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Concept
Quadrupôle magnétique
Résumé
Quadrupole magnets, abbreviated as Q-magnets, consist of groups of four magnets laid out so that in the planar multipole expansion of 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 field whose magnitude grows rapidly with the radial distance from its longitudinal axis. This is used in particle beam focusing.
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 will 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 Halbach cylinder.
In some designs of quadrupoles using electromagnets, there are four steel pole tips: two opposing magnetic north poles and two opposing magnetic south poles. The steel is magnetized by a large electric current in the coils of tubing wrapped around the poles. Another design is a Helmholtz coil layout but with the current in one of the coils reversed.
Strong focusing
At the particle speeds reached in high energy particle accelerators, the magnetic force term is larger than the electric term in the Lorentz force:
and thus magnetic deflection is more effective than electrostatic deflection. Therefore a 'lattice' of electromagnets is used 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. The image on the right shows an example of a quadrupole focusing in the vertical direction for a positively charged particle going into the image plane (forces above and below the center point towards the center) while defocusing in the horizontal direction (forces left and right of the center point away from the center).
Source officielle
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Concepts associés (15)
Accélérateur de particules
Un accélérateur de particules est un instrument qui utilise des champs électriques ou magnétiques pour amener des particules chargées électriquement à des vitesses élevées. En d'autres termes, il communique de l'énergie aux particules. On en distingue deux grandes catégories : les accélérateurs linéaires et les accélérateurs circulaires. En 2004, il y avait plus de dans le monde. Une centaine seulement sont de très grosses installations, nationales ou supranationales.
Quadrupôle magnétique
Un quadrupôle magnétique est une source de champ magnétique. La façon la plus simple de représenter un quadrupôle est l'addition de deux aimants droits parallèles, en sens inverse. Les composantes principales de leur champ magnétique s'annulent (l'ensemble a donc un dipôle magnétique nul), seules subsistent les composantes de l'ordre supérieur. Un quadrupôle magnétique possède deux pôles nord et deux pôles sud, placés aux faces alternées d'un carré.
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. The simplest example of a dipole magnet is a bar magnet. In particle accelerators, a dipole magnet is the electromagnet used to create a homogeneous magnetic field over some distance.