THEORETICAL-STUDY OF FACTORS INFLUENCING ARC EROSION OF CATHODE

A theoretical model has been formulated describing the influence of the arc condition and the cathode material and geometry on arc cathode erosion. To arrive at a self-consistent description for the entire arc cathode attachment region, a realistic, one-dimensional sheath model has been used. This sheath model is supplemented by an integral energy balance of the ionization zone between the sheath and the arc, and by a differential energy balance of the cathode. For the case of a tungsten cathode, it has been shown that arc constriction has a strong effect on the current density at the cathode spot but little influence on the cathode temperature. It has also been found that heat conduction within the cathode and radiation from the cathode surface control the energy transport from the cathode spot at low currents, and dissipation by thermionic electron release dominates at high currents. The conclusions are that for high-current applications erosion can be minimized by using a cathode material with a low vapor pressure, and that for these conditions the thermal design of the cathode plays a secondary role. In contrast, for low currents the erosion will be determined by the thermal characteristics of the cathode material.