An Experimental Investigation of Cathode Spot Motion in a Magnetically Rotating Arc Plasma Generator at Atmospheric Pressure

Cathode spots present complex forms in magnetically rotating arc plasma generators due to the coupling effect of the plasma flow and electromagnetic fields. In this paper, a magnetically rotating arc plasma generator is built to study cathode spot motion at atmospheric pressure. Cathode spot configuration is observed and discussed for different magnetic fields, arc currents, gas flow rates and cathode lengths. Results show that cathode spots with a slow rotation speed (less than 1Hz) and different rotation directions occur on the cathode end. For a low magnetic field, low arc current, high gas flow, short cathode rod, the rotation direction of the cathode spot is consistent with the arc column rotation. With an increase in the magnetic field, increase in the arc current, decrease in gas flow, or increase in the cathode rod length, the rotation speed declines, and the cathode spot can move in a reversed direction (i.e., against the arc column rotation). The cathode spot area also appears to expand gradually when the spot motion shifts from the normal direction to the reversed direction, resulting in less cathode erosion at the reversed motion. Further analysis indicates that the cathode spot motion is similar to the retrograde movement that occurs in a vacuum arc with an external magnetic field. The rotation of cathode spot may thus be induced by the radialcomponent of the magnetic field. The cathode spot area that affects the self-magnetic field in the cathode spot is potentially a direct determinant of spot motion.