Study of high-current vacuum arcs in axial magnetic fields

This paper presents an experimental investigation into the behaviour of metal-vapour arcs in vacuum with and without superimposed axial magnetic fields on copper (99.95 % pure electrolytic) electrodes. Properties of vacuum arcs have been investigated for a discharge current up to about 5 kA. Low-current arcing voltage and average current per spot have been measured. Voltage-current characteristics for high-current arc in axial magnetic field up to 0.1 T have been obtained. High-current arc voltage has been measured as a function of the arc current and the diameter of the expanding ring of cathode spots. From the measurements, an empirical relationship for the arc voltage has been derived for its linear behaviour. High-speed framing photography has been used in the investigation to record the cathode spots, plasma columns and gross features of discharge as a whole over a wide range of conditions. It has been found that there is an axial magnetic field that gives the critical plasma confinement. Cathode-spot motion has been studied and retrograde velocity vs. self-magnetic field data are reported for copper cathode.