Cathode erosion in high-current high-pressure arc

Cathode erosion rate was experimentally investigated for two types of arcs: one with tungsten cathode in nitrogen atmosphere and one with hafnium cathode in oxygen atmosphere. Conditions were typical for plasma arc cutting systems: gas pressure from 2 to 5 atm, are current from 200 to 400 A, gas flow rate from 50 to 130 litre min(-1). It was found that the actual cathode evaporation rate G is much lower than G(0), the evaporation rate that follows from the Hertz-Knudsen formula: G = nuG(0). The difference is because some of the evaporated particles return back to the cathode. For conditions of our experiments, the factor v could be as low as 0.01. It was shown experimentally that v depends strongly on the gas flow pattern close to the cathode. In particular, swirling the gas increases v many times. To explain the influence of gas swirling, model calculations of gas flows were performed. These calculations revealed difference between swirling and non-swirling flows: swirling the gas enhances the gas flow from the cathode. This enhanced flow sweeps the evaporated particles away from the cathode and thus increases the net evaporation rate. Electrode erosion associated with arc ignition (start erosion) was observed for both types of cathodes. Models of start erosion were suggested. In case of hafnium cathode the start erosion is due to cracking out the hafnium oxide layer, which was created during previous arc operation. In the case of the tungsten cathode the start erosion is produced by a cold cathode are (vacuum arc), which exists at the cathode during first moments after arc ignition.