Joule heating in emitting sites on various nonrefractory arc cathodes

Abstract

The most common and maximum radii of craters on cathodes of copper, mild steel, silver, silver cadmium oxide and aluminium having various oxide film thicknesses, are reported following 4.5 A arcing of duration between 3.8 ns and 3.5 μs. The increase in crater radius with arc duration is examined in the light of a simple joule-heating model in a hemispherical region from which material is lost to form a crater. All the results except one conform with adiabatic joule heating, the one exception involving heat conduction into the cathode being no longer negligible. A brief comparison with cratering in cathodes of vacuum arcs is made, showing that these are of the latter type where joule heating provides the dominant energy input and the crater radius tends to increase to a value where this input is balanced by thermal conduction into the bulk metal. Close agreement with the model confirms earlier estimates by other methods of mean emitting site currents, current densities and lifetimes for copper and steel, and then enables these quantities to be estimated for the first time for silver, silver cadmium oxide, aluminium and also for mild steel with a relatively thick oxide film.