Influence of the Cathode Surface on Arc Velocity

An arc cathode spot in a transverse magnetic field may move either in the Amperian (forward) direction or in the opposite, i.e., retrograde sense. The rate and direction of movement are dependent on a number of parameters such as arc current, magnetic field strength, and gas pressure at the cathode root. New experiments which demonstrate clearly the effect of cathode oxide layers on arc velocity are described. A simplified model of the cathode root region of the arc which is consistent with the experimental results is proposed. The predictions of this model, which is applicable to both Amperian and retrograde motion, are also in satisfactory agreement with results obtained by previous investigators.