Transient low-current arc cathode processes on metals having a very thin oxide film in air at atmospheric pressure

Twelve different metals, having oxide films of a few nanometres, were used as the cathode for a single 4 A arc in air at atmospheric pressure, about 30 ¿¿m long and of duration between 3.8 ns and 3.5 ¿¿s, and the surface was then examined by scanning electron microscopy. Three groups of metals were distinguished; those having semiconducting oxide leading to scattered craters; those with insulating oxide leading to dense cratering and considerable melting; and those with volatile oxide leading to little or no cratering. The growth with increasign arcing time of the circular areas containing these craters was measured and found to be remarkably similar, and, in the light of this, various aspects are examined. A number of simple models of the cathode region for these arcs are outlined. These include an examination of the gaseous and solid-state regions of the cathode fall and mechanisms of producing the high electric field required within the oxide, for electron emission by positive-ion charging or plasma jet biasing, and possible effects of microinclusions. Different assumptions about the current-density distribution are examined. It is concluded that positive-ion bombardment is likely to cause electron emission for aluminium cathodes, but that further work is needed for other metals.