Final answer:
The external structure of the X-Ray tube consists of the cathode, anode, and the enclosure that maintains a vacuum.
Step-by-step explanation:
The external structure of the X-Ray tube consists of three main parts: the cathode, which is the electron source; the anode, which is the target for the electrons; and the glass or metal enclosure that maintains a vacuum environment for electron acceleration and X-ray production. The cathode generates electrons, which are then rapidly accelerated toward the anode. Upon striking the anode, the deceleration of these electrons releases energy in the form of X-rays, which include both bremsstrahlung and characteristic X-rays specific to the anode material. The containment structure ensures that this process can occur in the absence of air, which would otherwise hinder the movement of electrons from the cathode to the anode.