LIQUID-METAL ARC SWITCHING DEVICE AND PROCESS

The electrical switch device has an envelope in which is mounted a force-fed liquid-metal cathode, an anode, a condenser which may or may not be subdivided for voltage grading purposes and, in the preferred embodiment, electrical shielding means for the condenser. The cathode is capable of very high electron-to-atom emission ratio. The required value for the electron-to-atom emission ratio is above 50 to 1. When arcing occurs from the liquid metal, a plasma jet of electrons, ions, and neutral particles is emitted from the arc spot. In addition, during arcing as well as non-arcing periods, some of the liquid metal evaporates from the cathode. This evaporation occurs into a much larger solid angle than that subtended by the plasma jet. The anode is mounted facing the cathode and it intercepts the plasma jet, thus permitting current conduction between anode and cathode with minimum voltage drop. The anode is kept at an elevated temperature, so that none of the ions and neutrals of the impinging plasma jet can remain condensed on it. They are immediately re-evaporated, including the ions after they have been neutralized. The condenser has a very much larger area than the exposed liquid metal area on the cathode, at least 100 times the exposed liquid metal area to dominate the equilibrium and it is kept at a low enough temperature to efficiently condense the liquid-metal vapor emitted by the cathode. With mercury used as the liquid metal, the condenser temperature is kept substantially below 0 DEG , preferably at about -35 DEG C, which is just above the melting point of mercury. The combination of the high electron-to-atom emission ratio of the cathode with the large, low temperature condenser results in an equilibrium background pressure (i.e., pressure outside the plasma jet) of at least as low as 103 Torr during arcing and lower than 104 Torr during non-arcing periods. This low background pressure, in turn, permits the essentially unperturbed propagation of the plasma jet between the cathode and the anode surface upon which it impinges. Such a discharge mode is commonly referred to as a "vacuum arc". The fact that the plasma jet is emitted only during arcing and that the pressure within the space surrounding this jet is kept low, results in the ability to hold off electric fields up to 50 kV per centimeter between anode and cathode immediately after cessation of arcing. Arcing may cease because of a zero in the current fed to the switching device, as i