Measurement of Electron Temperature in Low-Pressure Cesium Thermionic Energy Converters

Abstract

A dc probe was placed behind a small slit in the collector electrode in each of several cesium-vapor thermionic diodes. These systems were used to study the velocity distribution of the small fraction of the emitted electrons which passed through the collector slit. This technique overcomes several of the inherent disadvantages in the use of the dc Langmuir probe in cesium vapor. The error due to momentum defocusing in the collector slit is calculated with the presence of a plasma in the emitter—collector interspace. The probe log-current vs voltage curves could be accurately fitted to a Maxwell—Boltzmann velocity distribution over several cycles in log-current. In this manner, effective temperatures were defined as a measure of the randomness of the velocity component normal to the collector surface. No fully randomized, high-temperature electron group was detected in the unignited mode of these diodes. The effective electron temperature in the unignited mode became increasingly larger than the emitter temperature as the latter temperature was increased, but the departure was always less than 25%. The effective electron temperature increased rapidly with diode current as the ignited mode was entered and rose to be 470% higher than the emitter temperature.