Arc Cathode Processes in the Transition Region between Vacuum Arcs and Gaseous Arcs

Abstract

The cathode processes of electric arcs on cleaned Cu cathodes were investigated in the transition region between vacuum and atmospheric pressure (argon). The plasma density in the cathode plane was estimated by probe measurements to be n = (3 x 1012 A-l m-l) IA/r2 ( I A -arc current, r -distance from the spot). It was observed that several cathode spot parameters have an extremum at p cu 1oP Pa. The crater diameter has a minimum independently of the cathode temperature. The diffusion constant of the chaotic motion determined by framing photographs was found to have a maximum. Some additional, large displacements occurred a t that pressure. The diameter of the bright plasma cloud obtained by open-shutter photographs showed a maximum, the current per spot was found to decrease from 20 A in vacuum to 10 A at atmospheric pressure. It is thus concluded that the spot with the smallest crater radius and a low current per spot, occurring at -104 Pa, represents the single spot, whereas the spot a t higher pressures, and probably also in vacuum, has a complicated nature where the large craters are formed by a cooperation of single spots.

The dynamics of arc cathode spots in vacuum: new measurements

“…The periodic attraction and repulsion of the fragments results in a random displacement of the spot as a whole. This, among other factors, could explain the influence of external gas with pressures much smaller than the maximum plasma pressure: 100 mbar can be sufficient to change the spot velocity (Anders and Jüttner 1987) or the direction of the motion from retrograde to forwards in an external magnetic field (St John and Winans 1954). Between the fragments the plasma pressure is smaller, so external gas can reduce the interaction among them and influence the movement.…”