LHC Report: perhaps the end of the beginning

The 2015 6.5 TeV proton run will end on the morning of 4 November as we approach the close of an interesting but somewhat challenging year. Following machine development time and a technical stop, the LHC will restart operation with a proton-proton configuration at 2.51 TeV in the middle of November. Data from this special run will be used by the experiments as a reference point for the proton-lead and lead-lead collisions.  

The rate of UFOs per hour has dropped since the first high-intensity runs. Here it is seen after the first scrubbing run (SR1), through the second technical stop (TS2) until now (right end of the abscissae).

As of the end of October, the LHC is delivering luminosity in the order of 4.8x10³³ cm^-2s^-1 to ATLAS and CMS, 3x10³² cm^-2s^-1 to LHCb and 5x10³⁰ cm^-2s^-1 to ALICE, with an integrated luminosity of around 3.5 fb^-1 for both ATLAS and CMS. Looking back, the year’s operations can be roughly divided into four main phases, interspersed with technical stops, machine development periods and special physics runs.

Initial commissioning delivered the first beams at 6.5 TeV, followed by the first stable beams after two months of careful set-up and validation. The magnetic behaviour, optical properties and aperture were confirmed to be in good shape, and all the key beam-related systems were re-commissioned after the huge amount of work done during LS1.

The two scrubbing runs delivered good beam conditions at around 1500 bunches per beam after a concerted campaign to re-condition the beam vacuum system. However, electron cloud, which was anticipated to be more of a problem for the target 25 ns bunch-spacing beam, was still a significant issue at the end of the scrubbing campaign.

The initial 50 ns and 25 ns intensity ramp-up phase was tough going and had to contend with a number of issues, including earth faults, UFOs, the ULO (unidentified lying object) and radiation affecting electronics components in the tunnel. All these problems combined made operations difficult but nonetheless the LHC was able to operate with up to 460 bunches and to deliver some luminosity to the experiments, albeit with poor efficiency.

The second phase of the ramp-up following the second technical stop of the year was dominated by the e-cloud-generated heat load and the subsequent challenge for cryogenics, which had to wrestle with transients and operation close to their cooling power limit. The ramp-up in the number of bunches was thus slow but steady, culminating this week in a total of 2244 bunches in each beam. This result was achieved after a long campaign involving a lot of hard work and close collaboration between cryogenics, the scrubbing team and operations.

Importantly, the e-cloud generated during physics at 6.5 TeV scrubs slowly and so reduces the heat load for a given intensity. This opens a margin for more bunches, thus keeping the cryogenics close to the acceptable maximum.

The ULO has remained quiet and, happily, some conditioning of the UFOs has been seen. As a result, the rate of UFOs per hour has dropped noticeably, as shown in the figure. The overall machine’s availability for physics has remained reasonable with around 30 to 35% of the scheduled time spent in stable beams since the last technical stop.

All this bodes well for 2016.

by Mike Lamont for the LHC team