Electrons within a metal are normally bound by electrostatic forces to the positively charged atomic nuclei. However, if sufficient thermal energy is supplied, some electrons gain enough energy to overcome this binding force and escape from the metal surface.

This is thermionic emission.

In the X-ray tube, a low-voltage current passes through the tungsten filament, causing it to heat to temperatures exceeding 2000°C. At these temperatures, large numbers of electrons are liberated from the filament surface.

The rate of electron emission depends strongly on temperature. As filament temperature increases, electron emission increases exponentially. This means small increases in filament current can produce large increases in electron flow.

These freed electrons form a cloud near the filament, sometimes called a space charge. When a high voltage (kVp) is applied between the cathode and anode, the electrons are accelerated across the vacuum toward the target.

Thermionic emission controls the number of electrons available for acceleration but does not determine their kinetic energy (that depends on the applied kilovoltage).

Without thermionic emission, X-ray production would not be possible.