Anode Spot Threshold Current of Four Pure Metals Subjected to Uniform Axial Magnetic Field in High Current Vacuum Arcs

Abstract

High current vacuum arcs are essentially metal vapor arcs, and the contact material has a direct impact on the vacuum arcs. The objective of this paper is to experimentally determine the anode spot threshold current of vacuum arcs subjected to axial magnetic fields for four pure metals in vacuum arcs. The four pure metals are W, Mo, Cr, and Fe, which covers a wide range of thermal and electrical parameters. The experiments were performed in a demountable vacuum chamber, and a pair of Helmholtz coils was set in the chamber to supply uniform axial magnetic fields. The arc current in the experiments was in the range from 2 to 30 kA at 50 Hz. A high-speed camera was used to record the arc evolution through an observing window. The onset of anode spot phenomenon was determined by a combination of arc column images, arc voltage, and anode activity. From this combination criterion, the anode spot threshold current subjected to axial magnetic fields for the four pure metals was determined. The results show that the thermal characteristic T( $\text{k}\rho \text{c}$ ) $^{1/2}$ of the contact material has an important influence on the threshold current. Three cases of the thermal characteristic are considered. The ordering and proportionality between thermal characteristic and experimental data are studied. The results show that the axial magnetic flux density can significantly increase the threshold current. In addition, the anode spot threshold current for Mo and W increases more rapidly than that for Cr and Fe. Moreover, the anode emission of different metals is discussed.