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There are two basic types, linear and circular.
The particles are accelerated in a straight line, with the target at the end of it. Low energy accelerators such as cathode ray tubes and X-ray generators use a single pair of electrodes with a dc voltage of a few thousand volts between them. In an X-ray generator, the target itself is one of the electrodes.
Higher energy accelerators use a linear array of plates to which an alternating high energy field is applied. As the particles approach a plate they are accelerated towards it by an opposite polarity charge applied to the plate. As they pass through a hole in the plate, the polarity is switched so that the plate now repels them and they are now accelerated by it towards the next plate. Normally a stream bunches of particles are accelerated, so a carefully controlled AC voltage is applied to each plate to continuously repeat this for each bunch.
As the particles approach the speed of light the switching rate of the electric fields becomes so high that they operate at microwave frequencies, and so microwave cavities are used in higher energy machines instead of simple plates.
High energy linear accelerators are often called linacs.
Linear accelerators are very widely used - every cathode ray tube contains one, and they are also used to provide an initial low energy kick to particles before they are injected into circular accelerators. They also can produce proton beams, which can produce "proton-heavy" medical or research isotopes as opposed to the "neutron-heavy" ones made in reactors.
The largest is the Stanford Linear Accelerator, which is 2 miles long.
The accelerated particles move in a circle until they reach sufficient energy. The particle track is bent into a circle using dipole magnets. The advantage of circular accelerators over linacs is that components can be reused to accelerate the particles further, as the particle passes a given point many times. However they suffer a disadvantage in that the particles emit synchrotron radiation.
When any charged particle is accelerated, it emits electromagnetic radiation. As a particle travelling in a circle is always accelerating towards the centre of the circle, it continuously radiates. This has to be compensated for by some of the energy used to power the accelerating electric fields, which makes circular accelerators less efficient than linear ones. Some circular accelerators have been built to deliberately generate this radiation (called synchrotron light) as X-rays - for example the Diamond Light Source being built at the Rutherford Appleton Laboratory in England. High energy X-rays are useful for X-ray spectroscopy of proteinmyoglobin, showing coloured alpha helices. This protein was the first to have its structure solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, which led to them receiving a Nobel Prize in Chemistry. A protein is a complex,s for example.
Synchrotron radiation is more powerfully emitted by lighter particles, so these accelerators are invariably electronThe electron (also called negatron commonly represented as e&minus is a subatomic particle. In an atom the electrons surround the nucleus of protons and neutrons in an electron configuration. Electrons have the smallest electrical charge and when they mov accelerators. Consequently particle physicists are increasingly using heavier particles such as protons in their accelerators to get to higher energies. The downside is that these particles are composites of quarks and gluons which makes analysing the results of their interactions much more complicated.
The earliest circular accelerators were cyclotronA cyclotron is a machine designed to accelerate beams of charged particles by using a high frequency alternating voltage across a magnetic field to spiral the beam out and eventually deflect it once the beam's radius equals its container's. At this points, invented in 1929Centuries: 19th century 20th century 21st century Decades: 1870s 1880s 1890s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s Years: 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 See also 1929 in aviation 1929 in film 1929 in literature 1929 in mu 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.
Cyclotrons reach an energy limit because of the relativistic effects at high energies whereby particles gain mass rather than speed. As the Special theory of relativity means that nothing can travel faster than the speed of light does 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. The effect of the energy injected using the electric fields is therefore to markedly increase their mass rather than their speed. Doubling the energy might increase the speed a fraction of a percent closer to that of light but the main effect is to increase the relativistic mass of the particle.
Cyclotrons no longer accelerate an electrons when they have reached an energy of about 10 million electron volts. 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.
To push the energies even higher - into billions of electron volts, 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 track around the tube, and microwave cavities similarly distributed to accelerate them.
The size of Lawrence's first cyclotron was a mere 4 inches in diameter. Fermilab has a ring with a beam path of 4 miles. 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 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.
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