What is the primary source of free electrons in an X-ray tube?

In an X-ray tube, the primary source of free electrons is the cathode, which is a crucial component of the tube. The cathode typically consists of a heated filament, often made of tungsten. When the filament is heated, it undergoes a process called thermionic emission, where the heat energy causes some electrons to gain enough kinetic energy to escape from the surface of the filament. These free electrons are then attracted towards the anode due to the high voltage applied between the cathode and anode.

The vacuum chamber surrounding these components ensures that the free electrons can travel from the cathode to the anode without colliding with gas molecules, which would otherwise scatter the electrons and impede their movement. While the anode plays a role in receiving these electrons and producing X-rays, it is not the source of the free electrons themselves. Thus, the cathode is identified as the primary source of free electrons in the functioning of an X-ray tube.