When does thermionic emission occur in an electron gun?

Thermionic emission occurs when a filament is heated to a temperature that provides enough energy for electrons to escape from the material. In the context of an electron gun, the filament is typically made of a material that releases electrons when sufficiently heated. When a direct current (DC) supply is applied to the filament, it generates heat through resistance, raising the temperature of the filament.

As the temperature increases, electrons within the filament gain kinetic energy. Once this kinetic energy exceeds the work function of the material—which is the minimum energy needed for an electron to escape from the surface of the filament—electrons can be emitted into the vacuum. This process is fundamental in electron guns, where a focused stream of electrons is needed for various applications such as in cathode-ray tubes or electron microscopy.

In contrast to the correct reasoning, other scenarios such as exposure to magnetic fields, rapid cooling, or low-temperature vibrations do not facilitate the thermionic emission process. In fact, cooling would lower the kinetic energy of electrons, making it less likely for them to overcome the work function and be emitted. Low-temperature vibrations of the filament do not provide the necessary thermal energy required for electron emission, either.