A New Technique for Measurements of Radial Distributions of Excited Species in Plasmas and Its Application to Capillary Discharges in Argon

Abstract

This paper describes a novel technique for obtaining the radial profiles of individual emission lines from a plasma directly in the form of a recorded trace of intensity vs radial position. The method employs an ``end‐on'' collimation technique which eliminates the complicated data reduction treatment required by conventional ``side‐light'' methods. Spatial resolution and instrumental distortion effects are discussed. The method is applied in an experimental study of the capillary discharge in argon, and the relevance of the results to the excitation mechanisms of the cw AII laser is indicated. In particular, it is found that populations of excited AI states are substantially independent of the local electron density over wide ranges of plasma parameters, an effect which is attributed to the predominance of electron‐impact‐induced transfers over radiative transfers for the AI states. The populations of excited states of AII are found to have profiles which are in agreement with those predicted by a plasma model in which (a) the ion loss is controlled by ambipolar diffusion at higher pressures with a steady approach towards an essentially collisionless ``free‐fall'' motion occurring as the pressure is decreased, and (b) excitation to the AII excited levels proceeds stepwise via the AII ground state. The method is of general applicability to problems involving spatial variations in plasmas, and has been coupled with high‐resolution interferometric techniques to provide direct measurements of spatial dependence of spectral line‐broadening parameters.