Thermionic emission occurs at which part of the x-ray tube?

Thermionic emission refers to the process where electrons are emitted from the surface of a material when it is heated to high temperatures. In the context of an x-ray tube, this process occurs at the cathode. The cathode is typically composed of a filament that is heated, which causes the thermionic emission, allowing electrons to escape from the filament. These free electrons then accelerate towards the anode, which is crucial in the production of x-rays.

The anode is responsible for receiving these electrons and converting their kinetic energy into x-rays during the interaction. The focusing cup, which surrounds the cathode, plays a role in directing the electron beam towards the anode but does not itself emit electrons. The glass envelope of the x-ray tube serves primarily as a protective and vacuum-sealed environment to maintain the necessary conditions for x-ray generation, but it does not participate in the emission of electrons. Thus, the cathode is the key component in thermionic emission, making it the correct answer.