Particle accelerator

A particle accelerator is a device that uses electric and/or magnetic fields to propel electrically charged particles to high speeds.

Circular accelerators

In a circular accelerator, particles move in a circle until they reach sufficient energy. The particle track is typically bent into a circle using electromagnets. The advantage of circular accelerators over linear accelerators (linacs) is that the ring topology allows continued acceleration, as the particle can transit indefinitely. Another advantage is that a linac would have to be extremely long to have the equivalent power of a circular accelerator, which is impractical.

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Depending on the energy and particle being accelerated, circular accelerators suffer a disadvantage in that the particles emit synchrotron radiation. When any charged particle is accelerated, it emits electromagnetic radiation and secondary emissions. As a particle travelling in a circle is always accelerating towards the centre of the circle, it continuously radiates. This must be compensated for, which makes circular accelerators less efficient than linear ones.

Related Topics:
Synchrotron radiation - Electromagnetic radiation - Secondary emissions

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Synchrotron light

Some circular accelerators have been built to deliberately generate radiation (called synchrotron light) as X-rays also called synchrotron radiation , for example the Diamond Light Source being built at the Rutherford Appleton Laboratory in England or the Advanced Photon Source at Argonne National Laboratory in Illinois, USA. High energy X-rays are useful for X-ray spectroscopy of proteins or X-ray absorption fine structure (XAFS) for example.

Related Topics:
Synchrotron light - X-rays - Diamond Light Source - Rutherford Appleton Laboratory - England - Advanced Photon Source - Argonne National Laboratory - Illinois - X-ray spectroscopy - Protein - X-ray absorption fine structure

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Synchrotron radiation

Synchrotron radiation is more powerfully emitted by lighter particles, so these accelerators are invariably electron accelerators. Synchrotron radiation allows for better imaging as researched and developed at SLAC's SPEAR

Related Topics:
Electron - SLAC's SPEAR

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In contrast, particle physicists are increasingly using more massive particles such as protons in their accelerators to get to higher energies. These particles are composites of quarks and gluons, which makes analysing the results of their interactions much more complicated, and also of much scientific interest.

Related Topics:
Protons - Quarks - Gluons

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History of Cyclotrons and Defining

The earliest circular accelerators were cyclotrons, invented in 1929 by Ernest O. Lawrence. Cyclotrons have a single pair of hollow 'D'-shaped plates to accelerate the particles and a single dipole magnet to curve the track of the particles. The particles are injected in the centre of the circular machine and spiral outwards towards the circumference.

Related Topics:
Cyclotron - 1929 - Ernest O. Lawrence - Dipole magnet

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Cyclotrons reach an energy limit because of the relativistic effects at high energies whereby particles become more difficult to accelerate. Though the special theory of relativity precludes matter from traveling faster than the speed of light in a vacuum, the particles in an accelerator normally travel very close to the speed of light, perhaps 99.99%. In high energy accelerators, there is a diminishing return in speed as the particle approaches the speed of light. Therefore particle physicists do not generally concern themselves with speed, and speak of a particle's energy in electron volts (eV) instead.

Related Topics:
Relativistic effects - Special theory of relativity - Vacuum - Energy - Electron volt

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Cyclotrons no longer accelerate protons when they have reached an energy of about 10 million electron volts (10 MeV), because the protons get out of phase with the driving electric field. They continue to spiral outward to larger radius but, as explained above, no longer gain enough speed to complete the larger circle as quickly. There are ways for compensating for this to some extent - namely the synchrocyclotron and the isochronous cyclotron. They are nevertheless useful for "lower energy" applications.

Related Topics:
Synchrocyclotron - Cyclotron

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To push the energies even higher - into billions of electron volts (GeV), it is necessary to use a synchrotron. This is an accelerator in which the particles are contained in a donut-shaped tube, called a storage ring. The tube has many magnets distributed around it to focus the particles and curve their tracks around the tube, and microwave cavities similarly distributed to accelerate them.

Related Topics:
GeV - Synchrotron - Storage ring

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The size of Lawrence's first cyclotron was a mere 4 inches (100 mm) in diameter. Fermilab has a ring with a beam path of 4 miles (6 km). The largest ever built was the LEP at CERN with a diameter of 8.5 kilometers (circumference 26.6 km) which was an electron/positron collider. It has been dismantled and the underground tunnel is being reused for a proton/proton collider called the LHC, due to start operation in 2007. The aborted Superconducting Supercollider (SSC) in Texas would have had a circumference of 87 km. Construction was started but it was subsequently abandoned well before completion. Very large circular accelerators are invariably built in underground tunnels a few metres wide to minimise the disruption and cost of building such a structure on the surface, and to provide shielding against the intense synchrotron radiation.

Related Topics:
Fermilab - LEP - CERN - Positron - LHC - Superconducting Supercollider - Texas

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Current accelerators such as the Spallation Neutron Source, Relativistic Heavy Ion Collider, and upcoming Large Hadron Collider make use of superconducting magnets and RF cavity resonators to accelerate particles.

Related Topics:
Spallation Neutron Source - Relativistic Heavy Ion Collider - Large Hadron Collider - Superconducting - RF cavity resonators

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