Observation of different modes of cathodic arc attachment to HID electrodes in a model lamp

Electrodes made of pure and doped tungsten are operated in a special model lamp with a DC current of 1-10 A in argon or xenon atmosphere within the pressure range of 0.1-1 MPa. Cylindrical electrodes with different designs are investigated with regard to the mode of cathodic arc attachment. Three modes are observed: a diffuse mode, a spot mode and a super-spot mode. The major difference between the diffuse and the spot mode is the current density, which is low in the case of the diffuse mode and high in the spot mode. The diffuse mode is favoured by high current, low pressure and weak cooling of the electrode, the spot mode by the opposite conditions. In a transition region the cathode changes statistically between these modes. Whereas the global electrode temperature and the cathode fall of the diffuse and the spot mode differ slightly, the super-spot mode is associated with a significantly decreased global electrode temperature and cathode fall at similar parameters. SEM pictures show that the surface structure of the electrodes has wide influence on the mode of cathodic arc attachment. Due to the significant difference between the super-spot mode and the two other modes this paper is concentrated on the comparison between the spot and the diffuse mode.