Cathode spot motion in high-current vacuum arcs under self-generated azimuthal and applied axial magnetic fields

Abstract

The motion of vacuum arc cathode spots under the influence of self-generated azimuthal and externally applied axial magnetic fields has been investigated for several metals at currents up to about 10 kA. The spot motion was recorded by means of a high-speed framing camera, and the self-generated magnetic field was measured with an inductive magnetic probe. The axial magnetic field was applied by means of two coils round the vacuum chamber in an approximate Helmholtz arrangement. The data have been used to determine the relationships between the retrograde (anti-Amperian) spot velocity and the self and axial magnetic flux densities. At low self-generated fields, the velocity is directly proportional to the flux density but at high fields velocity saturation is observed in some cases. An axial magnetic field decreased the retrograde velocities on all metals investigated. The spontaneous spot formation observed with high rates of current rise is analysed; the velocity saturation phenomenon is explained semiquantitatively and the influence of the axial magnetic field on the cathode spot velocity discussed.