Cathode spots and arc structure in a dense, axial magnetic field-stabilised vacuum arc

The structures of cathode roots on AMF contacts at gap distances of a few millimetres are investigated by observation of the arc behaviour with a high-speed, high resolution electronic camera. While the current carried by a single cathode spot is almost independent of the arc current, the number of the cathode spots and the voltage drop between the contacts increase linearly with the total current. At the limit of the breaking capability of the contacts the average current density is about 1.8 kA/cm² (effective current), which conforms well to the experience with commercial vacuum interrupters. At this point the CuCr contact surface melts homogeneously over several tenths of square centimetres. Although the molten surface layer thickness is assumed to be a few tens to hundreds of microns at most, millimetre-sized protrusions grow rapidly, providing the impression of a “boiling” surface. The growth and dynamics of these surface structures is 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.