Measurements of the anode temperature in a vacuum arc with an asymmetric hot refractory Mo anode

The temperature of an asymmetric anode was measured at three points in the anode body of a Hot Refractory Anode Vacuum Arc (HRAVA) using high temperature thermocouple probes placed near the front and rear surface. Different anode geometries with inclined front surfaces were used for arc currents in the range 125-225 A and for gaps with electrode separations of 5-18 mm. The measurements show that the transition period to the Hot Anode mode in the HRAVA decreased with arc current. Photographic study of the interelectrode region indicated that the plasma plume in the transition period expands from the anode toward the cathode and filled the gap. The plasma plume was asymmetrically distributed on the asymmetric anode surface, resulting in an asymmetric anode surface temperature distribution. Asymmetry of the anode surface temperature distribution increased for more asymmetric anodes and gap geometries. Molybdenum anode temperatures increased with arc current and decreased with electrode separation. The anode surface steady state temperature exceeded 2200°K-2300°K when the arc currents were larger than 145 A. The observed surface temperature variations were caused by fluctuations of the heat flux to the anode.