A particle accelerator is a device that uses electric fields to propel electrically charged particles to high speeds. Everyday examples of particle accelerators are those found in television sets and X-ray generators. The largest and most powerful particle accelerators, such as the LHC and Tevatron, are used for experimental particle physics.
There are two basic types of particle accelerator: circular and linear.
In a circular accelerator, the 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 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.
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. 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. 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 protons when they have reached an energy of about 10 million electron volts, because the protons get out of phase with the driving electric field. They continue to spiral outward to larger redius 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.
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.
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. DC accelerator types capable of causing nuclear reactions are Cocroft-Waltons or voltage multipliers that convert AC to high voltage DC and Van de Graaffs that use static electricity carried by belts.
Higher energy accelerators use a linear array of plates (or drift tubes) 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 in the world is the Stanford Linear Accelerator, which is 2 miles long.
Except for synchrotron 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.
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. An example of such a next generation accelerator is the International Linear Collider, due to be constructed between 2015-2020.
As of 2005, it is believed that plasma wakefield accelerators in the form of electron-beam 'afterburners' and standalone laser pulsers will provide dramatic increases in efficiency within two to three decades. In plasma wakefield accelerators, the beam cavity is filled with a plasma (rather than vacuum). A short pulse of electrons or laser light eithers constitutes or immediately trails the particles that are being accelerated. The pulse disrupts the plasma, causing the charged particles in the plasma to integrate into and move toward the rear of the bunch of particles that are being accelerated. This process transfer energy to the particle bunch, accelerating it further, and continues as long as the pulse is coherent (Matthew Early Wright. (April 2005). "Riding the Plasma Wave of the Future". Symmetry: Dimensions of Particle Physics (Fermilab/SLAC), p. 12)
Energy gradients as steep as 200 GeV/m have been achieved over millimeter-scale distances using laser pulsers (Briezman, et al. [http://peaches.ph.utexas.edu/ifs/ifsreports/Self-focused762.pdf, "Self-Focused Particle Beam Drivers for Plasma Wakefield Accelerators"]. Retrieved 13 May 2005) and gradients approaching 1 GeV/m are being produced on the multi-centimeter-scale with electron-beam systems, in contrast to a limit of about 0.1 GeV/m for radiofrequency acceleration alone. Existing electron accelerators such as SLAC could use electron-beam afterburners to increase the intensity of their particle beams. Electron systems in general can provide tightly collimated, reliable beams; laser systems may offer more power and compactness. Thus, plasma wakefield accelerators could be used--if technical issues can be resolved--to both increase the maximum energy of the largest accelerators and to bring high energies into university laboratories and medical centers.
In next few decades, the possibility of black hole production at the highest energy accelerators may arise, if certain predictions of superstring theory are accurate (Scientific American, May 2005). If they were produced, it is thought that black holes would evaporate extremely quickly via Hawking radiation. However, the existence of Hawking radiation is controversial. (Adam D. Helfer, "Do black holes radiate?" Reports on Progress in Physics. Vol. 66 No. 6 (2003) pp. 943-1008 http://xxx.lanl.gov/abs/gr-qc/0304042 ) It is also thought that an analogy between colliders and cosmic rays demonstrates collider safety. If colliders can produce black holes, cosmic rays should have been producing them for aeons, and they have yet to harm us. However, this is also controversial. Models in which colliders cause trouble and cosmic rays do not have been proposed.
Black hole production would necessitate the development of new methods for investigating in a terrestrial accelerator the kinds of extremely massive particles that are thought to exist in dark matter and to have existed during the Big Bang.