Experimental investigation on the current of cathode spot of vacuum arc in axial magnetic fields

The splitting and threshold currents (I_s and I_thr, respectively) carried by a single cathode spot of vacuum arcs under the influence of an external axial magnetic field (AMF) up to 220mT are investigated experimentally. Experiments are conducted with OFHC copper and CuCr25 butt electrodes in a demountable vacuum chamber. The electrode separation is fixed at 6 mm. The spots are photographed using a high-speed framing camera with an exposure time of 2 μs. The uniform AMF (B_n) is applied by means of Nd-Fe-B permanent magnets placed symmetrically inside the chamber. It is found that for either cathode material, with increasing B_n, the splitting current I_s initially (0 ≤ B_n ≤ 20 mT) declines rapidly. It then remains basically constant. For both cathode materials, the threshold current I_thr exhibits a similar trend to the splitting current I_s. A possible explanation is proposed to account for the above results.