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

Abstract

Electrodes made of pure and doped tungsten are operated in a special model lamp with a DC current of 1-10 Ain argon or xenon atmosphere within the pressure range of 0.1-1 MPa. Cylindrical electrodes withdifferent 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 isthe current density, which is low in the case of the diffuse mode and high in the spot mode. The diffuse mode isfavoured by high current, low pressure and weak cooling of the electrode, the spot mode by the oppositeconditions. In a transition region the cathode changes statistically between these modes. Whereas the globalelectrode temperature and the cathode fall of the diffuse and the spot mode differ slightly, the super-spot modeis associated with a significantly decreased global electrode temperature and cathode fall at similarparameters. SEM pictures show that the surface structure of the electrodes has wide influence on the mode ofcathodic arc attachment. Due to the significant difference between the super-spot mode and the two other modesthis paper is concentrated on the comparison between the spot and the diffuse mode.