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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 synchronized with
the travelling particle beam. They were originally developed by Luis Walter Alvarez to study high-energy
particle physics.
History
Synchrotrons originated in the 1940s and have evolved progressively; the first serious exploitation of
synchrotron light was in the 1960s.
As scientists became aware of its great potential as a tool for conducting a wide variety of leading edge
experiments, development accelerated dramatically - particularly over the past fifteen years - with the
building of a number of second-generation and, from the early 1990s, third-generation facilities.
At present there are about 50 major facilities worldwide, with a number of others under construction or
planned.
Most scientifically developed countries have a local facility, including countries such as Brazil and India.
Japan has eight major installations (with a number of others privately operated), Germany seven and
Sweden two.
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 container 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
beam.
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. 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 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. 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.
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 2008. It is being built 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
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.
Applications
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Life sciences: protein and large molecule crystallography
Drug discovery and research
"Burning" computer chip designs into metal wafers
Studying molecule shapes and protein crystals
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
Synchrotron radiation has become an indispensable tool in a wide range of research fields.
Using the intense UV, soft X-ray and hard X-ray beams produced at synchrotron radiation facilities,
scientists can: determine the structure of materials and molecules, the electronic (chemical) structure of
surfaces and interfaces; analyse tiny trace element concentrations in micron-sized regions; measure local
molecular structures in disordered systems eg solutions and catalysts; obtain 3-D CAT scan images with
micron resolution, and so on.
One of the first techniques to make use of synchrotron radiation was X-ray crystallography: the
determination of atomic structure from X-ray diffraction data. X-ray crystallography has historically been
the primary tool used to investigate the structure of matter: the atomic and crystal structures of most
materials have been determined using these techniques.
With the high brightness X-ray beams produced by the new third generation synchrotron radiation facilities,
a number of totally new capabilities will become possible, further expanding the number of disciplines and
research communities making use of synchrotron radiation.
This structural knowledge is central to the development of many new technologies, e.g. pharmaceuticals and
nanotechnology.
Synchrotron radiation, because of the characteristics mentioned above, has not only allowed the extension of
conventional crystallographic techniques to the investigation of new materials and macro-molecules, but it
has also facilitated major advances in the understanding of the structure and dynamics of ceramics,
superconductors, polymers and the structure of surfaces and interfaces, even at the level of single layers of
molecules.
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The high brightness and intensity will make time-resolved crystallography and spectroscopy feasible
on a nano-second timescale, e.g. for the study of the dynamics of chemical reactions, which will
extend the utilisation of synchrotron radiation by the chemistry, physics and biostructure
communities
The high X-ray energies which will be available at high intensity for the first time will benefit the
geoscience community as the higher penetration of the radiation is well suited to high pressure
diffraction experiments
The high brightness of the third generation source is ideally suited to microbeam and imaging
applications, including X-ray microscopy, fluorescence micro-analysis and micro-diffraction. For
example, the fluorescence X-ray microprobes planned will have detection limits several orders of
magnitude lower than current techniques (a few parts per billion for most elements) and a resolution
of better than 0.1 microns. These microbeam techniques will be utilised by the environmental
science, geophysics and medical physics communities
Soft X-ray microscopy offers a resolution of about 100 Angstroms and the ability to image wet
biological samples. This is finding important applications in biology.
Synchrotron X-ray tomographic microscopy is a 3-D scanning technique that allows non-invasive high
definition scans of fossil embryos to be made. It brings out details as fine as 1,000th of a millimeter,
meaning it has two to three thousand times the resolution of a medical CT scan. Scientists feel this
technology has the potential to revolutionize the field of palaeontology. The first team to use the technique
have published their findings in Nature, which they believe "could roll back the evolutionary history of
arthropods like insects and spiders."
Advantages over conventional nuclear fission reactors
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Subcritical design means that the reaction could not run away — if anything went wrong, the
reaction would stop and the reactor would cool down. A meltdown could however occur if the
ability to cool the core was lost.
Thorium is an abundant element — much more so than uranium — reducing strategic and political
supply issues and eliminating costly and energy-intensive isotope separation. There is enough
thorium to generate energy for at least several thousand years at current consumption rates.
The energy amplifier would produce very little plutonium, so the design is believed to be more
proliferation-resistant than conventional nuclear power (although the question of uranium-233 as
nuclear weapon material must be assessed carefully).
The possibility exists of using the reactor to consume plutonium, reducing the dangerously large
world stockpile of the very-long-lived element.
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Less long-lived radioactive waste is produced — the waste material would decay after 500 years to
the radioactive level of coal ash.
No new science is required; the technologies to build the energy amplifier have all been
demonstrated in the laboratory. Building an energy amplifier requires only some engineering effort,
not fundamental research (unlike nuclear fusion proposals).
Power generation might be economical compared to current nuclear reactor designs if the total fuel
cycle and decommissioning costs are considered.
The design could work on a relatively small scale, making it more suitable for countries without a
well-developed power grid system
Inherent safety and safe fuel transport could make the technology more suitable for developing
countries as well as in densely populated areas.
Disadvantages
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General technical difficulties
Each reactor needs its own facility (synchrotron) to generate the high energy proton beam, which is
very costly.
No synchrotron of sufficient power has ever been built. Recently, however, the Spallation Neutron
Source with 1 GeV protons was completed for other experiments.