How do positive ions help electrons travel from an emitter to a collector?

In a thermionic power converter, the accumulation of emitted electrons between the electrodes creates a negative space-charge effect. Because these electrons are negatively charged, they repel one another, which disrupts the flow of current and limits the device's efficiency.1

To overcome this, positive ions are introduced into the space between the emitter and the collector. By mixing these positive ions with the cloud of negatively charged electrons, the device creates a plasma that maintains a relatively neutral space charge. This neutralization significantly reduces the electrostatic resistance force that would otherwise repel the emitted electrons, allowing them to travel from the emitter to the collector with much greater ease.1