Accuracy of Stark Broadening Calculations for Ionic Emitters

Abstract

The standard static-ion impact electron theory of line broadening is assessed with a calculation of the He II P a line over a broad range of plasma conditions. Ion dynamics, an improved electron collision operator, and accurate atomic physics are included in the computation. Simulations also have been performed to validate effects included in the calculation at higher densities. Calculated linewidths are compared with experiment over more than two decades in electron density, and broad agreement is found. This provides the first critical evaluation of the assumptions of Stark broadening theory. The profiles also display the first unambiguous evidence of ion dynamics. [S0031-9007(98)07936-8] PACS numbers: 52.70. Kz, 32.70.Jz, Stark-broadened line-shape calculations based on the standard static-ion impact-electron theory historically have been used to support the diagnostics of thermal plasmas [1]. In the following, the limits of validity of this theory will be assessed through a line profile computation of the He II Paschen-a (n 4 to 3) transition in a broad range of plasma conditions. The large number of effects involved in the calculation, the wide range of plasma parameters, and the extensive experimental line-shape data available suggest that P a is the appropriate test bed for these Stark broadening calculations. Moreover, benchmark simulations covering the plasma conditions are also available for comparison. The extensive range of available experimental data spans regions in the plasma parameter space for which almost all of the physical effects that can contribute to the line profile become important. These include the interference terms in the electronic collision operator, ion microfield fluctuations, and fine structure and detailed level splitting. Calculations of the atomic parameters of this transition are accessible, but not trivial, so that errors arising from the atomic physics are minimized, and attention can be focused on the line profile computation itself. All of the effects included in this computation have been studied singly or in combination in previous line shape calculations that were restricted to a smaller range of plasma parameters. The present work derives important new information from the range of plasma conditions covered and from the completeness of effects important for the standard Stark-broadening theory of ionic line profiles. It is for these reasons that the present calculations can be used to assess the accuracy of the standard model.The results presented here are based on calculations performed with PPP [2], a code that has been shown to produce rapid and accurate profiles of spectral lines emitted by multicharged ions in hot dense plasmas that are in excellent agreement with experimental data [3,4].It constitutes an important advance in the computation of Stark-broadened line shapes for the diagnostic of high density and temperature plasmas [5] such as those produced by lasers or pinches [6]. The calculated P a line profiles are compared with recent high density plasma expe...

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