The cathode is part on an x-ray tube and serves to expel the electrons from the circuit and focus them in a beam on the focal spot of the anode. It is a controlled source of electrons for the generation of X-ray beams. The electrons are produced by heating the filament, i.e., a coil of wire made from tungsten, placed within a cup-shaped structure, a highly polished nickel focusing cup, providing electrostatic focusing of the beam on the anode. In order to expel the electrons from the system, they need to be given the energy. Heat is used to expel the electrons from the cathode. The filament is crystallized during construction and its crystallized structure gives the filament stability. The process is called thermionic emission. The filament is heated with the electric current passing through it, to the glowing temperature and the electrons are, then expelled from the cathode.
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