Observation of Thermal Cathodic Hot Spots in a Magnetically Rotating Arc Plasma Generator

At atmospheric pressure, cathodic arc root tends to shrink to a luminous hot spot, which limits arc column expanding and accelerates cathode ablative rates. To obtain a nonconstricted cathodic arc root, we built a magnetically rotating arc plasma generator that mainly consists of a cylindrical graphite anode chamber, a concentric lanthanum tungsten cathode, and a solenoid coil for producing an axial magnetic field (AMF). Evolution of self-organized multihot spots on cathode end is observed in this study. Results show that with the AMF, arc currents, or/and cathode temperature increasing, the spot's quantity gradually increases, and at last multispots evolve into a diffuse annular one. When the arc column is constricted, the arc moves on the spots periodically. Thus, a single constricted cathodic root is formed. By controlling the axial gas flow, the constricted arc converts into a diffusive one which covers all spots simultaneously, and then multiroots or diffuse annular root are developed. The cathodic spots and roots formation mechanism are proposed, and experimental results support the prediction of nonlinear surface heating model.