Cathode Effects In Plasma Opening Opening Switches

Plasma as a conducting medium is injected into the anode - cathode gap of an opening switch. After a certain delay the generator voltage is applied to the gap and the switch conducts an increasing current until a fast opening happens, i.e. the switch resistance increases suddenly by about two orders of magnitude. Several different physical mechanisms have been proposed to explain the opening processes in short and long conduction time switches. The bipolar current model assumes that the limitation of the ion flow and ion erosion lead to the formation of a gap in the plasma. The hydrodynamics model assumes that j x B motion provides the opening by pushing the plasma out of the switch region. As a third possibility one has to consider that instabilities cause an increase of the resistivity to anomalous values. These models represent a blend of experimental observations, theoretical ideas, and numerical simulations. The plasma in the switch can be classified as the main " bulk" plasma and "near-electrode" plasma. In most discharges the vital processes occur at or near the cathode. Initially the injected gun plasma in contact with the cathode surface forms a Debye sheath to assure the quasineutrality of the bulk plasma. Without any external voltage applied a metal surface exposed to 20 plasma shots from a gun does not show any visible surface damage in contrast to the cathode surface of a plasma opening switch which shows many cathode spot craters. However, the streaming gun plasma induces desorption of weakly bound impurities. These neutrals expand with sound velocity. After a certain delay time the generator voltage pulse appears at the switch. The bulk plasma does not need a large potential drop to accommodate the electron flow.