Cathode Spot Dynamics and Arc Structure in a Dense Axial Magnetic-Field-Stabilized Vacuum Arc

The structures of cathode roots on axial magnetic field (AMF)-stabilized contacts at gap distances of a few millimeters are investigated by observation of the arc behavior with a high-speed high-resolution electronic camera. While the current that is carried by a single cathode spot is almost independent of the arc current, the number of cathode spots and the voltage drop between the contacts linearly increase with the total current. At the limit of the breaking capability of the contacts, the average current density is about 1.8 (effective current), which conforms well to the experience with commercial vacuum interrupters. At this point, the CuCr contact surface homogeneously melts over several tenths of square centimeters. Although the molten surface layer thickness is assumed to be a few tens to hundreds of micrometers at most, millimeter-sized protrusions rapidly grow, providing the impression of a “boiling” surface. The growth and dynamics of these surface structures are discussed and compared with the details of spatial-temporal measurements, and an explanation is given for the high rates of acceleration of droplets in the radial direction.