Particle accelerator

A particle accelerator is a device that uses electric and/or magnetic fields to propel electrically charged particles to high speeds. There are two types: linear (i.e., straight-line) accelerators and circular accelerators.

High-energy machines

Linear high-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 of "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 RF cavity resonators are used in higher energy machines instead of simple plates.

In an X-ray generator, the target itself is one of the electrodes. DC accelerator types capable of accelerating particles to speeds sufficient to cause nuclear reactions are Cockcroft-Walton generators or voltage multipliers, which convert AC to high voltage DC, or Van de Graaff generators that use static electricity carried by belts.

The largest and most powerful particle accelerators, such as the RHIC, the LHC (scheduled to start operation in 2007) and the Tevatron, are used for experimental particle physics. Particle accelerators can also produce proton beams, which can produce "proton-heavy" medical or research isotopes as opposed to the "neutron-heavy" ones made in reactors. An example of this type of machine is LANSCE at Los Alamos.

Low-energy machines

Everyday examples of particle accelerators are those found in television sets and X-ray generators. 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. A low-energy particle accelerator, an ion implanter is used in the manufacture of integrated circuits.

Linear particle accelerators

Main article: Linear particle accelerator

In a linear accelerator (linac), particles are accelerated in a straight line, with a target of interest at one end. Linacs 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 longest linac in the world is the Stanford Linear Accelerator, SLAC, which is 3 km (2 miles) long. SLAC is an electron-positron collider.

Circular accelerators

In the 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 continuous 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.

Depending on the energy and the 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 center of the circle, it continuously radiates towards the tangent of the circle. This radiation is called synchrotron light and depends highly on the mass of the accelerating particle. With this in mind, many high energy electron accelerators are linacs.

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.

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 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.

History of cyclotrons and defining

The earliest circular accelerators were cyclotrons, invented in 1929 by Ernest O. Lawrence at the University of California, Berkeley. 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.

Another type of circular accelerator, invented in 1940 for accelerating electrons, is the Betatron.

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. 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 think in terms of speed, but rather in terms of a particle's energy, usually measured in electron volts (eV), instead.

Cyclotrons can 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 a larger radius but, as explained above, no longer gain enough speed to complete the larger circle as quickly. They are nevertheless useful for "lower energy" applications. There are ways for compensating for this to some extent - namely the synchrocyclotron and the isochronous cyclotron.

To make the energies even higher, to 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.

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.

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

Tandem electrostatic accelerators

In a tandem accelerator, the negatively charged ion gains energy by attraction to the very high positive voltage at the geometric centre of the pressure vessel. When it arrives at the centre region, called the high voltage terminal, some electrons are stripped from the ion, which becomes positive. The ion is then accelerated away by the high positive voltage. Thus this type of accelerator is called a 'tandem' accelerator - it has two stages of acceleration, first pulling and then pushing the charged particles. An example of a tandem accelerator is ANTARES (Australian National Tandem Accelerator for Applied Research).

Targets and detectors

The output of a particle accelerator can generally be directed towards multiple lines of experiments, one at a given time, by means of a deviating electromagnet. This makes it possible to operate multiple experiments without needing to move things around or shutting down the entire accelerator beam.

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; 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 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, also called a storage ring 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 can increase the energy enormously; whereas in a fixed-target experiment the energy available to produce new particles is proportional to the square root of the beam energy, in a collider the available energy is linear.

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 10 times the current length. An example of such a next generation accelerator is the 40 km long International Linear Collider, due to be constructed between 2015-2020.

As of 2005, it is believed that plasma wakefield acceleration 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 either 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 transfers 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 pulsersBriezman, et al. "Self-Focused Particle Beam Drivers for Plasma Wakefield Accelerators." (PDF) 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 radio-frequency 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 centres.

Black hole production

In the 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 are 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 (2003). ""Do black holes radiate?" Rept. Prog. Phys. 66: 943. 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; some have suggested possible models in which black holes might be created by colliders but not cosmic rays.

External links

What are particle accelerators used for?
Particle Accelerators around the world
Wolfgang K. H. Panofsky: The Evolution of Particle Accelerators & Colliders, (PDF), Stanford, 1997
P.J. Bryant, A Brief History and Review of Accelerators (PDF), CERN, 1994.

Tandem accelerators

Amateur construction

References

Particle accelerators | Magnets

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