A particle accelerator is a device that uses electromagnetic fields to increase the kinetic energy of particles carrying electric charge and to control their motion as a beam. Accelerated particles include electrons, protons, and heavier ions. Machines range from compact industrial and medical equipment to large research installations. Their beams can strike targets, collide with other beams, or generate radiation used to examine matter. Accelerators are central instruments in particle physics and nuclear research, but their applications extend far beyond these fields. (energy.gov)
Physical principles
An electric field transfers energy to a charged particle. A magnetic field bends or focuses its trajectory; the magnetic part of the Lorentz force acts perpendicular to the particle’s velocity and does not itself increase kinetic energy. In many accelerators, radiofrequency cavities supply oscillating electric fields. Particles pass through these cavities at carefully controlled phases so that successive encounters increase their energy. (home.cern)
Particle energies are commonly expressed in electronvolts: one electronvolt is the energy gained by a particle with one elementary charge passing through a potential difference of one volt. Larger units include megaelectronvolts, gigaelectronvolts, and teraelectronvolts. At relativistic energies, additional energy produces progressively smaller increases in speed. According to special relativity, particles with nonzero rest mass approach, but cannot attain, the speed of light. Their energy and momentum can nevertheless continue to increase. (home.cern)
Main accelerator types
Accelerators are classified both by their geometry and by the method used to provide energy.
Electrostatic accelerators use a high, essentially steady potential difference. Early machines included voltage-multiplier systems and Van de Graaff generators. Such designs supplied controllable energetic particles for nuclear experiments, although producing and insulating sufficiently high voltages imposed practical limits. (www2.lbl.gov)
A linear accelerator, or linac, directs particles along a substantially straight path through accelerating structures. Unlike a circular machine, a conventional linac does not repeatedly reuse the same structures by circulating the beam. Linacs serve as independent research and medical machines and as injectors for larger accelerator complexes. (energy.gov)
A cyclotron uses a magnetic field to bend particles through successive turns while alternating electric fields repeatedly accelerate them. As their momentum increases, the particles generally follow an outward spiral. Cyclotrons are widely associated with ion acceleration and radionuclide production. (pub.iaea.org)
A synchrotron accelerates particles around a fixed design orbit. Its bending magnetic fields are increased in coordination with the particles’ rising momentum. This permits repeated passage through the accelerating cavities without the outward expansion characteristic of a cyclotron. The Large Hadron Collider at CERN is a synchrotron-based collider. (indico.cern.ch)
Beam production and control
An accelerator begins with a particle source, followed by systems for injection, acceleration, and beam transport. Large facilities often connect several machines in sequence: each raises the beam energy before transferring it to the next. A vacuum beam pipe minimizes collisions with residual gas that would disturb or remove particles from the beam. (energy.gov)
Different magnets perform distinct functions. Dipoles bend trajectories, whereas quadrupoles focus beams. Radiofrequency systems commonly organize particles into bunches rather than a continuous stream. Accurate synchronization keeps these bunches aligned with accelerating fields and, in colliders, with opposing bunches at interaction points. Some machines employ superconductivity to support powerful magnets, with associated low-temperature engineering requirements. (home.cern)
For circular machines, attainable momentum depends on both magnetic-field strength and bending radius. Another limitation is synchrotron radiation, emitted when charged particles follow curved trajectories. At equal energy and bending radius, this loss is much greater for electrons than for protons. Radiation losses therefore strongly influence high-energy electron-machine design, while dedicated light sources exploit the radiation itself. (cas.web.cern.ch)
Experiments and performance
In fixed-target experiments, an accelerated beam strikes stationary material. In colliders, two beams meet at selected interaction points. Surrounding detectors record products of scattering and other interactions, allowing researchers to infer particle properties and investigate the Standard Model. Collisions may produce new particles or initiate nuclear reactions; neutral secondary particles can be generated even though conventional acceleration acts on charged particles. (home.cern)
Energy is not the only measure of performance. For colliders, luminosity describes the encounter intensity of the beams. Multiplying luminosity by the cross-section of a particular process gives its expected event rate. High luminosity is especially important when investigating rare interactions. (home.cern)
Scientific, medical, and industrial uses
Accelerators investigate the structure of nuclei and the constituents of matter. Accelerator-based light sources provide intense X-ray beams for studying materials and biological structures. These facilities support research across chemistry, biology, Earth science, and materials science. (energy.gov)
Medical uses include producing radioactive isotopes and supplying beams for radiation therapy. Electron linacs can deliver electrons directly or generate therapeutic X-rays, while other systems accelerate protons or heavier ions. Industrial applications include medical-product sterilization, modification of plastics, surface treatment, and ion implantation in semiconductor manufacturing. (energy.gov)
Historical development
Modern accelerator development grew from efforts to obtain energetic particles for investigating the atomic nucleus. High-voltage machines and cyclotrons emerged as major approaches around 1930. Ernest Lawrence and M. Stanley Livingston demonstrated an early cyclotron at Berkeley in 1931; Lawrence received the 1939 Nobel Prize in Physics for its invention and development. Subsequent accelerator technology expanded both nuclear research and applications linking physics with chemistry, biology, engineering, and medicine. (www2.lbl.gov)