Quadrupole magnet

A magnet whose transverse field varies linearly with displacement. Its Lorentz force focuses a charged-particle beam in one transverse plane and defocuses it in the other. Oppositely oriented elements provide alternating-gradient focusing.

At fixed gradient and magnet length, the signed thin-lens focal length is proportional to particle momentum. For total energy error $\delta =(E-E_0)/E_0$, differentiating $p^2{c}^2=E^2-m^2{c}^4$ gives $d\ln p/d\ln E=1/{\beta }^2$. The ultrarelativistic result is $f=f_0(1+\delta )$ to first order. If energy means kinetic energy instead, the coefficient is $(K+m{c}^2)/(K+2m{c}^2)$.

An element of the strong focusing system near a collider interaction point. It helps form a small transverse beam waist there, matched to the surrounding periodic lattice. Small collision-point beta functions generally imply large envelopes and strong chromatic effects in nearby magnets.