Cyclotron, a device for accelerating protons, alpha particles (helium nuclei), and other electrically charged particles to high speeds. The high-speed particles are made to collide with the atoms of a target to probe the atomic nucleus and to produce certain radioactive isotopes for medicine, industry, and scientific research.

A cyclotron contains two hollow D-shaped electrodes, called dees, in a vacuum chamber between the poles of a powerful electromagnet. A source of electrically charged particles lies at the center of the chamber. In operation, the electric field between the dees draws charged particles from the source into one of the dees, and the magnetic field produced by the electromagnet makes the particles move in a circular path. The electric field between the dees alternates rapidly so that the particles are accelerated each time they cross the gap between the dees. As they speed up, the particles travel in ever-larger circles within the dees, yet the time they take to complete each revolution remains the same. At the outer edge of the dees, a deflector plate guides the particles toward the target.

The energy to which a cyclotron can accelerate particles is limited because the particles increase in mass as they reach high speed and can no longer be accelerated at an even rate. In the synchrocyclotron, the frequency of the alternating current that accelerates the particles between the dees is varied as the particles increase in mass, making it possible to increase the energy to which the particles are accelerated. In the isochronous cyclotron, a magnetic field is produced that increases toward the outer edge of the cyclotron in such a way that the particles can still be accelerated at an even rate as they increase in mass.

The cyclotron was invented in 1930-31 by Ernest O. Lawrence at the University of California. The principle of accelerating particles that move in circular paths was later used in other particle accelerators.