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. 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. The linear induction accelerator was invented by
Christofilos in the 1960s. Linear induction accelerators are capable of accelerating very high beam currents (>1000 A) in a single short pulse. They have been used to generate X-rays for flash radiography (e.g.
DARHT at
LANL), and have been considered as particle injectors for
magnetic confinement fusion and as drivers for
free electron lasers.
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. 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. 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 , multicell linear accelerator component. In a
linear particle accelerator (linac), particles are accelerated in a straight line with a target of interest at one end. They are often 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 long. SLAC was originally an
electron–
positron collider but is now a X-ray
Free-electron laser. 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 process 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
radio frequencies, and so
microwave cavities are used in higher energy machines instead of simple plates. Linear accelerators are also widely used in
medicine, for
radiotherapy and
radiosurgery. Medical grade linacs accelerate electrons using a
klystron and a complex bending magnet arrangement which produces a beam of energy . The electrons can be used directly or they can be collided with a target to produce a beam of
X-rays. The reliability, flexibility and accuracy of the radiation beam produced has largely supplanted the older use of
cobalt-60 therapy as a treatment tool.
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
synchrotron radiation. When any charged particle is accelerated, it emits
electromagnetic radiation and
secondary emissions. As a particle traveling 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. For this reason, many high energy electron accelerators are linacs. Certain accelerators (
synchrotrons) are however built specially for producing synchrotron light (
X-rays). Since the
special theory of relativity requires that matter always travels slower than the speed of light in
vacuum, in high-energy accelerators, as the energy increases the particle speed approaches the speed of light as a limit, but never attains it. Therefore, particle physicists do not generally think in terms of speed, but rather in terms of a particle's
energy or
momentum, usually measured in
electron volts (eV). An important principle for circular accelerators, and
particle beams in general, is that the
curvature of the particle trajectory is proportional to the particle charge and to the magnetic field, but inversely proportional to the (typically
relativistic)
momentum.
Cyclotrons Lawrence Radiation Laboratory, Berkeley, in August, 1939, the most powerful accelerator in the world at the time.
Glenn T. Seaborg and
Edwin McMillan (right) used it to discover
plutonium,
neptunium and many other transuranic elements and isotopes, for which they received the 1951
Nobel Prize in chemistry. The earliest operational circular accelerators were
cyclotrons, invented in 1929 by
Ernest Lawrence at the
University of California, Berkeley. Cyclotrons have a single pair of hollow D-shaped plates to accelerate the particles and a single large
dipole magnet to bend their path into a circular orbit. It is a characteristic property of charged particles in a uniform and constant magnetic field B that they orbit with a constant period, at a frequency called the
cyclotron frequency, so long as their speed is small compared to the speed of light
c. This means that the accelerating D's of a cyclotron can be driven at a constant frequency by a RF accelerating power source, as the beam spirals outwards continuously. The particles are injected in the center of the magnet and are extracted at the outer edge at their maximum energy. Cyclotrons reach an energy limit because of
relativistic effects whereby the particles effectively become more massive, so that their cyclotron frequency drops out of sync with the accelerating RF. Therefore, simple cyclotrons can accelerate protons only to an energy of around 15 million electron volts (15 MeV, corresponding to a speed of roughly 10% of
c), because the protons get out of phase with the driving electric field. If accelerated further, the beam would continue to spiral outward to a larger radius but the particles would no longer gain enough speed to complete the larger circle in step with the accelerating RF. To accommodate relativistic effects the magnetic field needs to be increased to higher radii as is done in
isochronous cyclotrons. An example of an isochronous cyclotron is the
PSI Ring cyclotron in Switzerland, which provides protons at the energy of 590 MeV which corresponds to roughly 80% of the speed of light. The advantage of such a cyclotron is the maximum achievable extracted proton current which is currently 2.2 mA. The energy and current correspond to 1.3 MW beam power which is the highest of any accelerator currently existing.
Synchrocyclotrons and isochronous cyclotrons proton therapy center 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 B, 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
isochronous cyclotron. In such a structure, the accelerating field's frequency (and the cyclotron resonance frequency) is kept constant for all energies by shaping the magnet poles so to increase magnetic field with radius. Thus, all particles get accelerated in
isochronous time intervals. Higher energy particles travel a shorter distance in each orbit than they would in a classical cyclotron, thus remaining in phase with the accelerating field. The advantage of the isochronous cyclotron is that it can deliver continuous beams of higher average intensity, which is useful for some applications. The main disadvantages are the size and cost of the large magnet needed, and the difficulty in achieving the high magnetic field values required at the outer edge of the structure. Synchrocyclotrons have not been built since the isochronous cyclotron was developed.
Synchrotrons (background ring) and Main Injector (foreground ring which is not actually circular) rings at
Fermilab. The Tevatron ring also contains Main Ring and a section of it is still used for downstream experiments. The Main Injector below (about half the diameter of the Tevatron) is for preliminary acceleration, beam cooling and storage, etc. To reach still higher energies, with relativistic mass approaching or exceeding the rest mass of the particles (for protons, billions of electron volts or
GeV), it is necessary to use a
synchrotron. This is an accelerator in which the particles are accelerated in a ring of constant radius. An immediate advantage over cyclotrons is that the magnetic field need only be present over the actual region of the particle orbits, which is much narrower than that of the ring. (The largest cyclotron built in the US had a magnet pole, whereas the diameter of synchrotrons such as the
LEP and
LHC is nearly 10 km. The aperture of the two beams of the LHC is of the order of a centimeter.) The LHC contains 16 RF cavities, 1232 superconducting dipole magnets for beam steering, and 24 quadrupoles for beam focusing. Even at this size, the LHC is limited by its ability to steer the particles without them going adrift. This limit is theorized to occur at 14 TeV. 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
RF cavity resonators used to drive the acceleration. 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
strong focusing concept. The focusing of the beam is handled independently by specialized
quadrupole magnets, while the acceleration itself is accomplished in separate RF sections, rather similar to short linear accelerators. Also, there is no necessity that cyclic machines be circular, but rather the beam pipe may have straight sections between magnets where beams may collide, be cooled, etc. This has developed into an entire separate subject, called "beam physics" or "beam optics". More complex modern synchrotrons such as the Tevatron,
LEP, and LHC may deliver the particle bunches into
storage rings of magnets with a constant magnetic field, where they can continue to orbit for long periods for experimentation or further acceleration. The highest-energy machines such as the Tevatron and LHC are actually accelerator complexes, with a cascade of specialized elements in series, including linear accelerators for initial beam creation, one or more low energy synchrotrons to reach intermediate energy, storage rings where beams can be accumulated or "cooled" (reducing the magnet aperture required and permitting tighter focusing; see
beam cooling), and a last large ring for final acceleration and experimentation.
Electron synchrotrons Circular electron accelerators fell out of favor for particle physics around the time that
SLAC's linear particle accelerator was constructed, because their synchrotron losses were considered economically prohibitive and because their beam intensity was lower than for the unpulsed linear machines. The
Cornell Electron Synchrotron, built at low cost in the late 1970s, was the first in a series of high-energy circular electron accelerators built for fundamental particle physics, the last being
LEP, built at CERN, which was used from 1989 until 2000. 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 light) as
X-rays also called synchrotron radiation, for example the
Diamond Light Source which has been 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. Synchrotron radiation allows for better imaging as researched and developed at
SLAC's SPEAR.
Fixed-field alternating gradient accelerators Fixed-Field Alternating Gradient accelerators (FFA)s, in which a magnetic field which is fixed in time, but with a radial variation to achieve
strong focusing, allows the beam to be accelerated with a high repetition rate but in a much smaller radial spread than in the cyclotron case. Isochronous FFAs, like isochronous cyclotrons, achieve continuous beam operation, but without the need for a huge dipole bending magnet covering the entire radius of the orbits. Some new developments in FFAs are covered in.
Rhodotron A Rhodotron is an industrial electron accelerator first proposed in 1987 by J. Pottier of the
French Atomic Energy Agency (CEA), 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.
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
synchrotron was the
Cosmotron at
Brookhaven National Laboratory, which accelerated
protons to about 3
GeV (1953–1968). The
Bevatron at Berkeley, completed in 1954, was specifically designed to accelerate protons to enough energy to create
antiprotons, and verify the
particle–antiparticle symmetry of nature, then only theorized. The
Alternating Gradient Synchrotron (AGS) at Brookhaven (1960–) was the first large synchrotron with alternating gradient, "
strong focusing" magnets, which greatly reduced the required aperture of the beam, and correspondingly the size and cost of the bending magnets. The
Proton Synchrotron, built at
CERN (1959–), was the first major European particle accelerator and generally similar to the AGS. The
Stanford Linear Accelerator, SLAC, became operational in 1966, accelerating electrons to 30 GeV in a 3 km long waveguide, buried in a tunnel and powered by hundreds of large
klystrons. It is still the largest linear accelerator in existence, and has been upgraded with the addition of storage rings and an electron-positron collider facility. It is also an X-ray and UV synchrotron photon source. The
Fermilab Tevatron has a ring with a beam path of . It has received several upgrades, and has functioned as a proton-antiproton collider until it was shut down due to budget cuts on September 30, 2011. The largest circular accelerator ever built was the
LEP synchrotron at CERN with a circumference 26.6 kilometers, which was an electron/
positron collider. It achieved an energy of 209 GeV before it was dismantled in 2000 so that the tunnel could be used for the
Large Hadron Collider (LHC). The LHC is a proton collider, and currently the world's largest and highest-energy accelerator, achieving 6.5 TeV energy per beam (13 TeV in total). 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
Spallation Neutron Source, incorporate superconducting
cryomodules. The
Relativistic Heavy Ion Collider, and
Large Hadron Collider also make use of
superconducting magnets and
RF cavity resonators to accelerate particles. == Targets ==