This invention is directed to a plasma device having a cathode structure having a high pressure region around the emitting surface of a barium impregnated cathode. Plasma in the cathode region protrudes and couples to a main plasma discharge operating at lower pressure and higher voltage.
The prior art includes the hollow cathode wherein tantalum foil is coated with emissive material which initialy comprises a mixture of barium carbonate and strontium carbonate. The foil is rolled and inserted into a cathode tube. An encapsulated heater is positioned around the tube and a cathode orifice is located at the end of the tube. Vapor flow through the cathode, for example mercury vapor, passes through the rolled foil and out of the cathode orifice. The hollow cathode has some problems. One problem is the dependence on low work function emission from the coated foil insert. Upon first heating the carbonates convert to oxides which are anhydrous and absorb water vapor when exposed to air. While this is a reversible process, the coating tends to lose contact wth the tantalum foil, so it may fall off and become ineffective. Another problem is ignition of the cathode. The cathode depends upon a cathode heater as a source for thermionic emission and for heating the mercury vapor in the cathode interior. Without a suitable heater which achieves thermionic emission, another appropriate ignition method must be employed. The control of emission has been a particular problem in hollow cathode neutralizers for ion thrusters. There has been a variation of the control characteristics with changes in beam current level, and there is a time variation of the control characteristics. These lead to control difficulties.
Harry J. King U.S. Pat. No. 3,515,932 discloses a different type of hollow cathode plasma generator. In the King patent an emissive coating is layered on the inside of the hollow cylinder of cathode 2 and a central tungsten heater coil 16 radiantly provides the heat to the emissive coating 14. The use of a filament type heater 16 is considered too fragile for ion sources for space thrusters. In other uses, an open tungsten wire heater 16 in the gas of the species to be ionized can only be used in cases where the gas is nonreactive with the heated tungsten wire. Furthermore, the phsycial spacing of the heated tungsten wire from the emissive material provides poor thermal coupling. Further, the quantity of the emissive material 14 that can be applied in the manner shown in the patent would appear to have a relatively short lifetime, when considering the lifetime needs of space thrusters and other long life electron sources.