The Chamonix workshop, held on 25–29 January, once again proved its worth as a place where all of the stakeholders in the LHC can come together, take difficult decisions and reach a consensus on important issues. This time the most important decision taken was to run the LHC for 18 to 24 months at a collision energy of 7 TeV (3.5 TeV per beam) before a long shutdown, which will allow time for all of the work necessary for the machine to reach the design collision energy of 14 TeV. As beam returns in the LHC this February it marks the start of the longest phase of accelerator operation in CERN’s history, running into summer or autumn 2011.
What is the reasoning behind this decision? First, the LHC a cryogenic facility, so each run is accompanied by lengthy cool-down and warm-up phases. Second, there is still essential work to be done to prepare the LHC for running at energies significantly higher than the collision energy of 7 TeV chosen for the first physics run. These facts led to a simple choice: run for a few months now and programme successive short shutdowns to step up in energy; or run for a long time now and schedule a single long shutdown before allowing a total energy of 14 TeV (7 TeV per beam). A long run gives the machine teams time to prepare carefully for the work that will be needed before running at 14 TeV. For the experiments, 18 to 24 months will bring enough data across all of the potential discovery areas.
Before the 2009 running period began, all of the necessary preparations to run the LHC at the collision energy of 1.18 TeV per beam had been carried out. The goal of the technical stop, scheduled to end in mid-February, was to prepare the machine for running at 3.5 TeV per beam, which requires a current of 6 kA in the LHC magnets.
The main work during the stop was on the new quench-protection system (nQPS), which is designed to improve the electrical reliability of the connection between the instrumentation feedthrough systems on the magnets and the nQPS equipment. There are around 500 of these connectors for each of the eight sectors in the LHC. An intensive effort ensured that this work was undertaken and completed in the first three weeks of January, so that the hardware-commissioning teams could proceed with testing the magnets up to 6 kA.
Several other teams took advantage of the stop to carry out other technical verifications and efficiency tests, for example, on some vacuum pumping units, the kicker system, the oxygen-deficiency hazard detectors, and on some ventilation components. At the same time as this work on the LHC, repairs took place on the water-cooling system of the CMS experiment.
All work, both in the LHC and in the CMS experiment, was scheduled to be completed by mid-February. The machine operations team will then begin to re-commission the LHC at 450 GeV per beam, building on the experience gained after the restart last year and completing investigations of machine parameters at this energy (CERN Courier January/February 2010 p24). The team will then prepare for the first ramps to 3.5 TeV per beam. Collisions at 3.5 TeV will follow, but only after the operators have established the appropriate running conditions.
