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Particle accelerator
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Particle accelerator

A particle accelerator uses electric fields to propel charged particles to great energies. Everyday applications are found in TV sets and X-ray generators. The particles are contained in an evacuated tube so that they do not get dispersed by hitting air molecules. In higher-energy accelerators, quadrupole magnets are used to focus the particles into a beam and prevent their mutual electrostatic repulsion from causing them to spread out.

There are two basic types, linear and circular.

Linear accelerators

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. The largest is the Stanford Linear Accelerator, which is 2 miles long.

Circular accelerators

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 proteins for example.

Synchrotron radiation is more powerfully emitted by lighter particles, so these accelerators are invariably electron 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 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.

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.

Targets

Except for syncrotron radiation sources, the purpose of an accelerator is to generate high energy particles for interaction with matter.

This is usually a fixed target, such as the phosphor coating on the back of the screen in the case of a television tube, or a piece of uranium in an accelerator designed as a neutron source, or a tungsten target for an X-ray generator. In a linac, the target is simply fitted to the at the end of the accelerator. The particle track in a cyclotron is a spiral outwards from the centre of the circular machine, so the accelerated particles emerge from a fixed point as for a linear accelerator.

For synchrotrons, the situation is more complex. Once the particles have been accelerated to the desired energy, a fast acting dipole magnet is used to switch the particles out of the circular synchrotron tube and towards the target.

A variation commonly used for particle physics research is a collider. Two circular synchrotons are built in close proximity - usually on top of each other and using the same magnets (which are then of more complicated design to accommodate both beam tubes). Bunches of particles travel in opposite directions around the two accelerators and collide at intersections between them. This doubles the energy of the collision compared to a fixed target accelerator for a small increase in cost.

Higher energies

At present the highest energy accelerators are all circular colliders, but it is likely that limits have been reached in respect of compensating for synchrotron radiation losses, and the next generation will probably be linear accelerators five or ten miles long.

See also