Anode spot threshold current of Cr and Mo under axial magnetic field in high current vacuum arcs

Contact materials have a significant impact on the anode spot phenomenon in vacuum arcs. The objective of this paper is to investigate the anode spot threshold current subjected to axial magnetic fields (AMFs) for Cr and Mo materials. The experiments were performed in a demountable vacuum chamber, in which a Helmholtz coil was set coaxially with a pair of butt-type contacts to supply axial magnetic fields (AMFs). The axial magnetic field was set at 37mT, 74 mT, and 110mT. Arc current in the experiment was at a range of 0-25 kA (rms) at 50 Hz. The contact diameter was 60mm. Contact separation gap was 28mm. A high-speed charge-coupled device video camera was used to record the vacuum arc evolution through the observing windows. Arc current and arc voltage were recorded simultaneously by an oscilloscope. The onset of anode spot phenomenon was determined from a combined determination of high-speed photographs, arc voltage and contact erosion pattern. The experimental results showed that the anode spot threshold current increased with higher AMFs. For Cr, the threshold currents were 11.9kA, 14.1kA and 17.2kA subjected to the three different AMFs (37mT, 74mT, 110mT). For Mo, the threshold currents were 15.0kA, 17.7kA and 22.8 kA subjected to the three different AMFs (37mT, 74mT, 110mT). Meanwhile, the threshold current for Mo was always higher than Cr because of different thermal and electrical parameters.