Arc-cathode craters on copper at high currents and with reduced gas pressures

The authors have recently obtained values for the currents, current densities and lifetimes of the emitting sites on copper cathodes of arcs in atmospheric air. This was done by measurements, using s.e.m. magnifications up to 100000 times, of the diameters, surface densities and rates of production of submicron sized craters, for stationary 4.5 A arcs of duration 3.8 ns to 3.5 ¿s and for arcs of up to 68 A moving at speeds in the range 19 to 241 m/s. The copper cathodes had copper oxide films ranging in thickness from 2.5 to 340 nm, and large differences were found between cathode phenomena below 10 nm thickness and those with thicker films. This work has now been extended to a wide range of arc current and of ambient gas conditions. For atmospheric pressure, measurements have now been made of cathode craters for both 2.5 nm and 100 nm films, over which arcs up to 1.1 kA moved at speeds up to 190 m/s. Cathodes with both oxide films have been examined after arcs of up to 300 A have moved along them at reduced pressures in air and in nitrogen down to 1 torr. Both forward and retrograde motion have been included. In all cases, the cathode-crater diameters and surface densities have been shown to be similar to those obtained previously for lower current arcs in atmospheric air. For 2.5 nm films, stationary short duration arcs have been ignited in nitrogen at reduced pressure, and their rate of production has been shown to be similar to that for atmospheric air. The variation in cathode track width with arc velocity and current has been explored for 2.5 nm films, and is shown to differ from that for thicker films. These results indicate that, when gross melting is avoided, the copper-cathode emitting-site currents, current densities and lifetimes, already established by the authors, can be applied to many high-current moving arcs and to arcs in a variety of gas conditions.