How does an auxiliary discharge thermionic converter produce positive ions?

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thermionic power converter Schematic of a basic thermionic converter.

In an auxiliary discharge thermionic converter, positive ions are produced by applying a voltage to a third electrode. This process utilizes an inert gas—such as neon, argon, or xenon—positioned between the emitter and collector electrodes.1

By generating these positive ions, the device creates a plasma that neutralizes the negative space-charge effect, which would otherwise repel emitted electrons and disrupt current flow. This configuration, often described as having a spark-plug-type design, allows the converter to function at lower temperatures compared to other models, enabling the use of conventional fossil fuels as a heat source.1