Why does a cloud of electrons form between the electrodes of a thermionic converter?

In a thermionic converter, a cloud of electrons forms between the electrodes because of the process of thermionic emission. When the emitter electrode (the hot plate) is heated to a sufficiently high temperature, electrons gain enough energy to overcome the material's electronic work function and escape the surface into the space between the electrodes.1

This accumulation of electrons creates a phenomenon known as the space-charge effect. Because electrons carry a negative charge, they repel one another, which disrupts the flow of current toward the collector electrode. To mitigate this, engineers often introduce positive ions into the electron cloud to neutralize the space charge, or they design the device with an extremely small gap between the electrodes to minimize the electrostatic resistance.1