High resolution X-ray spectra of stainless steel foils irradiated by femtosecond laser pulses with ultra-relativistic intensities

We report on the spectra of x-rays emitted from dense plasma generated via irradiation of thin stainless steel foils by ultra-relativistic femtosecond laser pulses (intensities ~3 × 1021 W/cm2). Kinetic modelling was used to estimate electron plasma density and temperature, demonstrating Te ~2.1 keV for Ne ~5 × 1022 cm−3 in the hottest emission region. Thus, it is experimentally demonstrated for the first time that the laser pulse of over 1021 W/cm2 intensity is absorbed neither in the solid density plasma nor in a pre-plasma of a common critical density, but in the matter of so called relativistic critical density.