Observation of an extremely constricted cathodic arc attachment to electrodes of high intensity discharge lamps

Abstract

Usually two modes of arc attachment to cathodes for high intensity discharge (HID) lamps are observed, the spot mode for high pressure, low currents and large electrode diameters and the diffuse mode for the opposite conditions. Recently, a very constricted attachment to cathodes was observed both in real HID lamps and in Bochum's model lamp. In the model lamp, the extremely constricted arc attachment was observed in argon and xenon atmosphere at a pressure p = 0.26 MPa and for currents I = 1-6 A. Whereas the global electrode temperature and the cathode fall of the diffuse and the spot mode differ only slightly, the extremely constricted arc attachment (so-called `super spot mode') is associated with a significantly decreased global electrode temperature at similar parameters. The reduction in electrode temperature implies low cathode falls and low electrode losses, respectively. Scanning electron micrographs show that the surface structure of the electrodes has substantial influence on the mode of cathodic arc attachment.