Britannica Sources used in this answer

Why must quadrupole magnets be arranged in pairs?

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.

Quadrupole magnets must be arranged in pairs because of their inherent focusing characteristics. A single quadrupole magnet focuses a particle beam in one plane while simultaneously causing it to spread, or defocus, in the perpendicular plane.1

To achieve proper focusing in both the horizontal and vertical directions, these magnets are used in pairs, with each member of the pair arranged to have the opposite effect of the other. This technique, known as alternating-gradient focusing, ensures that while one magnet may defocus the beam in a specific direction, the subsequent magnet in the pair compensates by focusing it in that same direction. By passing through this succession of lenses, the particle beam remains narrow and intense as it travels through the accelerator.12