The Cyclotron Principle: Reusing One Accelerating Gap

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.