De-mixing observations in a metal-halide arc lamp

Summary form only given. We have used high-energy X-ray induced fluorescence to measure the elemental distributions in a ceramic metal-halide arc lamp. These distributions have allowed us to examine elemental ratios in the arc, with the aim of improving the understanding of transport phenomena and their effect on the chemical equilibrium. Even in a relatively small arc (radius 0.45 cm), depletion of the metal additives from the axis is clearly seen. However, just outside the core, we observe relative excesses in the additive densities with respect to Hg and I. Measurements have been obtained near the mid-plane of the arc as well as close to the electrodes. X-ray induced fluorescence is particularly useful for making such measurements because no assumptions with respect to local thermodynamic equilibrium are required to obtain elemental densities. The ratios of elemental densities are unaffected by drifts in either the arc or the X-ray intensity when densities for different elements are acquired simultaneously. Furthermore, temperature profiles are retrieved from the Hg distribution, enabling a more stringent test of numerical predictions.