The anode region of low current arcs in high intensity discharge lamps

A survey is given of the behaviour of arc anodes which are operated under conditions being existent in high intensity discharge lamps. Experimental investigations are mainly performed at a special model lamp. They comprise optical observations of the anodic arc attachment, spatially resolved pyrometric measurements of the electrode temperature, electrical measurements of the anode fall and spatially resolved spectroscopic measurements of the electron temperature and density in front of the anode. A model of the anodic boundary layer is worked out composed of a constriction zone with an enhanced electrical power input and an anode sheath which may be an electrical generator. It is confirmed by comparing with the measured data that the power input into the anode is composed of a term linearly dependent on the arc current with a proportionality constant formed by the sum of the electrode work function and the electron enthalpy within the arc column and a small correction term being reduced with increasing anode temperature. A reduction in the work function by an anodic emitter effect is demonstrated for a high pressure sodium discharge, but in contrast to the cathode it is absent if thoriated electrodes are used. If HID electrodes are operated with switched dc or ac currents interferences between the anodic and cathodic phases occur. (topical review)