In recent years technology development programs for solar-heated thermionic space power systems were conducted.
Prototype combustion-heated thermionic systems for domestic heat and electric power cogeneration, and for rectification, have been developed.
All practical thermionic converters to date employ caesium vapor between the electrodes, which determines both the surface and plasma properties.
The energy of high temperature gases can be partly converted into electricity if the riser tubes of the boiler are provided cathode and anode of a thermionic generator with the interspace filled with ionized caesium vapor.
All the applications cited above have employed technology in which the basic physical understanding and performance of the thermionic converter were essentially the same as those achieved before 1970.
Similarly, improvements via use of oxygen-containing electrodes have been demonstrated in Russia along with design studies of systems employing the advanced thermionic converter performance.