A cosmic ray muon going through CMS with the magnet at full field. The line shows the path of the muon reconstructed from information recorded in the various detectors. (Image courtesy of CERN (European Organization For Nuclear Research))

Called the Barrel Toroid because of its shape, the magnet weighs in at about 100 tons and contains eight superconducting coils. It provides a powerful magnetic field for Atlas, one of the major particle detectors being prepared to take data at the European Organization for Nuclear Researchs (CERN) Large Hadron Collider (LHC), the new particle accelerator scheduled to go live in November.

The LHC sits inside a circular underground tunnel of 27-km circumference approximately 100-m beneath Switzerland and France, near Geneva. Scientists will use the LHC to recreate the conditions just after the Big Bang, by colliding two beams of protons travelling in opposite directions at close to the speed of light.

The eight coils in the Aylsd Barrel Toroid are shaped like round-cornered rectangles, 5 m wide, 25 m long, all aligned to millimeter precision. Unlike most particle detectors, the Atlas detector does not need large quantities of metal to contain the field because the field is contained within a doughnut shape defined by the coils. This reportedly will make measurements more precise.

Atlas has the largest volume or any detector ever constructed for particle physics (28,750 m3). Among the questions scientists hope to answer with Atlas are why particles have mass, what the unknown 96% of the universe is made of, and why nature prefers matter to antimatter.

The Atlas Barrel Toroid was first cooled down over six weeks to reach −269°C . It was then powered up incrementally to eventually reack 21 kA. This is 500 A more than the level needed to produce the nominal magnetic field. The resulting stored magnetic energy of 1.1 GJ, the equivalent of about 10,000 cars travelling at 70 km/hr, has now been safely dissipated, raising the cold mass of the magnet to −218°C.