Experimental observations of cathode-spot surface phenomena in the transition from a vacuum metal-vapor arc to a nitrogen arc

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Abstract
This paper provides experimental data on the crater dimensions associated with low-current (50 A) arcs burning for periods of 100 ms on copper cathodes. The cathodes were arced in metal-vapor and nitrogen ambients, and the resulting cathode surface contamination was measured for each arcing condition. As the ambient pressure increased from 10−6 to 100 Torr, the crater diameters in the heavily arced areas decreased from 48–58 to 7–17 μm, the current carried per cathode spot decreased, and the surface contamination increased. We consider that the smaller crater dimensions are associated with the reduction in the maximum current per spot and that the maximum cathode-spot current is influenced by the thin layer of surface contamination. We compare the trend of pressure-related crater-size reduction with previous observations of crater sizes as a function of current for vacuum arcs. We conclude that an individual cathode spot operating at a given current level in a gaseous ambient has many of the same basic characteristics as an individual cathode spot operating at the same current level in vacuum. An unexpected feature though, confirmed by our measurements, is that the erosion rate at higher pressures can be significantly lower than the minimum vacuum erosion rate associated with the characteristic 8– 10% ion current. Redeposition may be a plausible explanation.