Estimation of Critical Axial Magnetic Field to Prevent Anode Spots in Vacuum Interrupters

Previous works have shown a linear anode spot formation threshold current (I_th) dependence on the critical axial magnetic field (AMF), B^crit_AMF, in a vacuum under applied AMFs of up to ~110 mT. However, the effects of the contact design parameters (e.g., contact diameter, contact gap, and contact material) on this dependence have not yet been revealed. Here, we define this dependence as k, which signifies the critical AMF strength per unit arc current needed to prevent an anode spot in a vacuum arc. The objective of this paper is to determine k as a function of the contact design parameters in AMF vacuum interrupters (VIs), including the contact diameter, the arc extinguishing contact gap, and the contact materials, to prevent anode spot formation. Experiments were conducted in a demountable vacuum chamber, in which an external Helmholtz AMF coil was installed coaxially around a pair of butt-type contacts. The contact diameter D was in the 12-100 mm range and the arc extinguishing contact gap l was in the 12-24 mm range. Three contact materials were investigated: 1) oxygen-free high-conductivity Cu; 2) CuCr25 (25% weight of Cr); and 3) CuCr50 (50% weight of Cr). The BAMF applied by the external coil was uniform and varied from 0 to 110 mT. The experimental results showed that k = qD^-α, where D is the contact diameter, and q and α are constants. The k increased linearly but slowly with l, from 6.6 to 7.1 mT/kA when l increased from 12 to 24 mm. In addition, the k values obtained for the different contact materials were similar to each other. With D = 60 mm and l = 24 mm, k only varied from 6.3 to 7.1 mT/kA among the three contact materials. Therefore, the influence of l and the contact materials on k was insignificant when compared with that of D. This data could be useful for compact AMF VI designs in which anode spots do not form.