A particle accelerator is a machine whose purpose is to use electric and magnetic fields to accelerate particles to very high speeds, typically very close to the speed of light. Their primary use in physics is to impart a huge amount of kinetic energy to particles and then have them collide with a target, leading to a particle scattering with extremely high center-of-mass energy. Accelerators that do this are called colliders. Other use cases come from nuclear medicine where they are used for some diagnostic techniques and radiotherapy.
Particle accelerators come in two shapes: linear accelerators (often called linacs) accelerate in a straight line and circular ones accelerate in a curved trajectory. They accelerate a large number of particles in a beam, which is focused into a thin, collimated line using quadrupoles. The curvature, if necessary, is taken care of by dipoles instead. The most common particles used are electrons and protons because they are the easiest to obtain, but other particles are sometimes used, such as positrons in the medical technique of positron emission tomography (PET), used to visualize metabolic and biochemical activity.
Types#
Accelerators have gone through a lot of forms in the past. The original accelerator was the cyclotron (1932, due Ernest O. Lawrence), which got followed up by the synchrotron (1944, due Vladimir Veksler) and the storage ring (1961). That's not to say that first two are inherently worse: they are still in use today, especially in medical settings where cyclotrons are the most convenient due to their small size and low costs compared to building-sized storage rings.