Cathode erosion in inert gases: the importance of electrode contamination

Experimental results are presented for electrode erosion on Cu electrodes in magnetically rotated arcs in Ar and helium. Measurements were also made of the arc voltage and velocity. The effects due to the contamination of the electrode surface by either a native contaminant layer (copper oxide and carbon traces) or the continuous injection of very small amounts of various diatomic gases (nitrogen, oxygen, chlorine, and carbon monoxide) into the inert plasma gases were determined. The erosion rates for pure Ar were significantly higher than those for pure He (13.5 mu g/C for Ar and 1 mu g/C for He) and with both gases, very high arc velocities were measured initially ( > 60 m/s for Ar and > 160 m/s for He) when a natural contaminant layer was still present on the cathode. The removal of this layer resulted in lower velocities (2 m/s for Ar and 20 m/s for He) and higher erosion rates. The removal of the layer was much faster with Ar, due possibly to higher electrode surface current densities for Ar arcs. 23 ref.--AA