Simulation of electrode material deposition at the tube wall of a high-pressure mercury arc discharge lamp

In mercury arc discharge lamps electrodes consisting of a tungsten rod and emission material (sintered mixture of tungsten powder and alkaline-earth metal oxides) pressed on it in the form of a cylinder are used. In the process of lamp operation, the oxides are evaporated from the emission material surface, move in the background gas and are deposited at all sections of the discharge volume boundary including the lamp quartz tube wall, which results in lamp longevity decrease. Transport of evaporated material in the discharge volume was investigated earlier under the assumptions of its one-dimensionality and two- dimensionality in the vicinity of the electrode only. In this paper, a two-dimensional model of evaporated electrode material transport to the lamp tube wall based on the numerical solution of the diffusion equation is presented. Distribution of the material flow density along the tube wall is calculated and its dependence on the gas pressure is obtained. It is found that electrode material transport to the wall substantially decreases with background gas pressure increase in the tube. In particular, under the gas pressure 2660 Pa usual for cold lamps the material flow at the wall is about two orders higher than under the atmospheric pressure typical for heated lamps after mercury evaporation. Therefore, main material losses from the electrodes take place at the stage of lamp heating after its turning on, which agrees with experimental data.