After more than a year of repairs, the world's largest atom smasher will finally start blasting particles together this winter--but at only half of its maximum energy, officials at the European particle physics laboratory, CERN, near Geneva, Switzerland, announced today. That energy limit will prevent overloading of faulty electrical connections in the 27-kilometer-long, $5.5 billion Large Hadron Collider (LHC). Researchers at CERN are still finishing fixes to the ring-shaped subterranean accelerator, which suffered a catastrophic malfunction last fall only 9 days after it first circulated particles and before it produced any collisions.
CERN officials had considered further delaying the restart to fix the connections, so the news that the LHC will power up in mid-November after all came as a relief to experimenters working with the four enormous particle detectors that the LHC will feed. The LHC is designed to smash particles at energies seven times higher than have been achieved before, and physicists hope such collisions will produce exotic new particles or even open new dimensions of space. Starting at an energy only half that high still raises the possibility of a big discovery, says Thomas LeCompte of Argonne National Laboratory in Illinois, who works on the ATLAS particle detector. "It's very important to get started, and we're very excited that the machine is going to be running at three-and-a-half times higher energy than [ever reached] before," he says.
Currently, the Tevatron Collider at Fermi National Accelerator Laboratory in Batavia, Illinois, smashes particles with the most energy, about 2 teraelectron-volts (TeV). The LHC is designed to smash protons into protons at 14 TeV. But lab officials had previously set 10 TeV as the limit for this year, and they have now pegged the starting point at 7 TeV.
The energy limit should help ensure that the LHC does not suffer another failure like the one that occurred on 19 September, when the connection between two of its superconducting magnets melted. Since then, LHC researchers have found and repaired two more faulty connections of the same type.
The limit is needed to protect connections of a different type, however. In the September accident, the solder joint between two pieces of superconducting wire melted as 9000 amps coursed through it. The concern now centers on the nonsuperconducting sleeve called a copper stabilizer that surrounds each superconductor-to-superconductor splice (see diagram). The pieces in those stabilizers must be soldered to the copper cladding of the superconducting lines, and since May, CERN researchers have found weak copper-to-copper connections in about 80 of the machine's roughly 10,000 stabilizers.
Such a weak link could pose a problem if the superconducting wire inside were to warm up enough to lose its ability to carry current without resistance--a not-uncommon event in such an accelerator. If that were to happen within a faulty copper stabilizer, then current would continue to flow through the superconducting splice, which might then melt. The energy limit should keep the current low enough to prevent such a failure, says Stephen Myers, director of accelerators and technology at CERN. Running at 7 TeV, "we have a safety factor of 2 or 2.5," he says. "It's a very conservative approach."
As researchers gain experience with the machine, they may try to increase the energy next year, Myers says. "I'm still confident that we'll be able to get up to 9 or 10 TeV" later in the year, he says. In the meantime, LHC scientists are discussing how they might fix the faulty connection when the machine shuts down for maintenance in the winter of 2010.