Direct observation of de-mixing in a ceramic metal-halide arc lamp

De-mixing of additives in a vertically-operated, ceramic DyI 3 –CsI–Hg metal-halide arc lamp has been observed by examining the ratios of absolute elemental densities as a function of position. As the elemental densities have been obtained directly by x-ray induced fluorescence (Curry J J, Adler H G, Shastri S D and Lee W-K 2003 J. Appl. Phys. 93 2359), this approach reveals the extent of mixing (or de-mixing) with no intervening assumptions about equilibrium, the local temperature, or the accuracy of partition functions. The metal additives in the arc studied show complex de-mixing. Depletion of Dy and Cs from the core is attributed to ambipolar cataphoresis. These same additives also exhibit a relative enhancement in a layer just outside the core before decreasing again toward the wall. Thermochemical data are used to derive radial distributions of molecular species given the experimentally obtained elemental densities and the gas temperature assuming a lamp operating pressure of 10 6 Pa (10 atm).