Experimental investigation of anode arc attachment area in DC arc plasma torch at low pressures

Abstract

The anode area and anode erosion of a DC arc plasma torch with an external anode are experimentally investigated using different ambient pressures, arc currents, and anode configurations. A high-speed camera, high-voltage probe, electric probes, and a Schlieren system are utilized. The electric field in the anode area is measured using electric probes at arc electric currents lower than 100 A. It is found that a mean value of the arc electric field above the anode can be satisfactorily estimated in a non-intrusive way when the anode attachment is constricted. Anode erosion is proportional to a variable that can be compared in different experimental conditions just by quickly processing high-speed video footage, even while the plasma torch is in operation. Anode erosion decreases along with ambient pressure and it is lower when a diffuse anode arc attachment is present as opposed to a constricted anode attachment. The sources of plasma fluctuations inside and around the plasma jet are mainly the movement of the anode arc attachment and turbulent vortices. The average speed of the anode arc attachment increases with decreases in ambient pressure. On the contrary, the average period of the restrike process, or the distance between the exit nozzle and the average attachment’s position, decreases with reductions in ambient pressure. We report ambient pressure values for the transition between a constricted and diffuse anode arc attachment and for the transition between subsonic and supersonic plasma flow.