Quadrupole magnets consist of groups of four
magnets laid out so that in the multipole expansionof the field the dipole terms cancel and where the lowest significant terms in the field equations are quadrupole. Quadrupole magnets are useful as they create a magnetic fieldwhose magnitude grows rapidly with the radial distance from its longitudinal axis. This is used in particle beamfocusing.
The simplest magnetic quadrupole is two identical bar magnets parallel to each other such that the north pole of one is next to the south of the other and vice versa. Such a configuration would have no dipole moment, and its field will decrease at large distances faster than that of a dipole. A stronger version with very little external field involves using a k=3
In some designs of quadrupoles using
electromagnets there are four steelpole tips: two opposing magnetic north poles and two opposing magnetic south poles. The steel is magnetized by a large electric currentthat flows in the coils of tubing wrapped around the poles. Another design is a Helmholtz coillayout but with the current in one of the coils reversed. [ [http://hyperphysics.phy-astr.gsu.edu/Hbase/magnetic/magquad.html Quadrupole Magnetic Field ] ]
Quadrupoles in particle accelerators
At the speeds reached in high energy
particle accelerators, magnetic deflection is more powerful than electrostatic, and use of the magnetic term of the Lorentz force:
is enabled with various magnets that make up 'the lattice' required to bend, steer and focus a charged
particle beam. The quadrupoles in the lattice are of two types: 'F quadrupoles' (which are horizontally focusing but vertically defocusing) and 'D quadrupoles' (which are vertically focusing but horizontally defocusing). This situation is due to the laws of electromagnetism(the Maxwell equations) which show that it is impossible for a quadrupole to focus in both planes at the same time.
If an F quadrupole and a D quadrupole are placed immediately next to each other, their fields completely cancel out (in accordance with
Earnshaw's theorem). But if there is a space between them (and the length of this has been correctly chosen), the overall effect is focusing in both horizontal and vertical planes. A lattice can then be built up enabling the transport of the beam over long distances - for example round an entire ring. A common lattice is a FODO lattice consisting of a basis of a Focusing quadrupole, 'nothing' (often a bending magnet), a Defocusing quadrupole and another length of 'nothing'.
Charged particle beam
Wikimedia Foundation. 2010.
Look at other dictionaries:
Magnet — This article is about objects and devices that produce magnetic fields. For a description of magnetic materials, see magnetism. For other uses, see Magnet (disambiguation) … Wikipedia
quadrupole — noun a) a distribution of either electric charge or magnetization equivalent to two dipoles that point in opposite directions b) a magnet with two north poles and two south poles, used to focus a beam of particles … Wiktionary
Sextupole magnet — Sextupole magnets consist of groups of six magnets set out in an arrangement of alternating north and south magnetic poles arranged around an axis. They are used in particle beam focusing in particle accelerators.cite web|url=http://xfelinfo.desy … Wikipedia
Dipole magnet — from the Advanced Photon Source … Wikipedia
Storage ring — The 216 m circumference storage ring dominates this image of the interior of the Australian Synchrotron facility. In the middle of the storage ring is the booster ring and linac A storage ring is a type of circular particle accelerator in… … Wikipedia
Magnetic field — This article is about a scientific description of the magnetic influence of an electric current or magnetic material. For the physics of magnetic materials, see magnetism. For information about objects that create magnetic fields, see magnet. For … Wikipedia
Large Hadron Collider — LHC redirects here. For other uses, see LHC (disambiguation). Coordinates: 46°14′N 06°03′E / 46.233°N 6.05°E / 46.233; 6.05 … Wikipedia
Strong focusing — In accelerator physics strong focusing or alternating gradient focusing is the principle that the net effect on a particle beam of charged particles passing through alternating field gradients is to make the beam converge. By contrast Weak… … Wikipedia
Anneau de stockage — L anneau de stockage de 216 m. de circonférence domine cette vue de l intérieur des installations du synchrotron australien. L anneau d accélération et le linac sont visibles au milieu de l anneau de stockage. Un anneau de stockage est un… … Wikipédia en Français
Swiss Light Source — The Swiss Light Source is a synchrotron located at the Paul Scherrer Institute in Switzerland for producing synchrotron radiation of highest brightness. Planning started in 1991, project was approved in 1997, and first light from the storage ring … Wikipedia