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The LHC will consist of two "colliding" synchrotrons installed in the 27 km LEP tunnel. They will be filled with protons delivered from the SPS and its pre-accelerators at 0.45 TeV. Two superconducting magnetic channels will accelerate the protons to 7-on-7 TeV, after which the beams will counter-rotate for several hours, colliding at the experiments, until they become so degraded that the machine will have to emptied and refilled. The magnetic channels will be housed in the same yoke and cryostat, a unique configuration that not only saves space but also gives a 25 % cost saving over separate rings. High energy LHC beams need high magnetic bending fields, because the machine radius was not a parameter which could have been increased to provide gentle curves. To bend 7 TeV protons around the ring, the LHC dipoles must be able to produce fields of 8.36 Tesla, over five times those used a few years ago at the SPS proton-antiproton collider, and almost 100,000 times the earth's magnetic field. Superconductivity makes this possible. This is the ability of certain materials, usually at very low temperatures, to conduct electric current without resistance and power losses, and therefore produce high magnetic fields. For comparable power consumption, the LHC can delivery 25 times the energy and 10,000 times the luminosity of the SPS collider.
LHC magnet coils are made of copper-clad niobium-titanium cables. This technology, invented in the 1960s at the Rutherford-Appleton Laboratory, UK, was first used in a superconducting accelerator at the Fermilab Tevatron in the US in 1987. The Tevatron magnets reach peak fields of 4.5 Tesla at 4.2 K. The electron-proton collider magnets at HERA, at the DESY Laboratory, Germany, go somewhat higher, to around 5.5 Tesla. To get beyond this, LHC magnets will be operated at 1.9 K above absolute zero, that is almost 300¡ C below room temperature. This unusually low limit puts new demands on cable quality and coil assembly. European industry is already delivering cables that can carry 15,000 amps at 1.9 K and withstand forces which build up to hundreds of tons per metre in the coils as the field rises.
LHC magnet coils will be long, some 14 metres or more, and narrow, the inner diameter being 56 mm. Coil winding requires great care to prevent movements as the field changes. Friction can create normally-conducting "hot-spots" which "quench" the magnet out of its cold, superconducting state. A quench in any of the 5,000 LHC superconducting magnets will disrupt machine operation for several hours. Superconducting magnets have to be "trained" to reach higher and higher quench fields, as smaller and smaller "wrinkles" are removed from the coils. Individual training periods must be short for large scale production, for example for the 1,296 LHC dipoles. This puts tremendous demands on assembly control and testing. More energy is stored in a high magnetic field than in a low one, so the onset of a quench must be handled in a timely fashion. To design effective controls systems, safety engineers are using extremely advanced computer programmes to perform coupled mechanical-magnetic-thermal analyses of stresses induced by a quench. As well as dipoles, more than 2,500 other magnets are needed to guide and collide the LHC beams, ranging from small, normally conducting bending magnets to large, superconducting focusing quadrupoles. A special development programme is under-way in European laboratories and industry to build these novel, twin-channel quadrupoles. Operating superconducting installation on the scale of the LHC will provide valuable experience which could assist in some commercial developments where reliability is of vital concern. Superconducting cables could be used for low-loss transport of large amounts of electricity over long distances. The storage capacity of large superconducting coils operating at "comfortable" temperatures could be exploited to distribute the load on electricity generators more evenly between night-time and peak hours.
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Last update: 14-JAN-99 |