Cathode spot erosion and ionization phenomena in the transition from vacuum to atmospheric pressure arcs

Abstract

Cathode properties of arcs are investigated for ambient gas pressures of 10−3−725 Torr. Copper, silver, and carbon electrodes are used in controlled ambients of either nitrogen, argon, or helium. Experimental techniques include pressure-dependent observations of the ionized cathode vapor detected at cylindrical collectors surrounding the electrodes, arc photography, and measurements of the cathode erosion rate for copper at 100 A using the weight loss method. At pressures approaching atmospheric, it is shown that the measured erosion rate is reduced from that in vacuum by about an order of magnitude. Increasing the ambient pressure also decreases the distance from the cathode spots at which the 8% ion current can be detected. Data extrapolation shows that cathode spots at atmospheric pressure, particularly in the lighter gases, are still associated with the fundamental ion current. Comparison between the measured reductions in both the erosion rates and the ion currents indicates that vapor and ion redeposition on the cathode contribute to the decreased erosion. It is concluded that the cathode spots of low current arcs at pressures approaching atmospheric are basically similar to vacuum arc cathode spots. The final section of the paper compares current-dependent erosion data for copper arcs in vacuum and air, and shows the importance of collective cathode spot heating.