Transport effects in cesium thermionic converters

One of the primary reasons for the failure of thermionic converters to attain their ultimate theoretical performance is electron transport effects occurring in the interelectrode spacing. Two major types of transport effects, electron scattering and electron space charge, are discussed and theoretical results compared with experimental data. The uniform plasma theory is reviewed and experimental data show that for a monocrystal emitter this theory is consistent with the energy equation. Space charge neutralization is discussed. Both surface ionization and volume ionization are considered. Experimental results indicate that surface ionization is not as effective as would be expected by theory. This fact is responsible for the discrepancy between theoretically expected and experimentally obtained performance of converters. Under the conditions where surface ionization is inadequate for space charge neutralization, volume ionization can provide the necessary ions. Volume ionization, however, is achieved at the expense of the output power of the converter.