Observation of snow plow motion and surface plasma formation in plasma opening switches

The distribution of the inter-electrode plasma on an opening switch provides information about physical processes occurring during the conduction and opening phases of the switch. Two dimensional holographic interferometry has been used to measure the electron density distribution in the plasma injection region of the switch and the 11 cm long downstream region to the load. These measurements were complemented with streaked measurements of the luminous front propagation, spectroscopic measurements of its composition, and observations of the damage occurring on the cathode surface. The measurements reveal a snow plow front extending from the anode to the cathode which propagates toward the load. Dense surface plasmas are formed on the electrodes which propagate ahead of the snow plow front along the cathode surface. This is confirmed by the streak measurements which slow the cathode luminosity to propagate at a higher velocity than the front luminosity midway between the electrodes. Surface pitting of the cathode is observed near the load end of the injection region which extends 9 cm downstream of the switch region. The largest surface damage occurs here and is accompanied by discoloration of the electrode. The surface damage is characteristic of cathodic arc spots observed in vacuum arcs. It is direct evidence for the plasma generation processes occurring downstream of the injection region. The existence of a surface plasma up to the load during the conduction phase indicates that the switch never opened.