The European Organization for Nuclear Research, the Geneva-based particle physics laboratory, announced the Large Hadron Collider is operating once again. Following more than a year of repairs, CERN researchers said the particle accelerator, which cost $10 billion to construct, successfully circulated beams of protons in opposite directions at the same time. The project was brought to halt soon after the first tests began in September 2008 after a fault was discovered between two superconducting bending magnets. “It’s great to see beam circulating in the LHC again,” said CERN director general Rolf Heuer. “We’ve still got some way to go before physics can begin, but with this milestone we’re well on the way.”

Recommissioning the LHC began in the summer, and successive milestones have regularly been passed since then, CERN announced. The LHC reached its operating temperature of 1.9 Kelvin, or about -271 Celsius, on October 8. Particles were injected on October 23, but not circulated. A beam was steered through three octants of the machine on November 7, and circulating beams have now been re-established.

CERN said the next important milestone would be low-energy collisions, expected in about a week from now. These will give the experimental collaborations their first collision data, enabling important calibration work to be carried out. This is significant, the organization said, since up to now, all the data they have recorded comes from cosmic rays. Ramping the beams to high energy will follow in preparation for collisions at 7 TeV (3.5 TeV per beam) next year.

“The LHC is a far better understood machine than it was a year ago,” said CERN’s Director for Accelerators, Steve Myers. “We’ve learned from our experience, and engineered the technology that allows us to move on. That’s how progress is made. It’s been a Herculean effort to get to where we are today.”

Following the incident of September 19, 2008 that brought the LHC to a standstill, testing had focused on the 10,000 high-current superconducting electrical connections like the one that led to the fault. These consist of two parts: the superconductor itself, and a copper stabilizer that carries the current in case the superconductor warms up and stops superconducting, a so-called quench.

In their normal superconducting state, there is negligible electrical resistance across these connections, but in a small number of cases abnormally high resistances have been found in the superconductor. These have since been repaired, CERN announced in August. However, there remained a number of cases where the resistance in the copper stabilizer connections is higher than it should be for running at full energy, leading to the decision to run the LHC at an energy of 3.5 TeV per beam when it started.

CERN said the LHC will run at 3.5 TeV per beam until a significant data sample has been collected and the operations team has gained experience in running the machine. Thereafter, with the benefit of that experience, the energy will be taken towards 5 TeV per beam. At the end of 2010, the LHC will be run with lead ions for the first time. After that, the LHC will shut down and work will begin on moving the machine towards 7 TeV per beam.