Emitting-site lifetimes, currents and current densities on arc cathodes with 100 nm thick copper-oxide films

Static arcs of durations between 3.8ns and 40 ¿s have been initiated, in atmospheric-pressure air, on copper cathodes with an oxide film 100nm thick. By using a scanning electron microscope, the number and size of craters left by emitting sites and the total surface area (containing all of these many craters), from which the oxide had been stripped away, have been measured as a function of arc current as well as of arc duration. These results indicate that for this oxide film, a single cathode emitting site has an average lifetime which is only about 4.5 ns and this is therefore much smaller than has been thought to be the case for copper oxide, which indicates that the large increase in conduction through the oxide occurred by switching rather than by electroforming. The very small arc durations have yielded a close estimate of the number of coexisting sites, thus giving average site currents of about 11 mA at densities of about 1.4 × 1012 A/m2