What is the name of the structure on the cathode of an X-ray tube where the electron beam is formed?

The structure on the cathode of an X-ray tube where the electron beam is formed is indeed called the filament. The filament is a thin wire made of a material with a high melting point, typically tungsten. When an electric current passes through the filament, it heats up and emits electrons through a process known as thermionic emission. These emitted electrons then form a beam that is directed toward the target, which is located on the anode side of the X-ray tube.

The filament is essential because it generates the flow of electrons needed to produce X-rays. Without this component, there would be no electron beam to strike the target and generate X-ray photons. Understanding the role of the filament in the context of an X-ray tube is critical for anyone studying medical imaging and radiation physics.