Abstract
For the theoretical study of X and extreme-UV spectra of ions in plasmas, quantum mechanics brings more detailed results than statistical physics. However, it is impossible to handle individually the billions of levels that must be taken into account in order to properly describe hot plasmas. Such levels can be gathered into electronic configurations or superconfigurations (groups of configurations) and the corresponding calculations rely on appropriate statistical methods, for local or non-local thermodynamic equilibrium plasmas. In this article we present the basic principles of the Super-Transition-Array approach as well as its practical implementation. During the last decades, calculations performed with the SCO code (Superconfiguration Code for Opacity) have been compared to opacity measurements. The code includes static screening of ions by plasma and is well suited for studying plasma density effects (for example pressure ionization) on opacity and equation of state. The recently developed SCO-RCG code (Superconfiguration Code for Opacity combined with Robert Cowan’s “G” subroutine) combines statistical methods from SCO and fine-structure (detailed-level-accounting) calculations using subroutine RCG from Cowan’s code. SCO-RCG enables us to obtain very detailed spectra and to significantly improve the interpretation of experimental spectra. The Super-Transition-Array formalism is still the cornerstone of several opacity codes, and new ideas are emerging, such as the Configurationally Resolved-Super-Transition-Array approach or the extension of the Partially Resolved-Transition-Array concept to the superconfiguration method.
Highlights
Hot plasmas contain, for all their chemical constituents, atomic ions in different ionization stages, potentially spread over billions of energy levels, and subject to several atomic processes
This review article deals with a method of calculation of the photoabsorption cross-section in hot plasmas at local thermodynamic equilibrium (LTE)
The superconfiguration method has been introduced by BarShalom et al in order to calculate opacities in plasmas at local thermodynamic equilibrium
Summary
For all their chemical constituents, atomic ions in different ionization stages, potentially spread over billions of energy levels, and subject to several atomic processes. This review article deals with a method of calculation of the photoabsorption cross-section in hot plasmas at local thermodynamic equilibrium (LTE). We propose to discuss all the approximations of this method avoiding, as much as possible, technical details. With the development of more and more powerful computers, the application of precise atomic-physics methods to opacity computation became possible. This led us to develop the SCO-RCG code (Superconfiguration Code for Opacity combined with Robert Cowan’s “G” subroutine), combining statistical methods and Detailed-Level-Accounting (DLA) finestructure calculations of atomic structure and spectra.
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