X-ray radiation properties of plasma under interaction of femtosecond laser pulses with ∼ 10²² W/cm² intensities

Study of radiation properties of solid dense plasma irradiated by ultraintense lasers has a great interest both from fundamental physics and different application point of views. Recently upgraded petawatt J-KAREN-P laser together with precise focusing technique delivers 35 fs laser pulses of 10²² W/cm² intensity into a micron-size focal spot on target. For such unprecedented intensities the application of high-resolution X-ray spectroscopy allows to investigate the ionization mechanisms and to measure the parameters of relativistic plasma from front and rear sides of moderate (Al) and high Z (Ti, Fe,) thin foil targets. Kinetic modeling of the spectra is used to estimate electron plasma density and temperature, demonstrating T_e ∼2 keV for Ne ∼5e22 cm^-3 in the hottest emission region. Thus, it is experimentally demonstrated for the first time that the laser pulse of over 1e21 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. It is revealed how even small displacement of the target out of the optimal laser focus, as well the decrease in temporal contrast of the laser pulse, strongly reduce both the intensity of X-ray radiation and degree of plasma ionization. 2D PIC code simulations of femtosecond laser interaction with various materials are provided and compared with experimental results.