What is the function of the cathode in an x-ray tube?

• A. The cathode filters photons.
• B. The cathode accelerates electrons toward the ambient environment.
• C. The cathode converts kinetic energy to heat in the tube housing.
• D. The cathode provides thermionic emission of electrons and focuses them toward the anode through the focusing cup.

Answer: (D)

The cathode’s job is to supply the electron beam and guide it toward the anode. When the filament is heated, electrons are emitted from the cathode by thermionic emission, creating the electron cloud. The focusing cup, held at a negative potential, shapes and directs those electrons into a focused beam that travels toward the anode under the high voltage between cathode and anode. This combination—thermionic emission plus focusing toward the anode through the focusing cup—is what enables efficient x-ray production. Filtration happens outside the tube, the electrons aren’t aimed at the ambient environment, and the heat generated in the tube primarily comes from heating the filament and from electron interactions at the target, not from the cathode converting kinetic energy to heat in the housing.