Eight-month delay for LHC

[n2doc]

Broken magnets put particle collider in limbo.

Geoff Brumfiel

Details of last month's accident at the Large Hadron Collider (LHC), the world's premier particle accelerator, are emerging — and confirm that the machine will not restart before late May or early June 2009.

Officials at CERN, Europe's particle-physics laboratory near Geneva, Switzerland, say that the time is needed to overhaul a sector of the 27-kilometre-long machine, after an electrical failure on 19 September caused some 6 tonnes of ultra-cold liquid helium to leak into its tunnel. A preliminary report issued on 16 October says that as many as 29 of the nearly 10,000 magnets used to guide the accelerator's proton beam will need to be replaced. Further magnets may need to be removed and inspected, and modifications must also be made to prevent future accidents. "It's a serious incident," says James Gillies, a spokesman for the laboratory.

Still, CERN is confident it has the resources to make the repairs. No more than 24 dipole magnets and 5 quadrupole magnets were damaged; CERN has 30 dipole magnets — each weighing 35 tonnes — in reserve, as well as sufficient quadrupoles, says Gillies. Replacement magnets are already being tested in a facility above the buried accelerator tunnel. Nevertheless, Gillies says that the damage will take all of CERN's winter shutdown period to repair. Not including labour and the spares, the work will cost an estimated 100,000 Swiss francs (US$90,000), he says.

The LHC's superconducting magnets generate enormous fields by circulating huge electrical currents with virtually no resistance. To work correctly, they must be immersed in liquid helium and kept at a temperature of just 1.9 kelvin. During the 19 September test, the accident report says, a weld in a superconducting wire connecting two magnets heated above its operating temperature. That in effect turned the wire into a resistor — causing a massive 8.7 kiloamps of power to arc through the liquid helium and puncture into the surrounding vacuum vessel.

In just milliseconds, the arc managed to vaporize a "significant fraction" of the nearly metre-long connection between the two magnets, says Jim Strait, an accelerator physicist at the Fermi National Accelerator Laboratory in Batavia, Illinois, who has been consulting on the accident investigation. The liquid helium flowed through the hole and into an insulating region of vacuum, which was meant to work as a thermos to keep the magnets cool. Relief valves designed to allow the helium to escape were overwhelmed and, within seconds, the pressure in the machine became powerful enough to wrench magnets off their concrete supports.

more:
http://www.nature.com/news/2008/081017/full/4551015a.html

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