World’s largest superconducting magnet switches on

The ATLAS Barrel Toroid consists of eight superconducting coils, each in the shape of a round-cornered rectangle, 5m wide, 25m long and weighing 100 tonnes, all aligned to millimetre precision. It will work together with other magnets in ATLAS to bend the paths of charged particles produced in collisions at the LHC, enabling important properties to be measured. 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 increases the precision of the measurements it can make.

At 46m long, 25m wide and 25m high, ATLAS is the largest volume detector ever constructed for particle physics. Among the questions ATLAS will focus on are why particles have mass, what the unknown 96% of the Universe is made of, and why Nature prefers matter to antimatter. Some 1800 scientists from 165 universities and laboratories representing 35 countries are building the ATLAS detector and preparing to take data next year.

The ATLAS Barrel Toroid was first cooled down over a six-week period in July-August to reach –269oC . It was then powered up step-by-step to higher and higher currents, reaching 21 thousand amps for the first time during the night of 9 November. This is 500 amps above the current needed to produce the nominal magnetic field. Afterwards, the current was switched off and the stored magnetic energy of 1.1 GigaJoules, the equivalent of about 10 000 cars travelling at 70km/h, has now been safely dissipated, raising the cold mass of the magnet to –218oC.

“We can now say that the ATLAS Barrel Toroid is ready for physics,” said Herman ten Kate, ATLAS magnet system project leader.

The ATLAS Barrel Toroid is financed by the ATLAS Collaboration and has been built through close collaboration between the French CEA-DAPNIA laboratory (originator of the magnet’s design) , Italy’s INFN-LASA laboratory and CERN. Components have been contributed in-kind by national funding agencies from industries in France (CEA), Italy, Germany (BMBF), Spain, Sweden, Switzerland, Russia, and the Joint Institute for Nuclear Research (JINR), an international organization based near Moscow. The final integration and test of the coils at CERN, as well as assembly of the toroid in the ATLAS underground cavern, was done with JINR providing most of the manpower and heavy tooling.