The vacuum switch. Part 1: Properties of the vacuum arc

In any study of switching devices, an understanding of the current conduction processes in the steady or quasi-steady state must precede the investigation of the transient phenomena during the change from the conducting to the insulating state. The paper describes the results of an investigation into the nature of the arcs which are formed when the contacts of a vacuum switch separate. In many respects the vacuum arc is the simplest of all the high-current gas discharges, since there exists no ambient atmosphere to complicate the arcing processes, but this simplicity is only relative, and many assumptions have had to be made to enable a quantitative picture to be built up. The vacuum arc appears to have two very different states; in one, which seems to be the normal condition at currents below about 104A with copper electrodes, the arc is totally unlike high-pressure arcs, and burns as a set of completely separate arcs electrically in parallel. In the other states the arc much more nearly resembles high-pressure arcs. Most of the work described deals with the vacuum arc in the first state, and the density of neutral atoms, ions and electrons in the discharge is calculated as a function of current and position. The influence of the properties of the electrode materials on the arcing voltage and the stability of the arc is described, and the energy balance of the arc is computed. The data from the paper are used in a companion paper to investigate theoretically arc-extinction processes.