Why the Cyclotron Hit a Wall

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

The cyclotron principle that makes the cyclotron work depends on one assumption: that a particle's orbital period stays constant as it speeds up. That assumption is only approximately true, and it fails completely once a particle's speed becomes a significant fraction of the speed of light. As a particle approaches light speed, Einstein's special relativity says its effective mass rises, and a heavier particle takes measurably longer to complete each orbit even though the magnetic field pushing it around has not changed. The particle's orbital frequency gradually falls out of step with the cyclotron's fixed accelerating voltage, and once the two are far enough out of phase the particle stops gaining energy from the gap altogether. Classical cyclotrons are consequently limited to nonrelativistic particle speeds, and a later variant called the synchrocyclotron, which slowly reduces its accelerating frequency to track the slowing orbital rate, only pushed the ceiling up to about 350 MeV for protons before the same relativistic effects caught up again. The real solution required abandoning a fixed magnetic field entirely. In 1944 the Soviet physicist Vladimir Veksler proposed ramping the magnetic field strength in step with the particles' rising energy, so their orbital radius could be held constant instead of spiraling outward, letting an accelerating voltage of steadily changing frequency keep pace with the particles indefinitely. The American physicist Edwin McMillan proposed the identical idea independently in 1945, unaware of Veksler's earlier paper, and once McMillan learned of it he acknowledged Veksler's priority; the two later shared the 1963 Atoms for Peace Award for the invention now known as the synchrotron. Every major particle accelerator built since, including the Large Hadron Collider, is a descendant of that fix for the cyclotron's relativistic wall.