Synchrotron

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 synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant.[1] The honor of being the first to publish the idea went to Vladimir Veksler, and the first electron synchrotron was constructed by Edwin McMillan.

Characteristics

While a cyclotron uses a constant magnetic field and a constant-frequency applied electric field (one of these is varied in the synchrocyclotron), both of these fields 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 chamber for the particles to be a large thin torus. In reality it is easier to use some straight sections between the bending magnets and some bent sections within the magnets giving the torus the shape of a round-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 beams. Straight sections are required at spacings around a ring for both radiofrequency cavities, and in third generation setups space is allowed for insertion of energy extraction devices such as wigglers and undulators.

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.
The interior of the Australian Synchrotron facility. Dominating the image is the storage ring, showing the optical diagnostic beamline at front right. In the middle of the storage ring is the booster synchrotron and linac

In a cyclotron the maximum radius is quite limited as the particles start at the centre and spiral outward, thus the 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 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, using a narrow beam pipe which can be surrounded by much smaller and more tightly focusing 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.

Design and operation

Particles are injected into the main ring at substantial energies by either a linear accelerator or by an intermediate synchrotron which is in turn fed by a linear accelerator. The "linac" is in turn fed by particles accelerated to intermediate energy by a simple high voltage power supply, typically a Cockcroft-Walton generator.

Starting from an appropriate initial value determined by the injection velocity the magnetic field is then increased. The particles pass through an electrostatic accelerator driven by a high alternating voltage. At particle speeds not close to the speed of light the frequency of the accelerating voltage can be made roughly proportional to the current in the bending magnets. A finer control of the frequency is performed by a servo loop which responds to the detection of the passing of the traveling group of particles. At particle speeds approaching light speed the frequency becomes more nearly constant, while the current in the bending magnets continues to increase. The maximum energy that can be applied to the particles (for a given ring size and magnet count) is determined by the saturation of the cores of the bending magnets (the point at which increasing current does not produce additional magnetic field). One way to obtain additional power is to make the torus larger and add additional bending magnets. This allows the amount of particle redirection at saturation to be less and so the particles can be more energetic. Another means of obtaining higher power is to use superconducting magnets, these not being limited by core saturation.

Large synchrotrons
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.[2]

Until August 2008, the highest energy synchrotron 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.

However, 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
Tantalus	Madison, 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
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
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
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	IHEP, Protvino, Russia	70		1967
CAMD	LSU, Louisiana, US	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