Anatomy of a vertical metal halide discharge

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Abstract
Experimental measurements are compared with model calculations for a vertical metal halide discharge. The 400-W arctube has a gap of 4.3 cm, an inside diameter of 2.0 cm, and operates at about 5-atm Hg pressure with NaI and ScI3 additives. Emission spectroscopy is used to measure the discharge temperature and the densities of additive species. The model is based on the assumption of local thermodynamic equilibrium, and predicts the discharge behavior from first principles. Coupled equations describing the convective fluid flow, chemical equilibrium and diffusion, radiation transport, and discharge current flow are solved simultaneously so as to provide a self-consistent description of the discharge. Model results are compared with the experimental measurements. Discharge features such as the hot contraction regions near the electrode tips, and the depletion of additives from the upper parts of the discharge are discussed. Particular attention is paid to quantitative details of the Na diffusion.

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