A cyclotron is a type of particle accelerator designed to accelerate protons and ions to high velocities and then release them so as to strike a target. Observations of such collisions yield information about the nature of atomic particles. In contrast to the enormous particle accelerators used in particle physics today, the first cyclotron, built in 1930 by U.S. physicist E. O. Lawrence (1901–1958), measured just 4.5 in (12 cm) in diameter.
A charged particle moving at right angles to a magnetic field is subject to a force that is at right angles both to the field and to the charged particle's direction of motion at instant; this force follows the particle to follow a spiraling path. In a cyclotron, a pair of hollow, D-shaped pieces of metal are mounted above a powerful electromagnet, with their flat sides facing one another. One of the Ds is given a negative charge and the other is given a positive charge.
A charged particle, say a proton, is injected into this environment. With its positive charge, the proton is attracted by the negative D and repelled by the positive D; these forces start it into motion toward the negatively charged D. Once the particle is moving, the magnetic field deflects it into a curved path, back toward the positive D. Before the positive D can repel the proton, it is switched to a negative charge, thus attracting the proton rather than repelling it. Thus, the magnetic field keeps the particle on a circular path, while the alternating positive and negative charges on the D-shaped pieces of metal keep the proton chasing a negatively charged target indefinitely. As the proton circles inside the cyclotron it gains speed and thus energy; for a fixed magnetic-field strength, the size of the circle it travels increases correspondingly. Ultimately, before it can strike either of the metal Ds, it is propelled out of the cyclotron by a bending magnet and directed toward a target.
The cyclotron was a revolutionary device for its time, but has since been outmoded for particle-physics research purposes as cyclotrons are not capable of accelerating particles to the high speeds required for today's experiments in subatomic physics. High speeds are required for such research because, as Einstein proved, mass is proportional to energy. When an particle moves at high speed, say in a cyclotron, it has considerable energy of motion and its mass is therefore greatly increased. One way to boost the speed of the particle further is to switch the electrical polarities of the Ds at a gradually lower frequency. A more sophisticated version of the cyclotron, the synchrocyclotron, includes the complicated electronics necessary to do this. However, the most efficient method of compensating for the increased mass of high-energy particles is to increase the applied magnetic field as the particle speed increases. The class of device that does this is called a synchrotron, and includes the most powerful particle accelerators in existence today. These installations have rings more than 1.2 mi (2 km) in diameter, a far cry from Lawrence's first cyclotron.
Cyclotron-type warping of charged-particle paths occurs in nature as well as in cyclotrons: wherever charged particles move through a magnetic field (e.g., when charged particles from the Sun encounter the magnetic field of a planet), they are forced to follow spiraling paths. Since acceleration of a charged particle—any change in the particle's direction or velocity—causes it to emit electromagnetic radiation, charged particles encountering magnetic fields in space emit radiation. This radiation, termed cyclotron radiation, can reveal the interactions of particles and magnetic fields across the cosmos, and is of importance in astronomy.
Human-built cyclotrons of the fixed-field type are not used in physics research any more, but are increasingly important in medicine. Proton-beam therapy is a recent innovation in radiosurgery (surgery using radiation) in which protons accelerated by a cyclotron are beamed at a target in the human body, such as a tumor at the back of the eye. The energy of these protons can be carefully controlled, and their stopping distance inside living tissue (i.e., the depth at which they deposit their energy) precisely predicted. These features mean that tumors inside the body can be targeted while minimizing damage to healthy tissues.
De Martinis, C., et al. "Beam Tests on a Proton Linac Booster for Hadronotherapy." Proceedings of European Particle Accelerator Conference, Paris, France. 2002 (cited Feb. 6, 2003) <http:accelconf.web.cern.ch/AccelConf/e02/PAPERS/MOPRI095.pdf>.