A machine for accelerating subatomic particles, usually protons or electrons. The particles are guided through an evacuated pipe in a circular path by magnets, accelerated by radio-frequency (rf) electric fields. In a large machine, each pulse of particles may make tens of thousands of revolutions, being accelerated at each pass through the rf source. Magnetic field and rf frequency increase in a synchronized way with increasing particle velocity to maintain a circular path. Machines vary in size up to several kilometres diameter. They are a major experimental tool in particle physics.
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field (to turn the particles so they circulate) and the electric field (to accelerate the particles) are carefully synchronized with the travelling particle beam.
Characteristics
While a cyclotron uses a constant magnetic field and a constant-frequency applied electric field, and one of these is varied in the synchrocyclotron, both of these are varied in the synchrotron. By increasing these parameters appropriately as the particles gain energy, their path can be held constant as they are accelerated.
The maximum energy that a cyclic accelerator can impart is typically limited by the strength of the magnetic field(s) and the minimum radius (maximum curvature) of the particle path.
In a cyclotron the maximum radius is quite limited as the particles start at the center and spiral outward, thus this entire path must be a self-supporting disc-shaped evacuated chamber.
Synchrotrons overcome these limitations, allowing a narrow beam pipe which can be surrounded by much smaller and more tightly focused magnets. The ability of this device to accelerate particles is limited by the fact that the particles must be charged to be accelerated at all, but charged particles under acceleration emit photons (light), thereby losing energy. The limiting beam energy is reached when the energy lost to the lateral acceleration required to maintain the beam path in a circle equals the energy added each cycle. More powerful accelerators are built by using large radius paths and by using more numerous and more powerful microwave cavities to accelerate the particle beam between corners. Lighter particles (such as electrons) lose a larger fraction of their energy when turning. Practically speaking, the energy of electron/positron accelerators is limited by this radiation loss, while it does not play a significant role in the dynamics of proton or ion accelerators. The energy of those is limited strictly by the strength of magnets and by the cost.
Large synchrotrons
One of the early large synchrotrons, now retired, is the Bevatron, constructed in 1950 at the Lawrence Berkeley Laboratory.
Another early large synchrotron is the Cosmotron built at Brookhaven National Laboratory which reached 3.3 GeV in 1953.
Currently, the highest energy synchrotron in the world is the Tevatron, at the Fermi National Accelerator Laboratory, in the United States. It accelerates protons and antiprotons to slightly less than 1 TeV of kinetic energy and collides them together. The Large Hadron Collider (LHC), which is being built at the European Laboratory for High Energy Physics (CERN), will have roughly seven times this energy, and is scheduled to turn on in 2007.
The largest device of this type seriously proposed was the Superconducting Super Collider (SSC), which was to be built in the United States.
While there is still potential for yet more powerful proton and heavy particle cyclic accelerators, it appears that the next step up in electron beam energy must avoid losses due to synchrotron radiation.
However many scientists use synchrotron radiation (see synchrotron light) and for them the production of synchrotron radiation is the only purpose of a synchrotron.
Synchrotron radiation is useful for a wide range of applications and many synchrotrons have been built especially to produce synchrotron light. The highest energy of these is SPring-8 in Japan, accelerating electrons up to 8 GeV.
Synchrotrons which are useful for cutting edge research are large machines, costing tens or hundreds of millions of dollars to construct, and each beamline (there may be 20 to 50 at a large synchrotron) costs another two or three million dollars on average.
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