Editing Quadrupole (section)

=== Magnetic quadrupole ===
{{More citations needed|section|date=December 2024}}[[File:VFPt quadrupole coils 1.svg|left|thumb|Coils producing a quadrupole field]]

[[File:Magnetic quadrupole moment.svg|right|thumb|Schematic [[quadrupole magnet]] ("''four-pole''")]]

{{See also|Quadrupole magnet}}

All known magnetic sources give dipole fields. However, it is possible to make a magnetic quadrupole by placing four identical bar magnets perpendicular to each other such that the north pole of one is next to the south of the other. Such a configuration cancels the dipole moment and gives a quadrupole moment, and its field will decrease at large distances faster than that of a dipole.

An example of a magnetic quadrupole, involving permanent magnets, is depicted on the right. [[Electromagnet]]s of similar conceptual design (called [[quadrupole magnet]]s) are commonly used to focus [[Charged particle beam|beams of charged particles]] in [[particle accelerator]]s and beam transport lines, a method known as [[strong focusing]].  As a given quadrupole configuration deflects charged particles in one direction and focuses them in another, by using alternating quadrupole magnets a particle beam can be made to focus in the direction of travel.<ref>{{Cite book |last=Dattoli |first=Giuseppe |url=https://iopscience.iop.org/book/mono/978-0-7503-1239-4 |title=Charged Beam Dynamics, Particle Accelerators and Free Electron Lasers |last2=Doria |first2=Andrea |last3=Sabia |first3=Elio |last4=Artioli |first4=Marcello |date=2017-09-01 |publisher=IOP Publishing |isbn=978-0-7503-1239-4 |language=en}}</ref> 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]] that flows in the coils of tubing wrapped around the poles.

A changing magnetic quadrupole moment produces [[electromagnetic radiation]].