A synchrotron is a circular accelerator (sometimes called a synchrocyclotron) which has an electromagnetic resonant cavity (or perhaps a few placed at regular intervals around the ring) to accelerate the particles. Particles pass through each cavity many times as they circulate around the ring, each time receiving a small acceleration, or increase in energy. When either the energy or the field strength changes so does the radius of the path of the particles. Thus, as the particles increase in energy the strength of the magnetic field that is used to steer them must be changed with each turn to keep the particles moving in the same ring. The change in magnetic field must be carefully synchronized to the change in energy or the beam will be lost. Hence the name "synchrotron". The range of energies over which particles can be accelerated in a single ring is determined by the range of field strength available with high precision from a particular set of magnets.
To reach high energies, physicists sometimes use a sequence of different size synchrotrons, each one feeding the next bigger one. Particles are often pre-accelerated before entering the first ring, using a small linear accelerator or other device.