How does a thermionic power converter change heat directly into electricity?

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thermionic power converter Schematic of a basic thermionic converter.

A thermionic power converter functions as a solid-state device that converts heat directly into electricity through thermionic emission. It operates similarly to a diode and can be viewed as a heat engine utilizing an electron-rich gas as its working fluid.1

The process involves two electrodes:

Emitter (Hot Plate): One electrode is heated to a high temperature, providing the energy necessary for electrons to overcome the electronic work function and escape the surface.1
Collector: The second electrode is maintained at a significantly lower temperature to receive the emitted electrons.1

As electrons flow from the hot emitter to the cooler collector, the circuit is completed by an external load, which captures the electrical energy. Because emitted electrons carry a negative charge, they can repel one another—a phenomenon known as the space-charge effect—which is often mitigated by introducing positive ions into the gap to create a neutral plasma.1