Abstract
In hot plasmas, such as the ones encountered in astrophysics or laser-fusion studies, the number of ionic excited states may become huge, and the relevant electron configurations cannot always be handled individually. The Super Transition Array approach enables one to calculate the massic photo-absorption cross-section (or radiative opacity) in a statistical manner consisting of grouping configurations close in energy into superconfigurations. One of the main issues of the method, beyond its spectral resolution, is the determination of the most relevant configurations that contribute to opacity. In this work, we discuss different aspects of the generation of superconfigurations in a hot plasma and propose a new adaptive algorithm.
Highlights
The radiative opacity is an essential quantity governing the structure and evolution of stars
One may turn to the unresolved transition arrays (UTA) method [1], which assumes that all lines in the spectrum of each configurationto-configuration excitation merge into a single effective line, which can be depicted by a Gaussian function
Is a superconfiguration for which the first three supershells coincide with the normal lowest shells while each one of the others contains three shells supposed to be close in energy
Summary
The radiative opacity is an essential quantity governing the structure and evolution of stars. The numerical cost resulting from the huge number of transitions becomes prohibitive In this case, one may turn to the unresolved transition arrays (UTA) method [1], which assumes that all lines in the spectrum of each configurationto-configuration excitation merge into a single effective (super-) line, which can be depicted by a Gaussian function. The efficiency of the UTA method comes from the fact that compact formulas are available for the three lowest energy moments (orders 0, 1 and 2) of the line-strength weighted line energies of a transition array. Such moments are expressed in terms of reduced matrix elements of the dipole operator, Slater integrals, and subshell occupation numbers.
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