Investigations of Cathode Spots Behavior in Transverse Magnetic Fields

Investigations in an alternative current synthetic circuit have been performed in order to study the influence of transverse magnetic fields on the cathode spots behavior at a short gap (~3 mm) on CuCr contacts in vacuum. The experiments were conducted with currents from a few hundreds up to several thousands Amperes (8'000 A). With no magnetic field, it was observed that the cathode spots are located within a disk, whose radius grows with current and time (diffuse arc mode). Their motion is driven by the repulsive force between the spots. When a transverse magnetic field is imposed (>10 mT), the cathode spots had two opposite behaviors. First, the cathode spots located on the periphery of the disk in the anti-Amperian direction: those cathode spots keep their individual aspect and are quickly accelerated in a retrograde motion to the edge of the contacts, as expected in the literature. The other spots, the majority, tend to gather to form a group of spots in the Amperian direction. It has been observed that the higher the magnetic field strength, the denser the group of cathode spots become. It was also found that the concentration of cathode spots influences the plasma-electrode interaction at the anode. Indeed, it appears that the current at which the anode starts to melt is shifted to lower values in the presence of a transverse magnetic field.