Why do negatively charged electrons repel each other in the space-charge effect?

In the context of a thermionic power converter, the space-charge effect occurs because electrons emitted from the hot electrode form a cloud of negative charge that creates electrostatic resistance to further emission.1

The reason these electrons repel each other is due to the Coulomb force. According to the fundamental principles of electrostatics, like electric charges—in this case, negatively charged electrons—exert a repulsive force on one another along the line joining their centers. Because electrons possess a negative electric charge, this inherent repulsion causes the cloud of electrons to push against one another, which disrupts the flow of current between the electrodes.213

To mitigate this, engineers may reduce the spacing between electrodes or introduce positive ions to neutralize the negative space charge, thereby allowing electrons to travel more freely toward the collector.1