A storage ring is a type of circular

particle accelerator 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. Large accelerators are used for fundamental research in particle ...

in which a continuous or pulsed

particle beam A particle beam is a stream of charged or neutral particles. In particle accelerators, these particles can move with a velocity close to the speed of light. There is a difference between the creation and control of charged particle beams and ne ...

may be kept circulating typically for many hours. Storage of a particular

particle In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from ...

depends upon the

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...

momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass ...

and usually the charge of the particle to be stored. Storage rings most commonly store

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collide ...

s, or

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

s. Storage rings are most often used to store electrons that radiate

synchrotron radiation Synchrotron radiation (also known as magnetobremsstrahlung radiation) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in ...

. Over 50 facilities based on electron storage rings exist and are used for a variety of studies in chemistry and biology. Storage rings can also be used to produce polarized high-energy electron beams through the Sokolov-Ternov effect. The best-known application of storage rings is their use in

s and in particle colliders, where two counter-rotating beams of stored particles are brought into collision at discrete locations. The resulting subatomic interactions are then studied in a surrounding

particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing particles, such as those produced by nu ...

. Examples of such facilities are LHC, LEP,

PEP-II SLAC National Accelerator Laboratory, originally named the Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S. Departme ...

, KEKB,

RHIC The Relativistic Heavy Ion Collider (RHIC ) is the first and one of only two operating heavy-ion colliders, and the only spin-polarized proton collider ever built. Located at Brookhaven National Laboratory (BNL) in Upton, New York, and used by an ...

Tevatron The Tevatron was a circular particle accelerator (active until 2011) in the United States, at the Fermi National Accelerator Laboratory (also known as ''Fermilab''), east of Batavia, Illinois, and is the second highest energy particle collider ...

and

HERA In ancient Greek religion, Hera (; grc-gre, Ἥρα, Hḗrā; grc, Ἥρη, Hḗrē, label=none in Ionic and Homeric Greek) is the goddess of marriage, women and family, and the protector of women during childbirth. In Greek mythology, she ...

. A storage ring is a type of

synchrotron A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed ...

. While a conventional synchrotron serves to accelerate particles from a low to a high energy state with the aid of radio-frequency accelerating cavities, a storage ring keeps particles stored at a constant energy and radio-frequency cavities are only used to replace energy lost through synchrotron radiation and other processes. Gerard K. O'Neill proposed the use of storage rings as building blocks for a

collider A collider is a type of particle accelerator which brings two opposing particle beams together such that the particles collide. Colliders may either be ring accelerators or linear accelerators. Colliders are used as a research tool in particl ...

in 1956. A key benefit of storage rings in this context is that the storage ring can accumulate a high beam flux from an injection accelerator that achieves a much lower flux.

Important considerations for particle-beam storage

Magnets

A force must be applied to particles in such a way that they are constrained to move approximately in a circular path. This may be accomplished using either dipole electrostatic or dipole magnetic fields, but because most storage rings store relativistic charged particles it turns out that it is most practical to utilise magnetic fields produced by

dipole magnet A dipole magnet is the simplest type of magnet. It has two poles, one north and one south. Its magnetic field lines form simple closed loops which emerge from the north pole, re-enter at the south pole, then pass through the body of the magnet. ...

s. However, electrostatic accelerators have been built to store very low energy particles, and quadrupole fields may be used to store (uncharged)

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...

s; these are comparatively rare, however. Dipole magnets alone only provide what is called

weak focusing Weak focusing occurs in particle accelerators when charged particles are passing through uniform magnetic fields, causing them to move in circular paths due to the Lorentz force. Because of the circular movement, the orbits of two particles with s ...

, and a storage ring composed of only these sorts of magnetic elements results in the particles having a relatively large beam size. Interleaving dipole magnets with an appropriate arrangement of

quadrupole A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure refl ...

and sextupole magnets can give a suitable strong focusing system that can give a much smaller beam size. The FODO and Chasman-Green lattice structures are simple examples of strong focusing systems, but there are many others. Dipole and quadrupole magnets deflect different particle energies by differing amounts, a property called

chromaticity Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called ...

by analogy with physical

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...

. The spread of energies that is inherently present in any practical stored particle beam will therefore give rise to a spread of transverse and longitudinal focusing, as well as contributing to various particle beam instabilities. Sextupole magnets (and higher order magnets) are used to correct for this phenomenon, but this in turn gives rise to

nonlinear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many oth ...

motion that is one of the main problems facing designers of storage rings.

Vacuum

As the bunches will travel many millions of kilometers (considering that they will be moving at near the speed of light for many hours), any residual gas in the beam pipe will result in many, many collisions. This will have the effect of increasing the size of the bunch, and increasing the energy spread. Therefore, a better

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...

yields better beam dynamics. Also, single large-angle scattering events from either the residual gas, or from other particles in the bunch (

Touschek effect The Touschek effect describes the scattering and loss of charged particles in a storage ring. It was discovered by Bruno Touschek. It is determined by the average of the scattering rate around the ring : \frac = \frac\oint \frac(s) \, ds In fac ...

), can eject particles far enough that they are lost on the walls of the accelerator vacuum vessel. This gradual loss of particles is called beam lifetime, and means that storage rings must be periodically injected with a new complement of particles.

Particle injection and timing

Injection of particles into a storage ring may be accomplished in a number of ways, depending on the application of the storage ring. The simplest method uses one or more pulsed deflecting dipole magnets ( injection kicker magnets) to steer an incoming train of particles onto the stored beam path; the kicker magnets are turned off before the stored train returns to the injection point, thus resulting in a stored beam. This method is sometimes called single-turn injection. Multi-turn injection allows accumulation of many incoming trains of particles, for example if a large stored current is required. For particles such as protons where there is no significant beam damping, each injected pulse is placed onto a particular point in the stored beam transverse or longitudinal

phase space In dynamical system theory, a phase space is a space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. For mechanical systems, the phase space usuall ...

, taking care not to eject previously-injected trains by using a careful arrangement of beam deflection and coherent oscillations in the stored beam. If there is significant beam damping, for example radiation damping of electrons due to

, then an injected pulse may be placed on the edge of phase space and then left to damp in transverse phase space into the stored beam before injecting a further pulse. Typical damping times from synchrotron radiation are tens of milliseconds, allowing many pulses per second to be accumulated. If extraction of particles is required (for example in a chain of accelerators), then single-turn extraction may be performed analogously to injection. Resonant extraction may also be employed.

Beam dynamics

The particles must be stored for very large numbers of turns potentially larger than 10 billion. This long term stability is challenging, and one must combine the magnet design with tracking codes. and analytical tools in order to understand and optimize the long term stability. In the case of electron storage rings, radiation damping eases the stability problem by providing a non-Hamiltonian motion returning the electrons to the design orbit on the order of the thousands of turns. Together with diffusion from the fluctuations in the radiated photon energies, an equilibrium beam distribution is reached. One may look at for further details on some of these topics.

References

External links

* {{DEFAULTSORT:Storage Ring Accelerator physics Energy storage