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How does a synchrocyclotron overcome the energy limit of a classical cyclotron?

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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.

In a classical cyclotron, particles are limited by the relativistic mass increase that occurs as their speed approaches the speed of light. This increase causes the particle's orbital frequency to decrease, eventually causing it to fall out of phase with the constant-frequency accelerating voltage.12

A synchrocyclotron overcomes this limitation by modulating the frequency of the accelerating voltage to match the decreasing orbital frequency of the particles as they gain energy. Because the frequency is adjusted, the particles remain in phase with the accelerating voltage, allowing them to reach higher energies than those possible in a classical cyclotron. This process is further supported by the phenomenon of phase stability, which automatically corrects the trajectories of particles that arrive at the accelerating gap slightly out of sync, ensuring the beam remains stable during acceleration.1