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 current $I_{rm th}$ and 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 angle $Omega$ is related to arcing process, which is subtended by the anode from the cathode center or by a ratio of the anode diameter $D$ to the contact separation length $l$ as 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 gap $g$, no arcing process is considered. In the experiments, butt type contacts are used with contact material CuCr25 (25% Cr), and the contact diameters $D$ are 12, 25, 40, 60, and 80 mm. The contact separation length $l$ as 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 density $B_{rm AMF}$ adjusted 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 current $I_{rm th}$ is linearly dependent on the dynamic solid angle $D/l$ adjusted either by $D$ or $l$, respectively. And the threshold current $I_{rm th}$ is also linearly dependent on the AMF flux density $B_{rm AMF}$ applied. Contact diameter $D$ shows more significant influence on the threshold current $I_{rm th}$ than the contact separation length $l$ as arc extinguished. In a given contact diameter $D$ and contact separation length $l$ as arc extinguished, there is a critical peak arc voltage value, at which an anode spot first formed, regardless of arc current $I_{rm arc}$ and AMF flux density $B_{rm AMF}$ applied.