Breakdown processes in metal halide lamps

Metal halide lamps typically have cold fills of tens to a few hundred Torr of a rare gas and the vapour from the dosing of a metal halide solid and mercury. Breakdown and starting of the lamp occurs following application of multi-kV pulses across electrodes separated by a few centimetres. Restarting of warm lamps is often problematic as the available voltage is insufficient to break down the higher pressure (>many atm) of metal halide vapour. In this paper, fundamental processes during breakdown in cold and warm, idealized metal halide lamps in mixtures of Ar and Hg are investigated using a two-dimensional fluid model for plasma transport. We find that the capacitances of the walls of the discharge tube and adjacent ground planes are important in determining the breakdown voltage and avalanche characteristics. The prompt capacitance represented by, for example, external trigger wires provides a larger E/N to sustain ionization early in the avalanche. This effect is lost as the walls charge and shield the plasma from the ground planes. More rapid breakdown occurs in slightly warm lamps having small vapour pressures of Hg due to the resulting Penning mixture. Warmer lamps, having larger mole fractions of Hg, have less efficient breakdown as the increase in momentum transfer of the electrons is not offset by the additional ionization sources of the Penning mixture.