Dynamics of Arc Ignition and Cathode Spot Movement of Thermionically Emitting Cathode Surfaces

Abstract

If a thermionically emitting cathode is used in an arc discharge the heat feedback from the gaseous plasma to the solid surface provides the continuity of the arc process. The response time of the temperature fluctuation of the solid‐surface material is found to be at least an order‐of‐magnitude longer than that of the plasma, therefore, the rate process is necessarily controlled by the slowest branch in the process, namely, the solid material. Based on this logic, the initial stages of electric arc ignition and cathode spot growth are analyzed for a thermionically emitting cathode. A feedback heat‐transfer process which couples the plasma and the solid surface is hypothesized to govern these phenomena. While the model chosen has been simplified to the extent that quantitative significance may be lost, however, several previously observed qualitative features of arcs are explained, one of which is the need for a minimum current density for the persistence of a thermionic arc. The cathode spot growth can also be accounted for by this hypothetical model.