Hohlkathode mit einer Elektronenaustrittsoeffnung

1,020,233. Thermionic cathodes. C.S.F.- COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. Aug. 13, 1962 [Sept. 27, 1961], No. 30913/62. Heading H1D. A hollow cathode structure comprising a cavity 1 having an inner emissive coating 2 with associated heating means, and a restricted opening 3 for escape of electrons, is connected by a conduit 7 to a source of ionizable vapour which is ionized within cavity 1 to compensate the electron space charge therein by forming a plasma. As shown, cavity 1 is surrounded by a heating element 4, in a thermally-insulating envelope 5, and is connected to a heated caesium reservoir 10. Caesium vapour passes through porous member 9 and is deflected by barrier 13 and ionized by contact with a high work function emissive layer 2 of W, Re, Ta or Nb. The heating of the caesium reservoir and/or the cavity is regulated so that the electronic space charge is compensated by the caesium ions and a plasma is obtained, electrons being withdrawn therefrom by an external anode field. In an alternative arrangement, using a low work function emissive coating 2, the caesium vapour is ionized by contact with a directly-heated tungsten filament which replaces shield 13. Also, spherical cavity 1 may be replaced by a cylindrical cavity of circular or rectangular cross-section, the caesium vapour may be introduced at right angles to the orifice 3, and the ionizer may be a heated plate. Alternatively, the emissive layer may be heated by radiation from a heated ionizer or by electronic bombardment. A plurality of cavities may be arranged side-by-side to obtain not only a high density but also a large total current intensity.