Thermionic emission occurs in metals that are heated to a very high temperature. In other words, thermionic emission occurs, when large amount of external energy in the form of heat is supplied to the free electrons in the metals.

What is the source of thermionic emission?

Thermionic emission of electron

Thermionic sources rely on heat to generate electrons, similar to how light is produced by incandescent lightbulbs. As a current is applied to the filament (or crystal), it is progressively heated until its electrons have enough energy to escape the solid surface.

Where does thermionic emission of electrons occur?

Thermionic emission is the emission of electrons from a heated metal (cathode). This principle was first used in the Coolidge tube and then later in the modern day x-ray tubes. Before the discovery of the principle, gas tubes were used for x-ray production.