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Why can particle beams circulate and collide for several hours in a storage ring?

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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.

Particle beams can circulate and collide for several hours in a storage ring primarily because they travel through a high-quality vacuum. This environment is essential to ensure that particles do not collide with residual air molecules, which would cause them to be lost from the beam.1

By maintaining this near-perfect vacuum, the particles can complete many circuits of the machine, allowing the same bunches to intersect repeatedly. This is crucial because, unlike a target of solid or liquid matter, the density of particles within a beam is relatively low; therefore, the probability of two particles interacting during a single crossing is small. The ability to store these beams for extended periods enables researchers to maximize the number of interactions and conduct useful experiments.21

Additionally, magnetic fields are used to precisely guide and control the paths of these particles as they circulate. In some designs, such as particle-antiparticle colliders, the beams are bunched so that they only collide at specific interaction regions, further facilitating the stable, long-term storage of the beams.321