Time-Resolved High-Energy X-Ray Spectra of Laser-Irradiated Targets

Temporally resolved x-ray spectra in the range of 1-20 keV have been obtained from laser-irradiated high-$Z$ disk targets. We obtain the first time-resolved direct observation of the suprathermal x-ray tail resulting from hot-electron generation due to collective processes. The measurements indicated that the suprathermal temperature ${\theta }_H$ has a temporal behavior which essentially follows the laser pulse of intensity $I$, implying a relation ${\theta }_H\sim I^n$, $n\gtrsim 1$, an unexpected result.