Effect of copper-oxide thickness on the number and size of arc-cathode emitting sites

Static arcs of 12 ns¿42¿s duration have been initiated, in air at atmospheric pressure, on copper cathodes with oxide films between 2.5 and 340 nm thick. Using scanning-electron microscopy, the cathode crater diameters, surface densities and total number of craters in a given arcing duration have been measured. From these results, and from calculations of energy balance at a single emitting site, it has been possible to estimate emitting-site currents, current densities and lifetimes over the whole range of oxide thickness. The emitting-site current has been shown to remain virtually constant at about 10-15 mA over this range, but minima or maxima exist at about 25 nm in the curves of crater diameter, surface density, number of craters, site current density and site lifetime. This is clear evidence of major differences, as suggested earlier by the authors, between electrical conduction through the oxide and electron emission at copper-oxide films less than 25 nm thick and those of this and greater thicknesses. Further evidence exists in differences in the appearance of the craters. Current densities have been shown to range from 4×1011 to 4×1012A/m2, and the site lifetimes range from 1 to 100 ns. These times are too low for conduction through the oxide to have been by electroforming, involving a phase change, but they are quite consistent with switching.