Global approach to plasmas in LTE equilibrium

Abstract

The computation of a quantity such that the LTE opacity of a plasma by using a full detailed level accounting approach is, even in a limited spectral range, very demanding in terms of computational resources. For moderate- to high-Z plasmas, global concepts such as Unresolved Transition Arrays (between configurations) and Super Transition Arrays (between superconfigurations) can be very useful tools to handle large sets of radiative transitions. The relevance and the efficiency of these global methods are illustrated through the example of an absorption spectrum calculated in a line-by-line calculation and compared with the result of a superconfigurationaccounting approach. Also, the comparisons of these global calculations with experimental measurements of the plasma opacity in some selected energy ranges corresponding to specific nl - n´l´ transitions assess the usefulness of these methods. In particular, for moderate-Z plasmas, a UTA approach gives satisfactory results, notwithstanding the approximate accounting of the Boltzmann factors which can slightly alter the results at low temperatures. For higher-Z plasmas, the superconfiguration-accounting approach gives good results and makes tractable the treatment of very complex ions.