Consequences of geometry and material properties on breakdown in high pressure lamps

Optimizing breakdown and startup in high pressure lighting sources, such as metal-halide lamps, is of interest for both increasing electrode lifetime, by reducing voltage, and enabling re-ignition of a warm lamp. In this paper, we report on computational and experimental investigations of breakdown processes in real and idealized lamps with the goal of developing scaling laws for lamp design. The computational platform is a 2-dimensional plasma transport model using an unstructured mesh to resolve fine physical features of the lamp. Electric field emission, secondary electron emission and photoionization by radiation transport are included in addition to transport equations for all charged and neutral species. The experiments consist of an idealized coaxial lamp having an adjustable gap and cylindrical quartz tube. Diagnostics include I-V characteristics and optical emission. Scaling laws for breakdown processes, derived from the model, will be discussed for cold and warm Ar/Hg lamps having initial pressures of 10s-100s Torr and gaps of many cm. Emphasis will be placed on optimization of the lamp geometry and materials. To validate the model comparisons will be made between the model and experiments for simpler coaxial geometries for breakdown in Ar, while varying gap, pressure and geometry.