Analysis of the arc-cathode interaction of free-burning arcs

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 in an argon arc, it has been shown that the ion current density is almost 50% of the total current density at low arc currents, while it decreases to about 18% of the total current density and the thermionic electron current density increases to about 82% of the total current density at high currents. It has also been found that heat conduction within the cathode and radiation from the cathode surface control energy transport from the cathode spot at low currents, and that dissipation by thermionic electron release dominates at high currents.