Asynchrotron is a particular type of cyclicparticle accelerator, descended from thecyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The strength of themagnetic field which bends the particle beam into its closed path increases with time during the accelerating process, beingsynchronized to the increasingkinetic energy of the particles.[1]
The synchrotron is one of the first accelerator concepts to enable the construction of large-scale facilities, since bending, beam focusing and acceleration can be separated into different components. The most powerful modern particle accelerators use versions of the synchrotron design. The largest synchrotron-type accelerator, also the largest particle accelerator in the world, is the 27-kilometre-circumference (17 mi)Large Hadron Collider (LHC) near Geneva, Switzerland, built in 2008 by theEuropean Organization for Nuclear Research (CERN).[2] It can accelerate beams of protons to an energy of 7 tera electronvolts (TeV or 1012 eV).
The synchrotron principle was invented byVladimir Veksler in 1944.[3]Edwin McMillan constructed the first electron synchrotron in 1945, arriving at the idea independently, having missed Veksler's publication (which was only available in aSoviet journal, although in English).[4][5][6] The first proton synchrotron was designed bySir Marcus Oliphant[5][7] and built in 1952.[5]
Large synchrotrons usually have alinear accelerator (linac) to give the particles an initial acceleration, and a lower energy synchrotron which is sometimes called abooster to increase the energy of the particles before they are injected into the high energy synchrotron ring. Several specialized types of synchrotron machines are used today:
The synchrotron evolved from thecyclotron, the first cyclic particle accelerator. While a classicalcyclotron uses both a constant guidingmagnetic field and a constant-frequencyelectromagnetic field (and is working inclassical approximation), its successor, theisochronous cyclotron, works by local variations of the guiding magnetic field, adapting to the increasingrelativistic mass of particles during acceleration.[11]
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In a synchrotron, this adaptation is done by variation of the magnetic field strength in time, rather than in space. For particles that are not close to the speed oflight, the frequency of the applied electromagnetic field may also change to follow their non-constant circulation time. By increasing theseparameters accordingly as the particles gain energy, their circulation path can be held constant as they are accelerated. This allows the vacuum chamber for the particles to be a large thintorus, rather than a disk as in previous, compact accelerator designs. Also, the thin profile of the vacuum chamber allowed for a more efficient use of magnetic fields than in a cyclotron, enabling the cost-effective construction of larger synchrotrons.[citation needed]
While the first synchrotrons and storage rings like theCosmotron andADA strictly used the toroid shape, thestrong focusing principle independently discovered byErnest Courant et al.[12][13] andNicholas Christofilos[14] allowed the complete separation of the accelerator into components with specialized functions along the particle path, shaping the path into a round-cornered polygon. Some important components are given byradio frequency cavities for direct acceleration,dipole magnets (bending magnets) for deflection of particles (to close the path), andquadrupole /sextupole magnets for beam focusing.[15]
The combination of time-dependent guiding magnetic fields and the strong focusing principle enabled the design and operation of modern large-scale accelerator facilities likecolliders andsynchrotron light sources. The straight sections along the closed path in such facilities are not only required for radio frequency cavities, but also forparticle detectors (in colliders) and photon generation devices such aswigglers andundulators (in third generation synchrotron light sources).[citation needed]
The maximum energy that a cyclic accelerator can impart is typically limited by the maximum strength of the magnetic fields and the minimum radius (maximumcurvature) of the particle path. Thus one method for increasing the energy limit is to usesuperconducting magnets, these not being limited bymagnetic saturation.Electron/positron accelerators may also be limited by the emission ofsynchrotron radiation, resulting in a partial loss of the particle beam's kinetic 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.[citation needed]
More powerful accelerators are built by using large radius paths and by using more numerous and more powerful microwave cavities. Lighter particles (such as electrons) lose a larger fraction of their energy when deflected. Practically speaking, the energy ofelectron/positron accelerators is limited by this radiation loss, while this does not play a significant role in the dynamics ofproton orion accelerators. The energy of such accelerators is limited strictly by the strength of magnets and by the cost.[citation needed]
Unlike in a cyclotron, synchrotrons are unable to accelerate particles from zero kinetic energy; one of the obvious reasons for this is that its closed particle path would be cut by a device that emits particles. Thus, schemes were developed to inject pre-acceleratedparticle beams into a synchrotron. The pre-acceleration can be realized by a chain of other accelerator structures like alinac, amicrotron or another synchrotron; all of these in turn need to be fed by a particle source comprising a simple high voltage power supply, typically aCockcroft-Walton generator.[citation needed]
Starting from an appropriate initial value determined by the injection energy, the field strength of thedipole magnets is then increased. If the high energy particles are emitted at the end of the acceleration procedure, e.g. to a target or to another accelerator, the field strength is again decreased to injection level, starting a newinjection cycle. Depending on the method of magnet control used, the time interval for one cycle can vary substantially between different installations.[citation needed]
One of the early large synchrotrons, now retired, is theBevatron, constructed in 1950 at theLawrence Berkeley Laboratory. The name of thisproton accelerator comes from its power, in the range of 6.3GeV (then called BeV for billionelectron volts; the name predates the adoption of theSI prefixgiga-). A number oftransuranium elements, unseen in the natural world, were first created with this machine. This site is also the location of one of the first largebubble chambers used to examine the results of the atomic collisions produced here.[citation needed]
Another early large synchrotron is theCosmotron built atBrookhaven National Laboratory which reached 3.3 GeV in 1953.[16]
Among the few synchrotrons around the world, 16 are located in the United States. Many of them belong to national laboratories; few are located in universities.[17]
Until August 2008, the highest energy collider in the world was theTevatron, at theFermi National Accelerator Laboratory, in theUnited States. It acceleratedprotons andantiprotons to slightly less than 1TeV of kinetic energy and collided them together. TheLarge Hadron Collider (LHC), which has been built at the European Laboratory for High Energy Physics (CERN), has roughly seven times this energy (so proton-proton collisions occur at roughly 14 TeV). It is housed in the 27 km tunnel which formerly housed the Large Electron Positron (LEP) collider, so it will maintain the claim as the largest scientific device ever built. The LHC will also accelerate heavy ions (such aslead) up to an energy of 1.15PeV.[citation needed]
The largest device of this type seriously proposed was theSuperconducting Super Collider (SSC), which was to be built in theUnited States. This design, like others, usedsuperconducting magnets which allow more intense magnetic fields to be created without the limitations of core saturation. While construction was begun, the project was cancelled in 1994, citing excessivebudget overruns — this was due to naïve cost estimation and economic management issues rather than any basic engineering flaws. It can also be argued that the end of theCold War resulted in a change of scientific funding priorities that contributed to its ultimate cancellation. However, the tunnel built for its placement still remains, although empty. 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 tosynchrotron radiation. This will require a return to thelinear accelerator, but with devices significantly longer than those currently in use. There is at present a major effort to design and build theInternational Linear Collider (ILC), which will consist of two opposinglinear accelerators, one for electrons and one for positrons. These will collide at a totalcenter of mass energy of 0.5TeV.[citation needed]
Synchrotron radiation also has a wide range of applications (seesynchrotron light) and many 2nd and 3rd generation synchrotrons have been built especially to harness it. The largest of those 3rd generation synchrotron light sources are theEuropean Synchrotron Radiation Facility (ESRF) inGrenoble, France, the Advanced Photon Source (APS) near Chicago, United States, andSPring-8 inJapan, accelerating electrons up to 6, 7 and 8GeV, respectively.[citation needed]
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. These installations are mostly built by the science funding agencies of governments of developed countries, or by collaborations between several countries in a region, and operated as infrastructure facilities available to scientists from universities and research organisations throughout the country, region, or world. More compact models, however, have been developed, such as theCompact Light Source.[citation needed]
