Anode Spot Formation Threshold Current Dependent on Dynamic Solid Angle in Vacuum Subjected to Axial Magnetic Fields

The objective of this paper is to experimentally determine a relationship between an anode spot formation threshold currentIₜₕand a dynamic solid angle in drawn vacuum arcs subjected to axial magnetic fields (AMFs). Experiments are performed in a demountable vacuum chamber, in which a Helmholtz coil is installed coaxially with a pair of butt type contacts. Dynamic solid angleΩis related to arcing process, which is subtended by the anode from the cathode center or by a ratio of the anode diameterDto the contact separation lengthlas arc extinguishes$(D/l)$in a drawn vacuum arc. Although a traditional definition of solid angle$(D/g)$is related to a fixed full contact gapg, no arcing process is considered. In the experiments, butt type contacts are used with contact material CuCr25 (25% Cr), and the contact diametersDare 12, 25, 40, 60, and 80 mm. The contact separation lengthlas arc extinguished is controlled in a range of 12–24 mm determined by an opening velocity with an arcing time fixed at 10 ms. A dc AMF is applied to the pair of contact with the flux densityBAMFadjusted from 0 to 122 mT. Arc current in the tests is at a range of 0–30 kA rms at 50 Hz. The results show that anode spot formation threshold currentIₜₕis linearly dependent on the dynamic solid angle$D/l$adjusted either byDorl, respectively. And the threshold currentIₜₕis also linearly dependent on the AMF flux densityBAMFapplied. Contact diameterDshows more significant influence on the threshold currentIₜₕthan the contact separation lengthlas arc extinguished. In a given contact diameterDand contact separation lengthlas arc extinguished, there is a critical peak arc voltage value, at which an anode spot first formed, regardless of arc currentIarcand AMF flux densityBAMFapplied.