Particle accelerator
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A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.[1][2]
Large accelerators are used for fundamental research in
There are two basic classes of accelerators: electrostatic and electrodynamic (or electromagnetic) accelerators.
Because the target of the particle beams of early accelerators was usually the atoms of a piece of matter, with the goal being to create collisions with their nuclei in order to investigate nuclear structure, accelerators were commonly referred to as atom smashers in the 20th century.
Uses
Beams of high-energy particles are useful for fundamental and applied research in the sciences, and also in many technical and industrial fields unrelated to fundamental research.
Particle physics
For the most basic inquiries into the dynamics and structure of matter, space, and time, physicists seek the simplest kinds of interactions at the highest possible energies. These typically entail particle energies of many
The largest and highest-energy particle accelerator used for elementary particle physics is the Large Hadron Collider (LHC) at CERN, operating since 2009.[13]
Nuclear physics and isotope production
Particle accelerators can also produce proton beams, which can produce proton-rich medical or research
Synchrotron radiation
Electrons propagating through a magnetic field emit very bright and coherent photon beams via synchrotron radiation. It has numerous uses in the study of atomic structure, chemistry, condensed matter physics, biology, and technology. A large number of synchrotron light sources exist worldwide. Examples in the U.S. are SSRL at SLAC National Accelerator Laboratory, APS at Argonne National Laboratory, ALS at Lawrence Berkeley National Laboratory, and NSLS-II at Brookhaven National Laboratory. In Europe, there are MAX IV in Lund, Sweden, BESSY in Berlin, Germany, Diamond in Oxfordshire, UK, ESRF in Grenoble, France, the latter has been used to extract detailed 3-dimensional images of insects trapped in amber.[15]
Thus there is a great demand for electron accelerators of moderate (GeV) energy, high intensity and high beam quality to drive light sources.
Low-energy machines and particle therapy
Everyday examples of particle accelerators are
At lower energies, beams of accelerated nuclei are also used in medicine as particle therapy, for the treatment of cancer.
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.
Radiation sterilization of medical devices
Electrostatic particle accelerators
Historically, the first accelerators used simple technology of a single static high voltage to accelerate charged particles. The charged particle was accelerated through an evacuated tube with an electrode at either end, with the static potential across it. Since the particle passed only once through the potential difference, the output energy was limited to the accelerating voltage of the machine. While this method is still extremely popular today, with the electrostatic accelerators greatly out-numbering any other type, they are more suited to lower energy studies owing to the practical voltage limit of about 1 MV for air insulated machines, or 30 MV when the accelerator is operated in a tank of pressurized gas with high
The two main types of electrostatic accelerator are the
Electrodynamic (electromagnetic) particle accelerators
Due to the high voltage ceiling imposed by electrical discharge, in order to accelerate particles to higher energies, techniques involving dynamic fields rather than static fields are used. Electrodynamic acceleration can arise from either of two mechanisms: non-resonant magnetic induction, or resonant circuits or cavities excited by oscillating radio frequency (RF) fields.[19] Electrodynamic accelerators can be linear, with particles accelerating in a straight line, or circular, using magnetic fields to bend particles in a roughly circular orbit.
Magnetic induction accelerators
Magnetic induction accelerators accelerate particles by induction from an increasing magnetic field, as if the particles were the secondary winding in a transformer. The increasing magnetic field creates a circulating electric field which can be configured to accelerate the particles. Induction accelerators can be either linear or circular.
Linear induction accelerators
Linear induction accelerators utilize ferrite-loaded, non-resonant induction cavities. Each cavity can be thought of as two large washer-shaped disks connected by an outer cylindrical tube. Between the disks is a ferrite toroid. A voltage pulse applied between the two disks causes an increasing magnetic field which inductively couples power into the charged particle beam.[20]
The linear induction accelerator was invented by Christofilos in the 1960s.
Betatrons
The Betatron is a circular magnetic induction accelerator, invented by Donald Kerst in 1940 for accelerating electrons. The concept originates ultimately from Norwegian-German scientist Rolf Widerøe.[22][23] These machines, like synchrotrons, use a donut-shaped ring magnet (see below) with a cyclically increasing B field, but accelerate the particles by induction from the increasing magnetic field, as if they were the secondary winding in a transformer, due to the changing magnetic flux through the orbit.[24][25]
Achieving constant orbital radius while supplying the proper accelerating electric field requires that the magnetic flux linking the orbit be somewhat independent of the magnetic field on the orbit, bending the particles into a constant radius curve. These machines have in practice been limited by the large radiative losses suffered by the electrons moving at nearly the speed of light in a relatively small radius orbit.
Linear accelerators
In a
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
As the particles approach the speed of light the switching rate of the electric fields becomes so high that they operate at radio frequencies, and so microwave cavities are used in higher energy machines instead of simple plates.
Linear accelerators are also widely used in
Circular or cyclic RF accelerators
In the circular accelerator, particles move in a circle until they reach enough 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 circular accelerator is smaller than a linear accelerator of comparable power (i.e. 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
Since the
Cyclotrons
The earliest operational circular accelerators were
Cyclotrons reach an energy limit because of
Synchrocyclotrons and isochronous cyclotrons
A classic cyclotron can be modified to increase its energy limit. The historically first approach was the synchrocyclotron, which accelerates the particles in bunches. It uses a constant magnetic field , but reduces the accelerating field's frequency so as to keep the particles in step as they spiral outward, matching their mass-dependent cyclotron resonance frequency. This approach suffers from low average beam intensity due to the bunching, and again from the need for a huge magnet of large radius and constant field over the larger orbit demanded by high energy.
The second approach to the problem of accelerating relativistic particles is the
Synchrocyclotrons have not been built since the isochronous cyclotron was developed.
Synchrotrons
To reach still higher energies, with relativistic mass approaching or exceeding the rest mass of the particles (for protons, billions of electron volts or
However, since the particle momentum increases during acceleration, it is necessary to turn up the magnetic field B in proportion to maintain constant curvature of the orbit. In consequence, synchrotrons cannot accelerate particles continuously, as cyclotrons can, but must operate cyclically, supplying particles in bunches, which are delivered to a target or an external beam in beam "spills" typically every few seconds.
Since high energy synchrotrons do most of their work on particles that are already traveling at nearly the speed of light c, the time to complete one orbit of the ring is nearly constant, as is the frequency of the
In modern synchrotrons, the beam aperture is small and the magnetic field does not cover the entire area of the particle orbit as it does for a cyclotron, so several necessary functions can be separated. Instead of one huge magnet, one has a line of hundreds of bending magnets, enclosing (or enclosed by) vacuum connecting pipes. The design of synchrotrons was revolutionized in the early 1950s with the discovery of the
More complex modern synchrotrons such as the Tevatron,
Electron synchrotrons
Circular electron accelerators fell somewhat out of favor for particle physics around the time that
A large number of electron synchrotrons have been built in the past two decades, as part of synchrotron light sources that emit ultraviolet light and X rays; see below.
Synchrotron radiation sources
Some circular accelerators have been built to deliberately generate radiation (called
Synchrotron radiation is more powerfully emitted by lighter particles, so these accelerators are invariably
Fixed-field alternating gradient accelerators
Rhodotron
A Rhodotron is an industrial electron accelerator first proposed in 1987 by J. Pottier of the French Atomic Energy Agency (CEA),[34] manufactured by Belgian company Ion Beam Applications. It accelerates electrons by recirculating them across the diameter of a cylinder-shaped radiofrequency cavity. A Rhodotron has an electron gun, which emits an electron beam that is attracted to a pillar in the center of the cavity. The pillar has holes the electrons can pass through. The electron beam passes through the pillar via one of these holes and then travels through a hole in the wall of the cavity, and meets a bending magnet, the beam is then bent and sent back into the cavity, to another hole in the pillar, the electrons then again go across the pillar and pass though another part of the wall of the cavity and into another bending magnet, and so on, gradually increasing the energy of the beam until it is allowed to exit the cavity for use. The cylinder and pillar may be lined with copper on the inside.[35][36][37]
History
Ernest Lawrence's first cyclotron was a mere 4 inches (100 mm) in diameter. Later, in 1939, he built a machine with a 60-inch diameter pole face, and planned one with a 184-inch diameter in 1942, which was, however, taken over for World War II-related work connected with uranium isotope separation; after the war it continued in service for research and medicine over many years.
The first large proton
The
The
The aborted Superconducting Super Collider (SSC) in Texas would have had a circumference of 87 km. Construction was started in 1991, but abandoned in 1993. Very large circular accelerators are invariably built in tunnels a few metres wide to minimize the disruption and cost of building such a structure on the surface, and to provide shielding against intense secondary radiations that occur, which are extremely penetrating at high energies.
Current accelerators such as the
Targets
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. Particles are accelerated to the desired energy. Then, 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 synchrotrons 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.
Detectors
The detectors gather clues about the particles including their speed and charge using these the scientists can actually work on the particle. The process of detection is very complex it requires strong electromagnets and accelerators.
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Higher energies
At present the highest energy accelerators are all circular colliders, but both hadron accelerators and electron accelerators are running into limits. Higher energy hadron and ion cyclic accelerators will require accelerator tunnels of larger physical size due to the increased beam rigidity.
For cyclic electron accelerators, a limit on practical bend radius is placed by synchrotron radiation losses and the next generation will probably be linear accelerators 10 times the current length. An example of such a next generation electron accelerator is the proposed 40 km long International Linear Collider.
It is believed that plasma wakefield acceleration in the form of electron-beam "afterburners" and standalone laser pulsers might be able to provide dramatic increases in efficiency over RF accelerators 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 precedes 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.[38]
Energy gradients as steep as 200 GeV/m have been achieved over millimeter-scale distances using laser pulsers
Higher than 0.25 GeV/m gradients have been achieved by a dielectric laser accelerator,[40] which may present another viable approach to building compact high-energy accelerators.[41] Using femtosecond duration laser pulses, an electron accelerating gradient 0.69 GeV/m was recorded for dielectric laser accelerators.[42] Higher gradients of the order of 1 to 6 GeV/m are anticipated after further optimizations.[43]
Advanced Accelerator Concepts
Advanced Accelerator Concepts encompasses methods of beam acceleration with gradients beyond state of the art in operational facilities. This includes diagnostics methods, timing technology, special needs for injectors, beam matching, beam dynamics and development of adequate simulations. Workshops dedicated to this subject are being held in the US (alternating locations) and in Europe, mostly on Isola d'Elba. The series of Advanced Accelerator Concepts Workshops, held in the US,[44] started as an international series in 1982.[45] The European Advanced Accelerator Concepts Workshop series started in 2019.[46] Topics related to Advanced Accelerator Concepts:
- Laser Plasma Acceleration of electrons and positrons
- Laser and High-Gradient Structure-Based Acceleration
- Beam-Driven Acceleration
- Laser-Plasma Acceleration of Ions
- Beam Sources such as electron gun, Monitoring, and Control. See Accelerator physics
- Accelerator Physics
- Laser technology for particle acceleration
- Electromagnetic radiation Generation
- Muon collider
According to the Inverse scattering problem, any mechanism by which a particle produces radiation (where kinetic energy of the particle is transferred to the electromagnetic field), can be inverted such that the same radiation mechanism leads to the acceleration of the particle (energy of the radiation field is transferred to kinetic energy of the particle). The opposite is also true, any acceleration mechanism can be inverted to deposit the energy of the particle into a decelerating field, like in a kinetic energy recovery system. This is the idea enabling an energy recovery linac. This principle, which is also behind the plasma or dielectric wakefield accelerrators, led to a few other interesting developments in advanced accelerator concepts:
- Cherenkov radiation led to inverse Cherenkov radiation accelerator.[47]
- Free-electron laser led to the Inverse Free-electron laser accelerator.[48]
- A laser can also be inverted to produce acceleration of electrons.[49]
Black hole production and public safety concerns
In the future, the possibility of a black hole production at the highest energy accelerators may arise if certain predictions of
Accelerator operator
The use of advanced technologies such as superconductivity, cryogenics, and high powered radiofrequency amplifiers, as well as the presence of ionizing radiation, pose challenges for the safe operation of accelerator facilities.
See also
- Accelerator physics
- Atom smasher (disambiguation)
- Compact Linear Collider
- Dielectric wall accelerator
- Future Circular Collider
- International Linear Collider
- KALI
- Linear particle accelerator
- List of accelerators in particle physics
- Momentum compaction
- Nuclear transmutation
- Rolf Widerøe
- Superconducting Super Collider
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External links
- What are particle accelerators used for?
- Stanley Humphries (1999) Principles of Charged Particle Acceleration
- 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.
- ISBN 978-0-520-06426-3.
- David Kestenbaum, Massive Particle Accelerator Revving Up NPR's Morning Edition article on 9 April 2007
- Ragnar Hellborg, ed. (2005). Electrostatic Accelerators: Fundamentals and Applications. Springer. ISBN 978-3-540-23983-3.
- Annotated bibliography for particle accelerators from the Alsos Digital Library for Nuclear Issues Archived 2010-10-07 at the Wayback Machine
- Accelerators-for-Society.org, to know more about applications of accelerators for Research and Development, energy and environment, health and medicine, industry, material characterization.