Physical Effects and Numerical Simulation of X-Ray Transport in Plasmas

Abstract

As drivers, such as lasers and pulsed power generators, for laboratory plasmas have steadily increased in power and/or total deliverable energy, the increased size and density of the plasmas created by such devices has frequently resulted in higher plasma opacities. The effects of substantial optical depth upon plasmas can be divided into two general categories. First, the radiation diagnostics (primarily x-rays) are altered. Such quantities as line ratios and widths inevitably change from their respective optically thin values when the photons are subjected to reabsorption within the plasma. The second category of physical effects induced by radiation transport may be generally categorized as dynamic. Cooling rates and thermal balance within a plasma are directly influenced by the transport of photons within the plasma, as well as by the degree to which photons may escape the plasma without interaction, that is, the magnitude of the photon escape probability. Ionization fractions and level populations are also altered from their optically thin values by photon trapping. This has a direct effect upon the viability of certain x-ray laser schemes. A host of numerical techniques have been developed to treat radiation transport in plasmas. Many of these numerical algorithms received their initial impetus from the astrophysics community, and have been adapted to situations encountered in laboratory plasmas. In this article the physical effects of x-ray transport on laboratory plasmas are reviewed through reference to specific examples. Numerical techniques, both multifrequency (multigroup) and escape probability methods, are surveyed and assessed for effectiveness and appropriateness depending upon the total context of the numerical simulation as well as the specific plasma conditions expected to be encountered.