LHC dipole magnets start to roll off the production line

Fifteen metres long, thirty-tonnes in weight, and using several kilometres of superconducting cable, the magnet that has just arrived in hall 181 is a true colossus. It is the first pre-series dipole that will begin service in 2005 in the future Large Hadron Collider, LHC. Delivered by the French Alstom-Jeumont Industrie consortium, it is the first of 1248 magnets that will be manufactured over the coming five years. Needless to say, lavish attention has been devoted to this magnet by the engineers and technicians who accompanied it to CERN from Belfort in north east France.
The task of the dipole magnets will be to steer the LHC's proton beams on a circular trajectory around the LHC's 27 kilometre circumference. A magnetic field of 8.33 Tesla is required to guide the protons, accelerated to an energy of 7 TeV, around the ring. This is an extraordinarily high field intensity, 100 000 times higher than the earth's magnetic field. Only superconductors are able to meet the challenge. The active part of the magnet therefore consists of a coil of superconducting cable. "Each wire consists of 9 000 niobium-titanium filaments ten times finer than a hair," explains Jos Vlogaert, who is responsible for LHC dipole magnets at CERN. The proton beams circulate inside these coils. To achieve superconductivity, the coils are cooled to 1.9 Kelvin, about ­ 271°C. This extremely low temperature is achieved in a superfluid helium bath.
This architecture is the result of three successive generations of prototypes that have been built and tested since the beginning of the 1990s. "The last prototype has delivered exceptional results," says Vlogaert. But despite all these tests, it was no mean feat to move from the prototype to the pre-series magnet. As Vlogaert points out, "The magnet must be able to operate for 20 years". In short, no leak or faulty connection can be tolerated.
Three manufacturers are involved in this first phase: the Alstom-Jeumont Industrie consortium in France, Noell in Germany, and Ansaldo in Italy. For the moment these manufacturers have each been entrusted with an initial order for thirty magnets, which should allow them to optimise their know-how and estimate the cost of series production.

The manufacture of these technological marvels is the result of technology transfer between CERN and its suppliers. Thus Alstom-Jeumont Industrie has produced only the active part of this first magnet. Its casing will be built at CERN and will consist of two half-cylinders which have to be assembled using the huge press in Hall 181. A special welding technique is used to assemble the casing, known as surface tension transfer, which involves adapting the welding current to the melting conditions of the metal in real time. "We introduced it for our last prototype", underlines Vlogaert. A further technological challenge is that the immense cylinder has to be slightly bent to follow the curve of the tunnel. The maximum divergence between the ends and the centre must be 9 millimetres to an accuracy of 1 millimetre over a distance of 15 metres! This part of the work must be completed before the end of the year so that the first magnet can undergo tests, which are due to start next spring.
The rest of the series will be entirely built by the manufacturers who will have to learn these highly sophisticated production techniques at CERN. The Laboratory has invested in three presses similar to its own, which are being supplied to the dipole manufacturers, and is also financing the coil manufacturing and collaring lines.
After this pre-series production of 30 magnets each, the manufacturers will be invited to reply to a call for tenders for orders for the series production of the remaining 1158 dipole magnets.