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.

Arcing voltage characteristics of metal vapour arcs in vacuum for seven metals (copper, aluminium, tin, magnesium, zinc, cadmium and bismuth) of high purity and low gas content have been investigated. These metals cover a fairly wide range of thermophysical parameters. A triggered vacuum switch with circular butt contacts of 75 mm diameter and 15 mm separation was used and the discharge current was varied up to about 10 kA. The low-current arcing voltage and average current per cathode spot are reported for these metals. The empirical relationships for arcing voltages are derived for the linear range of the voltage-current characteristics. Voltage-current characteristics in axial magnetic fields up to 0.2 T are presented and the observed similarities and differences in the behaviour of the metals investigated are explained. Finally, the axial magnetic field necessary for giving a minimum arcing voltage is discussed.

No abstract

No abstract

No abstract