Cathode erosion mechanisms in an oxygen plasma cutting arc

Summary form only given, as follows. Erosion of a hafnium cathode imbedded in a water-cooled copper sleeve was investigated during arc ignition, steady state operation and arc shutdown using a high speed imaging system. The cathode was operated in a commercially available 200 A oxygen plasma cutting torch. Spectroscopic measurements have shown that the erosion by evaporation is very small for relatively short cutting cycles. Observation of the cathode during arc operation shows erosion of the hafnium is dominated by ejection of small metal droplets from the molten cathode surface. These droplet ejections occur predominantly during arc ignition and shutdown, and a special device allowed determination of the cathode erosion rate during the cutting cycle. The variation in erosion rate, droplet size and trajectory for the different times during the arc operation points towards different erosion mechanisms. Occurrence of the droplet ejection was correlated to the changes in the operating parameters of the cutting torch, such as current, voltage, gas pressure in the nozzle, plasma gas flow-rate and plasma gas type. A pressure imbalance on the molten cathode pool has been hypothesized as the cause of the ejections during arc ignition, while aerodynamic drag forces appear to be dominant during arc shutdown.