Evolution of the Cathode Spot Distribution in an Axial Magnetic Field Controlled Vacuum Arc at Long Contact Gap

The evolution of the cathode spot distribution is an important aspect of the behavior of vacuum arcs controlled with an axial magnetic field. Arcs of very high-current intensity (up to 43 kA) at a 50 Hz frequency have been generated in a vacuum chamber between two static CuCr25 electrodes spaced 20 mm and 30 mm apart. Arcs were initiated by means of an auxiliary electrode located at the circumferential periphery of the cathode and were controlled by an axial magnetic field (up to 270 mT) produced by external Helmholtz coils. Arcs were filmed with a high-speed camera.
An image processing method inspired by the literature has been developed to detect cathode spots on images of the cathode surface taken with the high-speed camera. The detection method has been applied to monitor the temporal evolution of the distribution of spots, from which various types of spot distributions have been identified according to the arc current intensity. The maps of detected spots have been used to reconstruct the distribution of the current density emitted at the cathode and to estimate the current intensity carried by a single spot.