Erosion of plasma torch electrodes

The erosion of copper electrodes in a concentric cylinder geometry with magnetically driven arcs was studied at steady state for currents up to 250 A in a variety of gases and gas mixtures, magnetic field strengths and gas flow rates. The effects of arc velocity, gas composition, current density and heat transfer to the cathode on erosion rates were examined.The arc velocity varied with the magnetic field strength and arc current accordingly to a newly developed equation, V $ alpha$ B$ sp{0.60}$ I$ sp{0.56}$, when the cathode surface was slightly contaminated with C, Cl, O or N. The composition of the surfaces was found using Auger and ESCA spectroscopy. A surface drag force, a new force opposing to the arc motion was proposed. In the case of clean and heavily contaminated surfaces (contaminant layers thicker than 10 microns), the surface drag becomes the major force opposing the arc movement. Work function measurements showed that surface drag increased as electron emission became more difficult.A novel technique was developed to determine the current distribution of the arc foot on the electrodes. A correlation between the arc foot current density and erosion rate was proposed. The effects of the surface composition, magnetic field and arc velocity on the arc current distribution were also examined.A conceptual model was developed for electrode erosion; the model was supported by experimental results and by the results of simulations using macroscopic and microscopic heat transfer models