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Why do electron linear accelerators use traveling electromagnetic waves?

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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.

Electron linear accelerators utilize traveling electromagnetic waves because of the unique physical properties of electrons. Due to their small mass, electrons reach speeds very close to the speed of light at relatively low energies, such as 5 megaelectron volts.12

Because these particles travel at a velocity nearly identical to that of the electromagnetic wave itself, they can effectively "ride the crest" of the wave as it moves through the accelerator structure. This allows the electrons to remain in a constant position relative to the wave, ensuring they continuously experience an accelerating electric field without falling out of phase. In contrast, heavier particles like protons require more complex designs, such as standing waves, because their speeds increase significantly over a longer distance before reaching a near-constant velocity.132