Quadrupole Magnetic Filed
To visualize the magnetic field of a quadrupole magnet.◙
Two horseshoe magnets with plastic box for iron filings, overhead projector.◙
Two large horseshoe magnets are arranged with their poles N to S and S to N, forming a "quadrupole." Quadrupole magnets are used in focusing of charged particle beams, where the beam moves through the field perpendicular to the pla ne of the magnets. A single quadrupole focuses in one direction and defocuses in the other direction, so quadrupole magnets are generally used in pairs as beam focusing magnets.
Richard L. Taylor, Focusing Electron Beams at SLAC, TPT 31, 18-26 (1993).