Cyclotron

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

Every particle accelerator built before the cyclotron faced the same basic problem: to push a charged particle to higher energy, you generally needed a longer straight path to accelerate it along, since each accelerating gap could only add so much energy per pass. Ernest Lawrence's 1929 to 1930 concept solved this without ever lengthening the machine, by reusing a single accelerating gap over and over as the particle spiraled outward inside a fixed magnetic field. The trick works because of a property now called the cyclotron principle: a charged particle moving in a fixed magnetic field completes one full orbit in the same amount of time no matter how large that orbit's radius is, as long as the particle's speed stays well below the speed of light. As the particle gains energy from each pass through the accelerating gap between two D-shaped electrodes, its orbital radius grows, but the time each orbit takes stays constant, so a single fixed-frequency alternating voltage keeps accelerating the particle correctly on every single pass without ever needing to be adjusted. The particle spirals outward from the center of the machine in a widening path, gaining a little more energy on each crossing of the gap, until it reaches the edge of the magnet and is extracted as a usable beam. Lawrence's graduate student M. Stanley Livingston did much of the engineering that turned the concept into a working machine, and the cyclotron went on to become the most powerful particle accelerator technology available for the next two decades, used to produce medical isotopes, probe the atomic nucleus, and eventually to synthesize new transuranium elements that do not occur naturally on Earth.

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.