Ideal Coulomb Plasma Approximation in Line Shape Models: Problematic Issues

Joel Rosato,Roland Stamm,Hubert Capes

doi:10.3390/atoms2020253

Joel Rosato, Roland Stamm + Show 1 more

Open Access

https://doi.org/10.3390/atoms2020253

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Abstract

In weakly coupled plasmas, it is common to describe the microfield using a Debye model. We examine here an “artificial” ideal one-component plasma with an infinite Debye length, which has been used for the test of line shape codes. We show that the infinite Debye length assumption can lead to a misinterpretation of numerical simulations results, in particular regarding the convergence of calculations. Our discussion is done within an analytical collision operator model developed for hydrogen line shapes in near-impact regimes. When properly employed, this model can serve as a reference for testing the convergence of simulations.

Highlights

The Spectral Line Shapes in Plamas Code Comparison Workshop (SLSP) [1] focuses on a set of standardized physical problems to be addressed using codes from different research groups/labs.Amongst these problems is the description of Stark line shapes with ion dynamics effects, referred to as the cases “1” and “2” in the first (2012) and second (2013) editions of the Workshop
The electrons and the ions are assumed to move along straight path trajectories and they produce unscreened Coulomb potentials
If the plasma is so weakly coupled that a calculation cannot be performed on a reasonable time scale, the box size is reduced and the corresponding number of particles is adjusted in such a way that a relevant statistical quantity is well reproduced within a few percents error bars. We suggest that this procedure does not suffice to obtain reference profiles in the case of infinite Debye length

Summary

Introduction

The Spectral Line Shapes in Plamas Code Comparison Workshop (SLSP) [1] focuses on a set of standardized physical problems to be addressed using codes from different research groups/labs. If the plasma is so weakly coupled that a calculation cannot be performed on a reasonable time scale, the box size is reduced and the corresponding number of particles is adjusted in such a way that a relevant statistical quantity (like the microfield probability density function or the microfield autocorrelation function) is well reproduced within a few percents error bars We suggest that this procedure does not suffice to obtain reference profiles in the case of infinite Debye length. It has been shown that the perturbers that effectively contribute to the line broadening are those which are located at a distance smaller than v/γ, where γis the line’s characteristic width, because of the finite lifetime of the emitter (see the discussions in [2,3]) This length may be interpreted as an upper cut-off in place of the Debye length for our “artificial” Coulomb plasma. In [2], this equation was solved numerically by iterations and it was shown that this method is quickly convergent

Questioning the Validity of Simulations

Conclusions