cyclic particle accelerator in which a charged particle—generally, a subatomic particle, such as an electron or a proton, or a heavy-ion particle, such as a gold ion—is accelerated to very high energies in the presence of an alternating electric field while confined to a constant circular orbit by a magnetic field. The magnetic field serves to bend or deflect the path of the charged particles. In order to maintain a constant trajectory within the cyclic accelerator, the magnetic field must gradually increase in strength as the particle's momentum increases. In addition the frequency of the accelerating electric field must be maintained or adjusted as necessary in order to be synchronous with the orbital frequency of the charged particles. The synchrotron is useful when the particle is accelerated to a speed approaching the speed of light—as in the acceleration of electrons or protons to extremely high energies—since, under such conditions, speed changes only slowly as the energy changes.

The basic principles of synchrotron design were proposed independently by Vladimir Veksler in the Soviet Union (1944) and Edwin McMillan (McMillan, Edwin Mattison) in the United States (1945). Synchrotron designs have been developed and optimized to accelerate different particles and are named accordingly. Thus, the electron synchrotron (particle accelerator) accelerates electrons, and the proton synchrotron (particle accelerator) accelerates protons. These types of accelerators are used to study subatomic particles in high-energy particle physics (physics) research. Electron synchrotrons are also used to produce synchrotron radiation. Heavy-ion synchrotrons are used primarily in nuclear physics (physics) research.

The highest particle energies ever achieved have been produced with the Tevatron—a superconducting proton synchrotron at the Fermi National Accelerator Laboratory, Batavia, Illinois—which accelerates protons to nearly one teraelectron volt (TeV; one trillion electron volts (electron volt)). The highest-energy electron synchrotron was at CERN in Geneva; it reached approximately 100 gigaelectron volts (GeV; 100 billion electron volts). Specialized electron synchrotrons, such as the Advanced Photon Source at Argonne National Laboratory, Argonne, Illinois, have been constructed to optimize the production of X-ray synchrotron radiation for structural studies of biological macromolecules and other complex materials.