X-ray conversion efficiency as a function of atomic number for 0.26-<i>μ</i>m-laser<i>–</i>irradiated targets

Abstract

Soft-x-ray low-resolution experimental spectra are obtained for a large set of targets irradiated at 2\ifmmode\times\else\texttimes\fi{}${10}^{14}$ W${\mathrm{?}}^{\mathrm{\ensuremath{-}}2}$ by 0.26-\ensuremath{\mu}m radiation. X-ray conversion efficiencies in various spectral ranges are studied as a function of atomic number. To calculate the emissivity, we develop a simple, multiple-Z atomic-physics model based on a screened-hydrogenic description for the atomic structure and on a non-local-thermodynamic-equilibrium modified Saha approach to plasma-ionization properties. We are able to replicate experimental soft-x-ray spectra by using a discrete summation of emissivities over a few temperatures with appropriate weighting factors determined from the experiment or through a separate hydrodynamic-code simulation. The modulations in the Z dependence of the x-ray conversion efficiency in various spectral ranges are well described and interpreted. The comparison of theory and experiment enables us to identify a few ``universal'' parameters independent of the atomic number for given irradiation conditions.