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4 TeV: the goal for 2012

27 March 2012
CCnew2_03_12

Running the LHC at 4 TeV per beam in 2012 was a key outcome of this year’s LHC Performance workshop in Chamonix. Announcing this in his concluding statement, Steve Myers, CERN’s Director for Accelerators and Technology, gave the main priorities for the year: delivering enough luminosity to the ATLAS and CMS experiments to allow them independently to discover or exclude the Higgs; the proton–lead-ion run; and a machine-development programme to target operation after the long technical shutdown (LS1) planned for 2013–2014. The 2012 integrated-luminosity target is to achieve more than 15 fb–1, and LHC progress will be monitored carefully with two checkpoints in the year to see if a run extension is needed to meet this target.

These conclusions derived from week-long discussions in Chamonix, which had begun with a critical review of 2011. Looking back on an excellent year for the machine and its experiments, the workshop identified possible improvements to critical systems – such as beam instrumentation and machine protection – to maximize the performance of the 2012 run.

The experiments provided their requirements for 2012, namely the need for at least 15 fb–1 either to discover the Higgs or to exclude it at 95% confidence level down to a mass of 115 GeV. Potential improvements to performance and machine availability include maximizing the time that the LHC delivers collisions to the experiments, as well as the potential of injectors to provide bunches with higher intensities and the smallest-possible beam size (translating directly into higher collision rates).

One of the big successes of 2011 was the “squeeze” – the reduction of the beam size at the interaction point – which was pushed in the latter part of the year. Further squeezing in 2012 might be possible in combination with the use of tighter collimator settings. This could give a peak luminosity of around 6 × 1033 cm–2 s–1, to be compared with a maximum of 3.6 × 1033 cm–2 s–1 in 2011. With a bunch spacing of 50 ns and a total of 1380 bunches (as in 2011), an integrated luminosity of 15 fb–1 seems to be in reach if the tighter collimator settings prove to be operationally robust and the impressive performance of the LHC’s many hardware systems continues.

While discussions took place in Chamonix, the full maintenance programme of the winter technical stop was nearing completion. The long operational periods now in place at the LHC allow only a few short technical stops between beam runs. This meant that time was tight for the much needed maintenance and upgrades during this winter stop.

When the 2011 beam run ended on 7 December, the cryogenics team emptied the magnets of helium to work on their full programme of maintaining and improving the already good level of availability. In addition, there were planned interventions to the essential technical-infrastructure systems, such as electricity, cooling and ventilation. An impressive list of maintenance included enhancements to vacuum, power converters, RF, beam instrumentation, safety, collimation, the beam dump and injection. To improve machine performance, measures were taken to mitigate the effects of radiation on equipment in and around the LHC tunnel, including the installation of additional shielding in points 1 and 5, as well as the relocation of radiation-sensitive electronics to less exposed areas.

Additional work was required around Point 5 to repair RF fingers at the connection of two beam-vacuum chambers in CMS. The repair was completed successfully and the sector was then put under vacuum. The cool-down to 1.9 K of all LHC sectors, which had been floating at about 80 K over the Christmas break, took place in February so that powering and cryogenic tests could occur before the machine restart in March. The tests included electrical qualification of the superconducting circuit, to check insulation and instrumentation integrity, followed by powering tests aimed at pushing the performance of all LHC circuits to their operational level. The tests injected current through the superconducting circuits while checking the correct behaviour of the protection mechanisms – an essential element for the safe operation of the machine. Much attention is needed to power the main dipole and quadrupole circuits at a different current level for operation at 4 TeV.

Following this impressive progress, the machine is set to run at 4 TeV. After operating at 3.5 TeV per beam for two years, the LHC is now entering another domain at a new energy level.

• For regular updates on the LHC, see the CERN Bulletin http://cern.ch/bulletin/.

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