Commissioning of the LHC came to an abrupt halt at midday on 19 September, when an incident occurred in sector 3-4 that resulted in a large helium leak in the LHC tunnel. The time necessary for the investigation and repairs precludes a restart before CERN’s obligatory winter-maintenance period, pushing the date for restart of the accelerator complex to early spring 2009.
The incident occurred only nine days after the successful “first-beam” day (LHC first beam: a day to remember). During a period with no beam, owing to the replacement of a faulty transformer, the commissioning team was completing work to allow the machine to run at 5 TeV per beam, originally planned for later this year. All but one of the eight sectors had already been commissioned to 5.5 TeV before start up, and it was while bringing the magnets in sector 3-4 up to the appropriate field strengths that the incident happened. Indeed, it was the last circuit to be tested, and it had reached a current equivalent to just higher than 5 TeV.
Preliminary investigations indicate that the most likely cause of the problem was a faulty electrical connection between two magnets, which probably melted at high current, leading to a rupture of the helium vessel and the release of high-pressure gas into the cyrostat. The gas then discharged into the tunnel through the pressure-relief valves designed for this purpose. At the same time, the quench-protection circuits on some 100 magnets fired, all working perfectly to protect the magnets as foreseen. A sector consists of 154 main superconducting dipoles plus straight sections with 40 main quadrupoles and various other magnets. CERN’s strict safety regulations ensured that at no time was there any risk to people.
The LHC, like other major particle accelerators, has been built at the cutting edge of technology but with unprecedented complexity, owing to its unique two-in-one superconducting magnet system. No fewer than 123,000 interconnections were needed for the 27 km ring, including 65,000 electrical connections with superconducting cables. All the other circuits had passed their tests to 9000 A with flying colours.
A full investigation of the incident is underway, but the whole sector must be warmed up to room temperature and the magnets involved opened up for inspection before this can be completed. Only at this stage will the extent of collateral damage caused by the sudden release of helium be fully known. The warm up is expected to be completed towards the end of October.