Device having thermionic cathode heated by field-emitted electrons

An electronic device comprising an evacuated envelope containing a main thermionic cathode having a first surface adapted to emit electrons when heated to elevated temperatures and an opposed second surface which is adapted to absorb energetic electrons and to convert the energy of said electrons to heat, an auxiliary field-emission cathode adapted to emit energetic electrons toward said second surface and spaced from said second surface, and means for applying a voltage between said main cathode and said auxiliary cathode, said voltage being of sufficient magnitude to cause a field emission of electrons from said auxiliary cathode with sufficient energy when absorbed by said second surface to heat said main cathode to cause thermionic electron emission from said first surface, there being no other means for heating said main cathode and said auxiliary cathode in said device.