ELECTRODE PHENOMENA IN DC ARCS AND THEIR INFLUENCE ON PLASMA TORCH DESIGN

Control over the physical phenomena, which characterize the arc — electrode interaction strongly, influences plasma torch design. Arc cathodes are characterized by their electron emission mechanism as thermionic (hot) cathodes or as cold cathodes with an explosive or evaporative emission, and by their geometrical configuration as rod, button or well type cathodes. The anode is, in most cases, a passive component of the electrical circuit, collecting electrons from the arc, and the characterization of this current transfer is intimately coupled with the thermal and flow fields in the anode region, i.e. the anode boundary layer. The anode surface can be perpendicular or parallel to the arc axis and the flow. Since the arc electrode surface is necessarily in contact with the high temperature plasma, cooling is necessary and some erosion usually unavoidable. Means for controlling the arc - electrode interaction will be discussed concentrating on the use of fluid dynamics to influence the boundary layers. Recent theoretical and experimental results will be presented, which demonstrate the effectiveness of such approaches.