Radiation Transfer in the Spectra of Short-Pulse Laser Heated Targets

Short pulse lasers are useful for creating high energy density (HED) plasmas.  Typical experiments have used ps-duration, 10² J pulses, focused to 10² µm diameter spots on thin targets to create plasmas with temperatures ~10⁷ K and densities ~3 g/cm^-3 [1].  HED plasmas are studied primarily by their emitted x-ray spectra. Comparisons of measured-to-simulated line strengths and widths are used to diagnose the temperature and density.  The validity of this method depends on accurate spectra modeling.  Many previous applications of such diagnostics have assumed optically thin plasmas.  However, some of the x-ray lines that are used, particularly the H- and He-like alpha lines of elements such as S, have optical depths of 1 to 10.  This calls for the inclusion of radiation transfer in the models.  To this end, coupled atomic kinetics/radiation transfer simulations have been carried out with the Cretin code [2].  Simulated spectra with and without radiation transfer effects are presented.  Comparisons to data obtained at the Orion Laser Facility are analyzed to determine the effect of radiation transfer on the inferred temperatures.