III.

Cyclotron

The American physicist Ernest O. Lawrence won the 1939 Nobel Prize for Physics for a breakthrough in accelerator design in the early 1930s. He developed the cyclotron, the first circular accelerator. A cyclotron is somewhat like a linac wrapped into a tight spiral. Instead of many tubes, the machine has only two hollow vacuum chambers, called dees, that are shaped like capital letter Ds back to back (thus: D). A magnetic field, produced by a powerful electromagnet, keeps the particles moving in a curved path. The potential difference between the dees constantly alternates in direction, so that every time the particles reach the gap they experience a forward acceleration. Within each dee the particles travel at constant speed during each half-revolution. As the particles gain energy, they spiral out towards the edge of the accelerator, where finally they exit.

When particles in a cyclotron approach the speed of light, they become appreciably more massive, as predicted by the theory of relativity. This makes it harder to accelerate them and throws the acceleration pulses at the gaps between the dees out of phase. A solution to this problem was suggested in 1945 by the Soviet physicist Vladimir I. Veksler and the American physicist Edwin M. McMillan. The solution, the synchrocyclotron, is sometimes called the frequency-modulated cyclotron. In this instrument, the oscillator (radio-frequency generator) that accelerates the particles around the dees is automatically adjusted to stay in step with the accelerated particles; as the particles gain mass, the frequency of accelerations is lowered slightly to keep in step with them. As the maximum energy of a synchrocyclotron increases, so must its size, for the particles must have more space in which to spiral. The largest synchrocyclotron is the 600-cm (236-in) phasotron at the Dubna Joint Institute for Nuclear Research in Russia; it accelerates protons to more than 700 MeV and has magnets weighing 6,984 tonnes.

The world’s most powerful cyclotron, the K1200, began operating in 1988 at the National Superconducting Cyclotron Laboratory at Michigan State University. The machine is capable of accelerating nuclei to an energy approaching 8 GeV.