laser interactions measured by K alpha fluorescence.

We present experimental results on fast-electron energy deposition into solid targets in ultrahigh intensity laser-matter interaction. X-ray $K\ensuremath{\alpha}$ emission spectroscopy with absolute photon counting served to diagnose fast-electron propagation in multilayered targets. Target heating was measured from ionization-shifted $K\ensuremath{\alpha}$ emission. Data show a $200\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ fast-electron range in solid Al. The relative intensities of spectrally shifted $\mathrm{Al}\phantom{\rule{0.2em}{0ex}}K\ensuremath{\alpha}$ lines imply a mean temperature of a few tens of eV up to a $100\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{m}$ depth. Experimental results suggest refluxing of the electron beam at target rear side. They were compared with the predictions of both a collisional Monte Carlo and a collisional-electromagnetic, particle-fluid transport code. The validity of the code modeling of heating in such highly transient conditions is discussed.