Line-broadening due to Plasma Fluctuations

Abstract

A wide variety of plasmas are e-ciently modelled by applying the Debye-Huckel model to account for the screening of the various electrostatic interactions in the system. In the present study, the model has been applied to a system which is close to but not quite at thermal equilibrium. It has been shown that small temporal ∞uctuations due to remaining ion motion can be interpreted as small local ∞uctuations within the system which may lead to sizeable contributions to line broadening. The observation of width and shape of spectral lines of atomic transitions from atoms or ions embedded in a plasma has early on been used as an important tool for the temperature and density diagnostics of plasmas (16). The relevance of such observations has stimulated a large number of studies of the physical processes that contribute to features of spectral lines which allow for the extraction of a wealth of plasma information. A selection of relevant publications in the area is given by the following list (1,8,5,11,6,15). The present study draws attention to a contribution to line broadening which so far has not yet been documented and which will be called ∞uctuation line broadening in this study. While recent work of ours dealt with a plasma situation for which this efiect | though larger by three orders of magnitude than the natural line width | is very small compared to other broadening mechanisms, we focus here on plasmas close to the pressure ionization limit. In this region ∞uctuation line broadening is much larger. Our study is based on weakly coupled plasmas which are customarily described by the Debye- Huckel screening model (3). This particular screening model is convenient to obtain numerical results. All screening models predict a decrease in binding with increasing plasma efiects. However, difierent electronic states respond difierently to increasing plasma conditions, and these difierences, while shifting all transition energies downward, leave footprints of their varying amounts. The efiects are not noticeable in a plasma in which each radiating atom or ion experiences exactly the same screening condition. With some amount of temporal or local ∞uctuations of the screening environment, however, a related distribution of transition energies results. The omission of one sizable contribution leads to incorrect assignments for temperature, density or pressure. A number of important experimental plasmas fall into the realm of Debye-Huckel plasmas: Gas- discharge plasmas, plasmas of thermonuclear fusion experiments, plasmas in the solar chromosphere and plasmas in interstellar space. These are weakly coupled plasmas with coupling constants much less than unity. The importance of properly including Debye screening in the evaluation of astrophysical plasma data has been emphasized by Spatschek (15) and, more recently, by Saha etal.