superconducting magnet

An electromagnet is just a coil of wire carrying a current — the more current, the stronger the field. The catch with ordinary wire is that it resists the current, heating up like a toaster element, so there is a practical limit to how much current you can push through before the wire melts or the power bill becomes absurd. A superconducting magnet sidesteps this entirely by using a special material that, when chilled to extreme cold, carries electric current with literally zero resistance.

Zero resistance means no heating and no energy lost to keep the current flowing, so you can drive enormous currents through the coil and produce magnetic fields far stronger than any ordinary magnet could sustain. The price is the cold: the superconducting cables in a big accelerator must be kept at a few degrees above absolute zero, colder than outer space, using liquid helium. There is also a dramatic failure mode called a quench — if a tiny patch of the cable warms up and stops superconducting, the stored magnetic energy can dump into that spot as heat very fast, so the magnets need careful protection systems.

Superconducting magnets are what make today's highest-energy machines possible. Stronger bending magnets let a ring of a given size hold a higher-energy beam, so the leap to superconducting dipoles was a direct leap in energy reach. The LHC runs on them, the Tevatron pioneered them, and any proposed bigger collider depends on pushing magnet technology even further. The same superconducting magnet technology, incidentally, is what powers the MRI scanners in hospitals.