Why is a gas-filled thermionic converter also called a plasma converter?

A gas-filled thermionic converter is referred to as a plasma converter because it is specifically designed to generate and maintain a plasma within the space between the emitter and the collector electrodes.1

In these devices, positively charged ions are continuously produced—typically by using an easily ionizable vapour such as cesium—and mixed with the negatively charged electrons emitted from the hot electrode. This mixture creates a state of relatively neutral space charge. By neutralizing the negative charge of the electron cloud, the plasma significantly reduces the electrostatic resistance that would otherwise hinder the flow of electrons from the emitter to the collector.1