bending and focusing magnets
/ DY-pohl, KWOD-roo-pohl /
A beam of charged particles will not steer itself. Two jobs have to be done over and over: bend the beam so it stays on the ring instead of flying off in a straight line, and squeeze the beam so it stays a thin, dense thread instead of spreading out and fading away. These two jobs are done by two different kinds of magnet, and a big accelerator is, in large part, a long chain of them.
The bending magnet is a dipole — a magnet with a simple north-and-south field that pushes the whole beam sideways, curving its path. String enough dipoles around a circle and the beam follows a closed loop. The focusing magnet is a quadrupole — four poles arranged so the field is zero at the center and grows stronger toward the edges, acting like a lens that pulls stray particles back toward the middle. A single quadrupole squeezes the beam in one direction while letting it spread in the other (think of squeezing a balloon: pinch it one way and it bulges the other), so they are used in alternating pairs that net out to focusing in both directions, a scheme called strong focusing. Dipoles set the path; quadrupoles keep the beam tight.
These magnets are why accelerators have their characteristic look — kilometre after kilometre of magnet, the dipoles slightly bending and the quadrupoles slightly squeezing. The strength of the bending dipoles directly limits the machine: the harder you can bend the beam, the higher the energy you can hold on a ring of a given size. That is exactly why the highest-energy machines use the most powerful magnets available, which in practice means superconducting ones.
The LHC ring carries 1,232 superconducting dipole magnets to bend its proton beams and hundreds of quadrupoles to focus them; without the dipoles the protons would simply shoot off in a straight line out of the tunnel.
Dipoles bend the path; quadrupoles act as lenses that keep the beam thin.
A single quadrupole cannot focus in both directions at once — it focuses one way and defocuses the other — so accelerators alternate them in a pattern that nets out to focusing overall; one quadrupole alone would make the beam worse, not better.