Erosion mechanisms of hafnium cathodes at high current

An investigation of cathode erosion has been conducted for conditions similar to those encountered in a plasma cutting process. A hafnium insert in a water-cooled copper sleeve serves as the cathode. Modifications to the torch allowed the observation of the cathode surface during operation and measurement of material loss from the cathode during different phases of an operating cycle. Erosion has been found to be predominantly due to ejection of molten material droplets. Most ejection events are associated with changes in the conditions of the plasma, e.g. during start-up, change of gas flow and shutdown. The ejections can be explained by imbalances of the forces acting on the molten surface, those associated with the arc current, those due to surface tension, and those associated with the fluid dynamics within the torch.