How do positive ions reduce space-charge effects in gas-filled thermionic converters?

In a thermionic power converter, the emission of electrons from the hot emitter creates a cloud of negative charge in the space between the electrodes. Because these electrons repel one another, they create a space-charge effect that disrupts the flow of current to the collector.1

To mitigate this, gas-filled or plasma converters introduce positive ions into the region between the electrodes. By mixing these positive ions with the negatively charged electron cloud, the device creates a plasma that maintains a relatively neutral space charge. This neutralization significantly reduces the electrostatic resistance force that would otherwise repel emitted electrons, allowing them to pass more easily from the emitter to the collector.1

This process is typically achieved using an easily ionizable vapour, such as cesium, which is chosen for its low ionization potential.1