A comparison between detailed and configuration-averaged collisional-radiative codes applied to non-local thermal equilibrium plasma

TL;DR

This study compares detailed and configuration-averaged collisional-radiative models for non-LTE plasmas, revealing the conditions under which the simpler configuration-averaged approach remains valid and identifying its limitations at low temperatures.

Contribution

It introduces and evaluates a configuration-averaged collisional-radiative model, comparing it to a detailed version, and proposes criteria for its validity in low-temperature plasma modeling.

Findings

Configuration-averaged code is accurate at Te=10 eV or higher.

Fails at Te=1 eV, showing limitations at low temperatures.

Autoionization cascades can cause significant errors in the averaged model.

Abstract

A collisional-radiative model describing nonlocal-thermodynamic-equilibrium plasmas is developed. It is based on the HULLAC (Hebrew University Lawrence Livermore Atomic Code) suite for the transitions rates, in the zero-temperature radiation field hypothesis. Two variants of the model are presented: the first one is configuration averaged, while the second one is a detailed level version. Comparisons are made between them in the case of a carbon plasma; they show that the configuration-averaged code gives correct results for an electronic temperature Te=10 eV (or higher) but fails at lower temperatures such as Te=1 eV. The validity of the configuration-averaged approximation is discussed: the intuitive criterion requiring that the average configuration-energy dispersion must be less than the electron thermal energy turns out to be a necessary but far from sufficient condition. Another condition based on the resolution of a modified rate-equation system is proposed. Its efficiency is emphasized in the case of low-temperature plasmas. Finally, it is shown that near-threshold autoionization cascade processes may induce a severe failure of the configuration-average formalism.