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SynchrotronThis article is concerned with the synchrotron device - a sub-atomic particle accelerator. For applications of the synchrotron radiation produced by cyclic particle accelerators see synchrotron light. A synchrotron is a particular type of cyclic particle accelerator in which the magentic field (to turn the particles so they circulate) and the electric field (to accelerate the particles) are carefully synchronized with the travelling particle beam. 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. This allows the vacuum container for the particles to be a large thin torus (commonly described as a "doughnut shape"). In reality it is easier to use some straight sections and some bent sections giving the "doughnut shape" the shape of a rounded cornered polygon. A path of large effective radius may thus be constructed using simple straight and curved pipe segments, unlike the disc-shaped chamber of the cyclotron type devices. The shape also allows and requires the use of multiple magnets to bend the particle beam. The maximum energy that a cyclic accelerator can impart is typically limited by the strength of the magnetic field(s) and the maximum radius 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. Since the radius is limited, the power of the machine becomes limited by the strength of the magnetic field. In the case of an ordinary (not superconducting) electromagnet the field strength is limited by the saturation of the core (when all magnetic domains are aligned the field may not be further increased to any practical extent). The arrangement of the single pair of magnets the full width of the device also limits the economic size of the device. 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, so the highest-energy synchrotrons accelerate larger particles; protons or atomic nuclei. One of the early large synchrotrons, still operating, is the Bevatron, constructed in 1950 at the Lawrence Berkeley Laboratory. The name of this proton accelerator comes from its power, in the range of 6.3 BeV (billion electron volts; the name predates the adoption of the SI prefix Giga.). A number of heavy elements, unseen in the natural world, were first created with this machine. This site is also the location of one of the first large bubble chambers used to examine the results of the atomic collisions produced here. The largest device of this type yet proposed was the Superconducting Super Collider (SSC), to be built in the United States. This design uses superconducting magnets which allow more intense magnetic fields to be created without the limitations of core saturation. While construction was begun this machine was not completed owing to the great expense — this appears to be a failure of economic management rather than an engineering failure. It appears that expense is the limiting factor in proton and heavy particle accelerators. CERN, in Europe is currently developing somewhat less ambitious accelerators that will significantly advance the state of the art in machine power. While there is still potential for yet more powerful proton and heavy particle cyclic accelerators, it appears that the next step up in electron acceleration power must avoid losses due to synchrotron radiation. This will require a return to the linear accelerator, but with devices significantly longer than those currently in use. 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" - see the article linked for some applications. List of synchrotrons
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