Geneva, 25 August 2005. Important milestones have been successfully reached today in the installation of the two largest magnets ever built for experiments at CERN1. At one side of the 27 km ring of the future Large Hadron Collider (LHC), the 230 tonne solenoid magnet for the CMS experiment has been rotated through 90° prior to insertion into its cryostat – the jacket that will cool the magnet to 4.2 K (-269° C). At the opposite side of the ring, in the underground cavern where the ATLAS detector is being constructed, the last of eight 25-m long toroid magnet coils has been put into place, to complete a huge magnetic “barrel” that forms a major part of the detector.
The LHC, currently under construction, will accelerate two beams of particles in opposite directions to close to the speed of light, reaching an energy of 7 teraelectronvolts (TeV). The beams will collide head on in the middle of four big particle detectors – ALICE, ATLAS, CMS, and LHCb – located at different points around the LHC ring. The collisions will be at the highest energies ever observed in laboratory conditions and physicists are eager to see what they will reveal. ATLAS and CMS are the largest of the detectors, all of which will explore new territory in matter and energy and take us to a deeper understanding of the Universe.
The CMS solenoid, which represents the "S" in CMS (Compact Muon Solenoid), is a cylindrical coil of superconductor: at low temperature it conducts electricity without resistance, allowing high magnetic fields to be achieved. When running, it will generate a magnetic field of 4 tesla and have a stored energy of 2.6 gigajoules – enough to melt 18 tonnes of gold. This is a world record for energy stored in a magnet. With an inside diameter of 6.3 m and a length of 12.5 m, the solenoid was built in five sections, each 2.5 m long and weighing 45 tonnes. This allowed transportation from the fabrication site in Italy to CERN, where the sections have been assembled into the final solenoid. Practical considerations made assembly easier with the cylinder vertical. In the final detector it will be horizontal, surrounding the pipe where the particle beams in the LHC will collide head on. Today’s delicate operation involved rotating the complete solenoid from vertical to horizontal – a manoeuvre that took about one hour in all after several years of detailed studies and preparation.
Weighing 7000 tonnes, the ATLAS detector is not as heavy as CMS, which weighs 12 500 tonnes, but it is much bigger: at 46 m long, 25 m wide and 25 m high it will be the largest-volume detector ever constructed for high-energy physics. The relatively light weight of ATLAS is due to the design of its superconducting magnet system, which is based largely on air-core “barrel” and “endcap” toroid magnets that provide the magnetic field out to the edges of the complete detector. The barrel toroid magnet consists of eight superconducting coils, each in the shape of a round-cornered rectangle, 5 m wide, 25 m long and weighing 100 tonnes. The components - the superconductors, windings, coil casings, thermal shields, supports and vacuum vessels, from various companies and institute workshops around the world - have been assembled into their cryostats and tested at CERN over the past four years. In October 2004, the first of the barrel coils was lowered down the 18-m diameter shaft into the cavern where the ATLAS detector is being assembled, 100 m below ground. Now the eighth and final barrel toroid coil has been installed in its final position, at the top of the main structure of ATLAS. The barrel will be closed by two “end cap” toroid magnets, which are due to be installed in 2006.
Superconducting magnets also form a major part of the LHC itself. The collider consists predominantly of about 1800 superconducting magnet systems, which will guide the two particle beams in opposite directions around the 27-km ring. The first of 1232 dipole magnets for steering the particles was lowered into the tunnel earlier this year on 7 March. Now more than 80 magnet systems have been installed and the machine continues to be on course to start up in 2007.