Synchrotron

This article is about the synchrotron, a particle accelerator. For applications of the synchrotron radiation produced by cyclic particle accelerators see synchrotron light source.

A synchrotron is a particular type of cyclic particle accelerator originating from the cyclotron in which the guiding magnetic field (bending the particles into a closed path) is time-dependent, being synchronized to a particle beam of increasing kinetic energy. The synchrotron is one of the first accelerator concepts that enable the construction of large-scale facilities, since bending, beam focusing and acceleration can be separated into different components.

The first electron synchrotron was constructed by Edwin McMillan in 1945, although the principle had already been published (unknown to him) in a Russian journal by Vladimir Veksler. The first proton synchrotron was designed by Sir Marcus Oliphant and built in 1952.

Differentiation

A storage ring is a special type of synchrotron in which the kinetic energy of the particles is kept constant.

A synchrotron light source is a combination of different accelerator types, including a storage ring with beamlines and usually a synchrotron (which is sometimes called booster in this context). Synchrotron light sources in their entirety are sometimes called "synchrotrons", although this is technically incorrect.

A cyclic collider is also a combination of different accelerator types, including two intersecting storage rings and the respective pre-accelerators.

Principle of operation

While a classical cyclotron uses both a constant guiding magnetic field and a constant-frequency electromagnetic field (and is working in classical approximation), its successor, the isochronous cyclotron, works by local variations of the guiding magnetic field, adapting the increasing relativistic mass of particles during acceleration.

A drawing of the Cosmotron

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 ultrarelativistic, the frequency of the applied electromagnetic field may also change to accompany their non-constant circulation time. By increasing these parameters appropriately 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 thin torus, 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.

While the first synchrotrons and storage rings like the Cosmotron and ADA strictly used the toroid shape, the strong focusing principle independently discovered by Ernest Courant et al. and Nicholas Christofilos 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 by radio frequency cavities for direct acceleration, dipole magnets (bending magnets) for deflection of particles (to close the path), and quadrupole / sextupole magnets for beam focusing.

The interior of the Australian Synchrotron facility, a synchrotron light source. Dominating the image is the storage ring, showing a beamline at front right. The storage ring's interior includes a synchrotron and a linac.

The combination of time-dependent guiding magnetic fields and the strong focusing principle enabled the design and operation of modern large-scale accelerator facilities like colliders and synchrotron light sources. The straight sections along the closed path in such facilities are not only required for radio frequency cavities, but also for particle detectors (in colliders) and photon generation devices such as wigglers and undulators (in third generation synchrotron light sources).

The maximum energy that a cyclic accelerator can impart is typically limited by the maximum strength of the magnetic fields and the minimum radius (maximum curvature) of the particle path. Thus one method for increasing the energy limit is to use superconducting magnets, these not being limited by magnetic saturation. electron/positron accelerators may also be limited by the emission of synchrotron 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.

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 of electron/positron accelerators is limited by this radiation loss, while this does not play a significant role in the dynamics of proton or ion accelerators. The energy of such accelerators is limited strictly by the strength of magnets and by the cost.

Injection procedure

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-accelerated particle beams into a synchrotron. The pre-acceleration can be realized by a chain of other accelerator structures like a linac, a microtron or another synchrotron; all of these in turn need to be fed by a particle source comprising a simple high voltage power supply, typically a Cockcroft-Walton generator.

Starting from an appropriate initial value determined by the injection energy, the field strength of the dipole magnets is then increased. If the high energy particle 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 new injection cycle. Depending on the method of magnet control used, the time interval for one cycle can vary substantially between different installations.

Synchrotrons in large-scale facilities

Modern industrial-scale synchrotrons can be very large (here, Soleil near Paris)

One of the early large synchrotrons, now retired, 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 GeV (then called BeV for 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.

Another early large synchrotron is the Cosmotron built at Brookhaven National Laboratory which reached 3.3 GeV in 1953.

As part of colliders

Until August 2008, the highest energy collider in the world was 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 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 as lead) up to an energy of 1.15 PeV.

The largest device of this type seriously proposed was the Superconducting Super Collider (SSC), which was to be built in the United States. This design, like others, used superconducting 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 excessive budget 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 the Cold War resulted in a change of scientific funding priorities that contributed to its ultimate cancellation. 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. This will require a return to the linear accelerator, but with devices significantly longer than those currently in use. There is at present a major effort to design and build the International Linear Collider (ILC), which will consist of two opposing linear accelerators, one for electrons and one for positrons. These will collide at a total center of mass energy of 0.5 TeV.

As part of synchrotron light sources

Synchrotron radiation also has a wide range of applications (see synchrotron 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 the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, the Advanced Photon Source (APS) near Chicago, USA, and SPring-8 in Japan, accelerating electrons up to 6, 7 and 8 GeV, respectively.

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 the Compact Light Source.

List of installations

Synchrotron	Location & Country	Energy (GeV)	Circumference (m)	Commissioned	Decommissioned
Advanced Photon Source (APS)	Argonne National Laboratory, USA	7.0	1104	1995
ALBA	Cerdanyola del Vallès near Barcelona, Spain	3	270	2010
Canadian Light Source	Saskatoon, Canada	2.9	174	2004
Tantalus	Madison, Wisconsin, USA	.2	9.38	1968	1995
ISIS	Rutherford Appleton Laboratory, UK	0.8	163	1985
Australian Synchrotron	Melbourne, Australia	3	216	2006
ANKA	Karlsruhe Institute of Technology, Germany	2.5	110.4	2000
LNLS	Campinas, Brazil	1.37	93.2	1997
SESAME	Allaan, Jordan	2.5	125	Under Design
Bevatron	Lawrence Berkeley Laboratory, USA	6	114	1954	1993
Birmingham synchrotron	University of Birmingham, UK	1	-	1953
Advanced Light Source	Lawrence Berkeley Laboratory, USA	1.9	196.8	1993
Cosmotron	Brookhaven National Laboratory, USA	3	72	1953	1968
National Synchrotron Light Source	Brookhaven National Laboratory, USA	2.8	170	1982
Nimrod	Rutherford Appleton Laboratory, UK	7		1957	1978
Alternating Gradient Synchrotron (AGS)	Brookhaven National Laboratory, USA	33	800	1960
Stanford Synchrotron Radiation Lightsource	SLAC National Accelerator Laboratory, USA	3	234	1973
Synchrotron Radiation Center (SRC)	Madison, USA	1	121	1987
Cornell High Energy Synchrotron Source (CHESS)	Cornell University, USA	5.5	768	1979
Soleil	Paris, France	3	354	2006
Shanghai Synchrotron Radiation Facility (SSRF)	Shanghai, China	3.5	432	2007
Proton Synchrotron	CERN, Switzerland	28	628.3	1959
Tevatron	Fermi National Accelerator Laboratory, USA	1000	6300	1983	2011
Swiss Light Source	Paul Scherrer Institute, Switzerland	2.8	288	2001
Large Hadron Collider (LHC)	CERN, Switzerland	7000	26659	2008
BESSY II	Helmholtz-Zentrum Berlin in Berlin, Germany	1.7	240	1998
European Synchrotron Radiation Facility (ESRF)	Grenoble, France	6	844	1992
MAX-I	MAX-lab, Sweden	0.55	30	1986
MAX-II	MAX-lab, Sweden	1.5	90	1997
MAX-III	MAX-lab, Sweden	0.7	36	2008
ELETTRA	Trieste, Italy	2-2.4	260	1993
Synchrotron Radiation Source	Daresbury Laboratory, UK	2	96	1980	2008
ASTRID	Aarhus University, Denmark	0.58	40	1991
Diamond Light Source	Oxfordshire, UK	3	561.6	2006
DORIS III	DESY, Germany	4.5	289	1980
PETRA II	DESY, Germany	12	2304	1995	2007
PETRA III	DESY, Germany	6.5	2304	2009
Canadian Light Source	University of Saskatchewan, Canada	2.9	171	2002
SPring-8	RIKEN, Japan	8	1436	1997
KEK	Tsukuba, Japan	12	3016
National Synchrotron Radiation Research Center	Hsinchu Science Park, Taiwan	3.3	518.4	2008
Synchrotron Light Research Institute (SLRI)	Nakhon Ratchasima, Thailand	1.2	81.4	2004
Indus 1	Raja Ramanna Centre for Advanced Technology, Indore, India	0.45	18.96	1999
Indus 2	Raja Ramanna Centre for Advanced Technology, Indore, India	2.5	36	2005
Synchrophasotron	JINR, Dubna, Russia	10	180	1957	2005
U-70 synchrotron	Institute for High Energy Physics, Protvino, Russia	70		1967
Schermland-B10	PSG Da Vinci College, Purmerend, Netherlands	70		2012
CAMD	LSU, Louisiana, United States	1.5	-	-
PLS	PAL, Pohang, Korea	2.5	280.56	1994

• Note: in the case of colliders, the quoted energy is often double what is shown here. The above table shows the energy of one beam but if two opposing beams collide head on, the centre of mass energy is double the beam energy shown.

Applications

• Life sciences: protein and large molecule crystallography
• LIGA based microfabrication
• Drug discovery and research
• "Burning" computer chip designs into metal wafers
• Analysing chemicals to determine their composition
• Observing the reaction of living cells to drugs
• Inorganic material crystallography and microanalysis
• Fluorescence studies
• Semiconductor material analysis and structural studies
• Geological material analysis
• Medical imaging
• Proton therapy to treat some forms of cancer

See also

• List of synchrotron radiation facilities
• Synchrotron X-ray tomographic microscopy
• Energy amplifier
• Superconducting Radio Frequency

External links

• Canadian Light Source
• Australian Synchrotron
• Diamond UK Synchrotron
• Lightsources.org
• CERN Large Hadron Collider
• Synchrotron Light Sources of the World
• A Miniature Synchrotron: room-size synchrotron offers scientists a new way to perform high-quality x-ray experiments in their own labs, Technology Review, February 4, 2008
• Brazilian Synchrotron Light Laboratory
• Podcast interview with a scientist at the European Synchrotron Radiation Facility
• Indian SRS
• Sameen Ahmed Khan, Synchrotron Radiation (in Asia), ATIP Report, No. ATIP02.034, 28 pages (21 August 2002). (ATIP: The Asian Technology Information Program, Tokyo, Japan, 2002). Complete Report.
• ALBA Light Source