Closing the gap: descent of the last LHC magnet

The last magnet for the LHC is lowered down the access shaft (Image: CERN)

Geneva, 26 April 2007. A ceremony was held at CERN1 today to mark the end of a crucial phase of installation of the Large Hadron Collider (LHC). A large dipole magnet was symbolically lowered into the tunnel at 12:00. This completes the basic installation of the more than 1700 magnets that make up the collider, which measures 27 km in circumference and is scheduled to be commissioned at the end of 2007.

The superconducting dipole magnets are the most complex components of the LHC machine. Their superconducting coils allow them to convey extremely high electric currents without any loss of energy. This enables them to produce the high magnetic fields necessary to force the trajectory of protons to follow a 27 km circular path at nearly the speed of light. The collisions between the protons will reach energies of 14 teraelectronvolts (TeV) - 70 times higher than those of the former LEP collider for which the 27 km tunnel was originally built - making the LHC the world’s most powerful accelerator. If the LHC had been made of conventional magnets, it would have needed to be 120 km long to reach the same energies, and its electricity consumption would have been phenomenal.

Like the hundreds of magnets that came before it, the final magnet was lowered 50 metres below the Earth's surface through a custom-built shaft with an oval cross-section. It was then conveyed via a transfer tunnel to the LHC tunnel itself, which lies between 50 and 150 metres underground; once below ground, specially designed transport vehicles delivered the magnet to its final destination at 3 km an hour. The narrowness of the tunnel complicated these handling operations, making it impossible, for example, for two loads to pass each other.
“More than 35 000 tonnes of material has been safely lowered underground, transported up to 15 km inside the tunnel and positioned with an accuracy of a tenth of a millimetre,” said LHC project leader Lyn Evans. “It is a fantastic achievement.”

Once in position, the magnets are connected to the cryogenic system to form a large string operating in superfluid helium, which will maintain the accelerator at a temperature just two degrees above absolute zero (-271°C). The cryogenic capabilities of the superconducting magnets were tested at CERN between 2004 and early this year, with the last dipole magnet passing its cryogenics testing on 1 March.

“Installation of these large components in the LHC tunnel has been successfully achieved on schedule, thanks to the competence and motivation of the large team in charge, working day and, mostly, night and weekends,” said CERN Director General Robert Aymar. “They deserve our sincere congratulations.”

The manufacture of these superconducting magnets was a huge technical and industrial challenge both for CERN and for European industry. Over 1000 tonnes of niobium-titanium superconducting cable had to be produced. Around a hundred companies in Europe manufactured the magnet components, and three companies, Babcock Noell Nuclear in Germany, Alstom in France, and Ansaldo in Italy, were responsible for their assembly. At the height of production, the three industrial sites were able to manufacture between nine and ten magnets a week.



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