Super-transition-arrays: A model for the spectral analysis of hot, dense plasma

TLDR

Detailed configuration accounting is impractical and the average‑atom method fails to capture the spectrum’s fine structure under these conditions. The study introduces a method for calculating bound‑bound emission from LTE plasmas and discusses its application to laser‑produced plasma experiments. The method represents each single‑electron transition array with a small number of super‑transition‑arrays computed in optimized potentials, enabling rapid convergence. Analytic expressions for STA moments are derived, the approach smoothly bridges average‑atom and UTA results, and comparisons reveal that low‑probability transitions can significantly contribute to emission under certain plasma conditions.

Abstract

A method is presented for calculating the bound-bound emission from a local thermodynamic equilibrium plasma. The total transition array of a specific single-electron transition, including all possible contributing configurations, is described by only a small number of super-transition-arrays (STA's). Exact analytic expressions are given for the first few moments of an STA. The method is shown to interpolate smoothly between the average-atom (AA) results and the detailed configuration accounting that underlies the unresolved transition array (UTA) method. Each STA is calculated in its own, optimized potential, and the model achieves rapid convergence in the number of STA's included. Comparisons of predicted STA spectra with the results of the AA and UTA methods are presented. It is shown that under certain plasma conditions the contributions of low-probability transitions can accumulate into an important component of the emission. In these cases, detailed configuration accounting is impractical. On the other hand, the detailed structure of the spectrum under such conditions is not described by the AA method. The application of the STA method to laser-produced plasma experiments is discussed.