Radiation Transport in Dense Plasmas.

Abstract

Radiation transport in dense plasmas can be an important process in plasma dynamics and diagnostics, hence it has received much attention in recent years. The inclusion of photon processes can alter the electron temperature and atomic level populations directly, and subsequently effect other plasma parameters. Radiation also facilitates the measurement of relevant plasma parameters, particularly when scale lengths of temperature and density gradients are too short for probe measurements. A radiation transport model for line radiation has been developed for inclusion in a magneto-hydrodynamic computer code. The transport model combines escape and transmission probability theory for optically thin ((tau) > 1) or strongly absorbing ((SIGMA)(,a) > (SIGMA)(,s)) regions with Eddington theory in optically thick, scattering dominated regions. Escape probabilities have their foundations in transport theory for strongly absorbing media, and the theory was developed to a large extent by Case, DeHoffman, and Placzek.('1) The Eddington theory models problems with one or several isolated diffusive regions between optically thin zones, or a variable Eddington calculation throughout the entire plasma. The two theories are coupled by means of albedo boundary conditions. The magneto-hydrodynamic code models a one dimensional cylindrically symmetric Z-pinch or exploding wire plasma, solving the MHD equations in the Lagrangian frame. A laser heated aluminum exploding wire plasma is considered. The non-equilibrium atomic rate equations are solved to obtain the effective charge of the plasma and the transparent line emission which is used as the source function in the radiation transport model. Only the dominant lines of AlXI, XII, and XIII are considered. Decoupling of the electron temperature equation, the atomic rate equations, and the radiation transport is accomplished by assuming that the radiation field does not vary substantially from one time step to another. Simulations with and without radiation transport are compared. The addition of the radiation transport calculations causes a decrease in emission. There is some redistribution of radiant energy due to the fact that several excited states are capable of undergoing radiative decay to several levels. The emission decreases slightly for an optically thin plasma to as much as 2-5 times for an optically thick plasma. The decrease is due to self-absorption during transport. The self-absorption affects the plasma dynamics in several ways, although they are much less dramatic than the emission differences. The electron temperatures, the plasma radius and the effective charge for the case with radiation transport are larger than the respective values from the transparent plasma. At the same time the ion temperatures are slightly lower for the case with transport. (') ('1)Case, K. M., F. DeHoffmann, and G. Placzec, Introduction to the Theory of Neutron Diffusion, Volume 1, Los Alamos Scientific Laboratory, June 1953.