How does contact ionization work in a cesium thermionic converter?

In a cesium thermionic converter, contact ionization is a process used to neutralize the space-charge effect—the buildup of negatively charged electrons that inhibits current flow between the emitter and the collector.1

This phenomenon occurs when the ionization potential of the cesium vapor is lower than the electronic work function of the emitter material. When cesium atoms strike the hot emitter surface, they lose an electron to the surface, becoming positively charged ions. These positive ions are then released into the space between the electrodes, where they mix with the cloud of emitted electrons. By creating a plasma with a relatively neutral space charge, these ions significantly reduce the electrostatic resistance that would otherwise impede the flow of electrons toward the collector.1

Tungsten is frequently used as the emitter material in these devices because it can withstand the high temperatures necessary for efficient operation while facilitating this ionization process.1