Influence of AMF on the Expansion Speed of Cathode Spots in High-Current Triggered Vacuum Arc

In this paper, the influence of an axial magnetic field (AMF) on the expansion speed of cathode spots (CSs) in the initial expansion stage of high-current triggered vacuum arc is investigated experimentally in a demountable vacuum chamber. Butt contacts with contact plate material of oxygen-free high-thermal-conductivity Cu and CuCr25 were used in experiments. Images of CSs are photographed with a high-speed digital camera with an exposure time of 2 $\mu{\rm s}$ . The uniform constant AMF in the intercontact region is supplied by external magnetic field coil. Experimental results indicate that, in an arc stabilized by AMF, CS expansion speed also increases with the increase of $di/dt$ , which is the same as that in arc burning freely. In addition, quasi-quantitative analysis of the experimental results show that the bending and lengthening of CS motion trajectory in AMF are not sufficient to explain the decrease of CS expansion speed with AMF. Another reason might be related to the inhibitory effect of AMF on the loss of arc plasma, which could also contribute to the decrease of CS expansion speed.