Electrostatic Sheath Phenomena in Thermionic Converters.

A generalized theoretical description of electrostatic sheath phenomena in thermionic converters is developed. The analysis goes beyond existing treatments in taking into account discontinuous distribution functions for emitted, plasma, and trapped particles; non-equilibrium particle trapping; sheath electric fields consistent with particle densities and temperatures within the sheath; and Schottky effects at the electrode surface. The resulting sheath criteria and models are applicable to both monotonic and double sheaths, and are expressed in a form suitable for incorporation into an existing digital computer program for thermionic converter analysis. The improved treatment has been applied to thermionic converters. The results indicate the following: non-monotonic sheaths at the emitter in the lower region of the ignited mode for electron rich conditions--this is in agreement with previous investigations; the ignition region hysteresis is probably associated with a bistable sheath at the collector; The collector sheath is non-monotonic for nearly all operating conditions of practical interest. The latter provides a possible explanation of reported collector work function anomalies. (Author)