K-shell spectroscopic diagnosis of suprathermal electrons at fusion-relevant environmental conditions

Fast-electron-induced changes in the ion charge distribution lead to a substantial modification of radiative properties of x-ray emitting systems. We report Cu K-shell spectra emitted from 1.5-μm-thick Cu foils irradiated by laser intensities up to 5×10¹⁶ W/cm2 using the fundamental and frequency tripled radiation of the Prague iodine laser system PALS, i.e., at conditions characteristic for laser spikes anticipated in the shock ignition scheme. The emission of the lower-ionization-stage ions observed as a fine structure merged with the Cu Kα₁₂ doublet and spectral lines corresponding to transitions in He-like to Ne-like Cu atoms were identified. The spectra analysis based on a combination of MULTI2D hydrodynamic modelling of the plasma evolution and its emissivity synthesized by collisional-radiative code FLYCHK provides information on distribution of hot electron fractions at low plasma temperatures. The procedure used for interpretation of experimental data contributes to a development of alternative methods for diagnosis of suprathermal electrons.