How does the space-charge effect limit the current in a thermionic power converter?

In a thermionic power converter, the space-charge effect limits current by creating a cloud of negatively charged electrons in the region between the emitter and the collector. Because these electrons are all negatively charged, they repel one another and exert an electrostatic force that pushes low-energy electrons back toward the emitter, thereby disrupting and restricting the flow of current to the collector.12

This phenomenon, known as space-charge-limited operation, acts as a significant barrier to achieving higher current densities. To mitigate this limitation, engineers have developed two primary strategies:12

• Reducing Electrode Spacing: By minimizing the gap between the emitter and collector to the order of micrometres, the buildup of the electron cloud is restricted, though this approach is often difficult to manufacture.1
• Neutralizing the Charge: The most feasible method involves introducing positive ions into the space between the electrodes. By mixing these ions with the emitted electrons, the region becomes a plasma with a relatively neutral space-charge, allowing electrons to travel to the collector with minimal electrostatic resistance.1