Langmuir probe measurement of cesium plasma in thermionic energy converter

In order to determine the optimum conditions for thermionic energy converter (TEC) operation, a Langmuir probe has been applied to investigate a cesium plasma in a TEC operating in an unignited mode, which has a large electrode gap, and is operating at a relatively low emitter temperature. In the analysis of the probe characteristics, the work functions of and the electron emission from the emitter, collector and probe have been taken into account. It is confirmed by the present experiment that the floating and emitter sheath potentials estimated using parameters such as the electrode and probe work functions, thermionic emission current density and ion current density, produced by surface ionization together with measured electron and ion saturation currents, are in good agreement with those analyzed from the measured probe characteristics. Moreover, it is verified that the space-charge neutrality near the emitter affects the plasma density in the space between two electrodes more significantly than the thermionic electron current from the emitter. This leads to the conclusion that the relaxation of the negative space potential by the effective auxiliary discharge is important for the improvement of TEC output characteristics.