Model predictions of arc cathode erosion rate dependence on plasma gas and on cathode material

Abstract

We have previously reported results of a theoretical study which predict that high current arc cathode erosion is predominantly dependent on the work function and the vapor pressure of the cathode material, and that the thermal design plays a secondary role. These results have been obtained with a newly developed self-consistent model of the cathode region including a realistic one-dimensional sheath model. The results have been obtained for an argon arc and a tungsten cathode. The model has now been extended and results have been obtained for different arc gases and different electrode materials. The arc gas has a strong effect because it affects not only the temperature at the boundary between the arc and the cathode region, but also the electron density in the cathode region and at the cathode. The results of the calculations show that the cathode material plays a dominant role in terms of the cathode spot temperature and the associated mass loss rate by evaporation of cathode material. Since the addition of thorium oxide to tungsten reduces the work function of the cathode material, the cathode spot temperature as well as the mass loss rate by evaporation are reduced. For the same cathode material, hydrogen leads to the highest cathode spot temperature and mass loss rate, followed by nitrogen and argon. The current density at the cathode spot, the cathode spot size, and the percentages of the energy fluxes removed from the cathode spot are mainly determined by the plasma gas rather than by the cathode material.