In recent years, the alloy of choice for accelerator magnets has been niobium-titanium. Superconducting magnets made from this alloy operate in all of today's most powerful machines and will be used in the Large Hadron Collider (LHC). The LHC magnets are expected to operate at a field strength of 8.36 Tesla which is approaching the 10 Tesla mark that is considered to be the upper limit of niobium-titanium accelerator magnets. Dipole magnets are used to bend the path of accelerating particle beams and to keep them on course. The stronger the magentic field, the tighter the arc of the beam. With stronger dipole magnets, an accelerator can push particles to much higher relativistic energies around the same-sized circular beam path. The use of strong field superconducting electromagnets has always posed technical challenge because superconductivity weakens or vanishes in the presence of a strong magnetic field. Electromagnets cannot attain a dipole field strength much above 2 Tesla and so development has continued in the search for new and better superconducting alloys.
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 experimental points on the ring. The beams "degrade" so the machine will have to be emptied and refilled after several hours. 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.
LHC magnet coils will be 14 metres or more in length, and the inner diameter will be 56 mm. Coil winding must not allow movements as the field changes as 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. More energy can be 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 use extremely advanced computer programmes to analyse quench induced stresses. 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.
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.
The cryogenic technology chosen for the LHC uses superfluid helium, which has unusually efficient heat transfer properties, allowing kilowatts of refrigeration to be transported over more than a kilometre with a temperature drop of less than 0.1 K. LHC superconducting magnets will sit in a 1.9 K bath of superfluid helium at atmospheric pressure. This bath will be cooled by low pressure liquid helium flowing in heat exchanger tubes threaded along the string of magnets. The LHC cryogenic system is very large as well as very cold. Refrigeration power equivalent to over 140 kW at 4.5 K is distributed around the 27 km ring. To save costs, the four existing LEPII 12 kW, 4.5 K cryoplants will be reused. Their cooling power will be increased by 50% and 1.9 K stages will be added. In all, LHC cryogenics will need 40,000 leak-tight pipe junctions. 12 million litres of liquid nitrogen will be vaporised during the initial cooldown of 31,000 tons of material. The total inventory of liquid helium will be 700,000 litres.
The LHC is not only of interest to scientists. 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.
The world record for field strength in a dipole magnet was shattered in April 1997, by researchers at the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab). A one-meter long superconducting electromagnet, featuring coils wound out of 14 miles of niobium-tin wire, reached a field strength as high as 13.5 Tesla. The previous high of 11.03 Tesla was set by a Dutch group in 1995.
The field strength achieved by Scanlan and his group is about a quarter of a million times stronger than the magnetic field of Earth. It is about triple the strength of the superconducting dipole magnets at the Tevatron, the highest energy particle accelerator in the world.