Synchrotrons

The final stages of acceleration in the major accelerator facilities are usually synchrotrons. They consist of large circular devices where charged particles travel in evacuated pipes under the influence of magnets which are positioned around the circumference of the circle. Acceleration is achieved by the application of radio frequency electric fields at RF cavities along the circumference of the ring. The magnetic fields must be increased synchronously with the acceleration in order to keep the particles on the constant radius path. Such accelerators can be used with protons or electrons, and even with heavier positive ions.

Major technological hurdles had to be overcome to achieve the present state where the synchrotrons can produce a pencil-thin beam of particles with over 100,000 times the energy of the most energetic natural radioactive emission. A major step involved the development of what is called "weak focusing" by V. Veksler and E. M. McMillan around 1945. Particles which have orbit radii which are slightly larger or smaller than the optimum radius of the center of the beam tube can be managed by designing the fringe magnetic fields so that these particles arrive in phase with the accelerating mechanism. A second major step was the development of "strong focusing", attributed to N. Christophilos in 1950 and Courant, Livingston and Snyder in 1952. A combination of dipole and quadrupole magnets alternately focus and defocus the beam in both the horizontal and vertical directions. The combination of magnets can be arranged to achieve a net collimation or "focusing" of the beam. Strong focusing is important in the higher energy accelerators to keep the beam size small for savings on the cost of the magnets.

For proton synchrotrons, the high energy limit is set by the strength of the bending magnets. For a given bending magnet strength, higher energies can be achieved only by making the radius larger. The largest proton synchrotrons are the Main Ring (500 GeV) and Tevatron (1 TeV) at Fermilab and the Super Proton Synchrotron (SPS, 450 GeV) at CERN.

The world's largest electron synchrotron is the Large Electron-Positron Collider (LEP) at CERN. It has a radius of about 4 km. For the electron synchrotrons, the maximum energy is limited by the losses to synchrotron radiation which increases with the fourth power of the particle energy. Since those losses are inversely proportional to the orbit radius, these accelerators are made as large as possible.

Storage Rings

A storage ring consists of an evacuated pipe passing through a ring of magnets where the magnetic field can be kept constant. Charged particles can then circulate in the ring indefinitely. The geometry is the same as that described for the synchrotron; in fact a synchrotron can serve as a storage ring. For colliding beam experiments, you could have two storage rings which held the two different particle species until such time as you directed them together to collide. A storage ring may hold particles for hours, whereas the process of acceleration of particles up to their design energies may take only seconds. A stringent requirement for a storage ring is the vacuum; it must be a much better vacuum than for routine synchrotron operation.

Storage rings for electrons and positrons make up the Stanford Positron Electron Accelerator Rings (SPEAR). This facility is notable for the discovery of the charm quark and the tau lepton.