Anode Erosion Pattern Caused by Blowing Effect in Constricted Vacuum Arcs Subjected to Axial Magnetic Field

The objective of this paper is to investigate the anode erosion pattern caused by the blowing effect in constricted vacuum arcs under an axial magnetic field (AMF). The blowing effect was reflected from the microstructure of the melt layer on the anode. A pair of 42-mm-diameter AMF contacts were installed inside a detachable vacuum chamber. The contact material was CuCr25 (25% Cr). The arc modes were observed with a high-speed charge-coupled device video camera. The anode surface temperature after current zero was measured with a two-color pyrometer. The microstructure of the cross section of the anode surface melt layer was observed by scanning electron microscopy. The experimental arc currents increased from 10.5 to 17.8 kA (rms) with an interval of 2 kA. The results show that the anode erosion pattern in constricted arcs is caused by an interactive process of thermal and mechanical effects. The constricted arc columns melted the anode surface material and blew the molten material from the center to the peripheral region. The redistribution of the anode molten layer caused by this blowing effect formed two erosion regions on the anode surface. The depth of the melt layer in the central region was less than 100 mu m, but in the peripheral region it was in the range of 100-400 mu m. In these different erosion regions of the anode melt layer, there was considerable variation in both size and shape of the Cr sphere. Moreover, the thermal and mechanical effects of the constricted arc redistributed the Cr content of the molten layer on the anode surface. The Cr content in the central region (Erosion Region I) increased to 31% and in the periphery region (Erosion Region II) it decreased to 18%, respectively, from 25% before the experiments.