Britannica Sources used in this answer

Why do drift tubes become longer along a proton linear accelerator?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of a linear proton resonance accelerator, or linac, showing protons entering from a source, passing through a series of labeled drift tubes powered by a radio-frequency source, and exiting as a beam through a cavity. — particle accelerator The accelerator is a large-diameter tube within which an electric field oscillates at a high radio frequency. Within the accelerator tube are smaller diameter metallic drift tubes, which are carefully sized and spaced to shield the protons from decelerating oscillations of the electric field. In the spaces between the drift tubes, the electric field is oriented properly to accelerate the protons in their direction of travel.

In a proton linear accelerator, drift tubes are used to shield particles from the electric field during intervals when the field’s direction would otherwise decelerate them. Because the protons gain energy and increase in speed as they travel through the accelerator, they cover more distance during each half-cycle of the alternating field.12

To ensure that the protons cross the gaps between the tubes only when the field is in the correct orientation to provide acceleration, the lengths of the drift tubes must be proportional to the speed of the particles passing through them. Consequently, as the proton energy and velocity increase along the length of the accelerator, the drift tubes are made progressively longer to maintain this synchronism.1