Why is cesium used in many gas-filled thermionic power converters?

In gas-filled thermionic power converters, cesium is used primarily to address the space-charge effect, which occurs when the negative charges of emitted electrons repel one another and disrupt the flow of current between the electrodes.

To mitigate this, positive ions are introduced into the space between the emitter and the collector to create a plasma with a relatively neutral space charge. This allows electrons to pass from the emitter to the collector with minimal electrostatic resistance. Cesium is preferred for this purpose in the most efficient converters because of its low ionization potential of 3.89 electron volts, which facilitates the generation of these necessary positive ions.1