Quadrupole Magnet

These magnets are also called "Focusing," "Defocusing," or "Low Beta" magnets. This style of magnet has two North poles and two South poles. The best explanation of their function is an optical one. Picture a magnifying glass; it has two convex surfaces that gather divergent rays of light together and focuses them to a spot—that's a focusing quadrupole. A defocusing lens does just the opposite; it spreads rays of light apart—that's a defocusing quadrupole. Low Beta Magnets are what the Tevatron uses to collide protons and antiprotons at the CDF and D0 experiments.