Escape Methods Vs Diffusion for Radiation Transport in Z-Pinch Simulations

Abstract

Summary form only given. In Z-pinch loads designed for X-ray production the imploding kinetic and some of the electromagnetic energy is converted into radiation. Hence radiation and atomic processes control the plasma conditions of such loads at the time of assembly on axis. Simulations of Z-pinch dynamics need to properly account for this microphysics to be a viable analysis tool and accurate predictor of advanced loads. Several methods for radiation transport and population dynamics in simulation models are compared and contrasted for their advantages and limitations. Collisional radiative equilibrium (CRE) along with escape methods for various emission features offer the most accurate treatment with high spectral fidelity, but are presently limited to the assumption of 1-D cylindrical symmetry in time dependent calculations. On the other hand, the method of radiation diffusion can address multi-dimensional structure resulting from instabilities. A multi-group formulation can even provide gross spectral information on the plasma emission. However the diffusion approach often indicates lower electron temperatures and higher charge states than the CRE/escape method approach, and the latter is more in conformity with experimental data. Comparison of the above two methods within 1-D implosions will be presented for Ti wire array loads on the Z-generator. Part of the difference can be attributed to the typical assumption of local thermal equilibrium (LTE) conditions for the species populations in calculating the opacities for diffusive transport. To address this limitation a revised approach to radiation diffusion using non-LTE multi-group Planck and Rosseland opacities will be developed. The formulation requires the use of a calculated source function to replace the usual blackbody approximation in the radiation energy transport equation. Simulation results from this method will be contrasted with the CRE/escape and LTE diffusion approaches