Emission physics of the thermionic energy converter

Emission processes establish the upper limit of the performance obtainable in the thermionic converter. This paper briefly introduces the basic factors which must be considered to achieve this limit and outlines recent developments in the related technology. The basic elements of thermal excitation common to these processes are outlined and the results are used to illustrate the interdependence of the atom, electron and ion emission processes. The constraints imposed by this interdependence imply the existence of a natural limitation on vaporization lifetime for efficient operation using elementary emitter materials. This limitation is overcome by the use of emitters with an adsorbed layer of cesium and constraints imposed by the interdependence of the emission processes prescribe regions of efficient operation of these emitters. Consideration of the basic processes occurring in the adsorbed layer permits semiquantitative evaluation of the relative merit of different materials for use as emitters and collectors. The effect of nonuniform surfaces is reviewed and work in progress to understand the nature and origin of the nonuniformities is discussed. Substantial improvement in converter performance and reproducibility is possible through the development of uniform and optimum electrode surfaces.