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.
Source: Stanford Linear Accelerator Center