Why is the collector kept cooler than the emitter in a thermionic power converter?

In a thermionic power converter, the collector must be kept at a significantly lower temperature than the emitter to facilitate the conversion of thermal energy into electricity.

The process functions as a heat engine that utilizes an electron-rich gas as its working fluid. Thermal energy is supplied to the emitter to liberate electrons, which then flow toward the collector. Upon striking the collector, a portion of the energy is used to drive current through an external load. The remaining energy is converted into heat at the collector, which must be removed to maintain the necessary temperature differential and ensure the device continues to function effectively.1