Chamonix 2016: setting the future course for the LHC and the accelerator complex

The LHC Performance Workshop took place in Chamonix between Tuesday, 25 and Thursday, 28 January. The programme included a review of the machine’s performance in 2015, a forward look at Run 2, and discussion of the status of the LHC injectors upgrade (LIU) and HL-LHC projects. The final session was dedicated to the 2019-2020 long shutdown (LS2).

The 2016 LHC Performance Workshop participants.

Last year was the first year of operations following the major maintenance work of the 2013 – 2014 long shutdown (LS1). It was a tough but ultimately successful year. An analysis of operations and efficiency was performed with the aim of identifying possible improvements for 2016. The performance of key systems – e.g. machine protection, collimation, RF, transverse damper, magnetic circuits and beam diagnostics – has been good but nonetheless efforts are still being made to provide, for example, better reliability, improved functionality and monitoring.

A number of challenges also arose in 2015 – some familiar, some new – and these were covered by dedicated talks. The list includes the now famous Unidentified Falling Objects (UFOs) and an unidentified aperture restriction in an arc dipole, the so-called ULO - Unidentified Lying Object. The UFO rate has fallen during 2015 and it seems that it will remain acceptable in 2016. The ULO proved not to be a major problem in 2015 and is expected to remain under control this year. The overheads involved in carrying out an intervention in a cold part of the machine to remove the ULO would be prohibitive at this stage.

A dominating issue of 2015 was the electron cloud. A systematic scrubbing campaign mitigated the worst effects. A strategy for continued scrubbing at 6.5 TeV was deployed and allowed 2244 bunches per beam and promising luminosity performance to be established. Electron cloud will still be present in 2016 and the plans for tackling it were outlined. There was also discussion of the machine configuration, planning and potential performance for 2016.

Machine availability is absolutely key for efficient luminosity production and a day was spent examining availability tracking and the performance of all key systems. Possible areas for improvement in the short and medium term were identified.

The LIU project aims to improve the LHC injectors in order to deliver the extremely ambitious beams required for the HL-LHC. The deployment of LIU is scheduled to take place during LS2 and will consist of an impressive and extensive upgrade programme for the Booster, the PS and the SPS, as well as the completion of Linac4. Additionally, the LIU team is also planning upgrades of Linac3, LEIR, PS and SPS for the ion runs.

An in-depth survey of the potential performance limitations of the HL-LHC and ways to mitigate them were discussed. Electron cloud will remain an issue: a number of measures were proposed, including in-situ amorphous carbon coating and in-situ Laser-Engineered Surface Structures (LESS) to address electron cloud in the magnets of the insertion regions.

Key upgrades to the collimation systems and the radiofrequency (RF) cavities are also required. The preparation for an installation of novel crab cavities in the SPS is well under way. This will allow their performance to be tested with protons before the concept is used in the LHC.

Ions will be an integral part of the HL-LHC programme and the means to deliver the required beams and luminosity are taking shape. The recent successful Pb-Pb run at 5.02 TeV centre-of-mass energy per colliding nucleon pair and the quench tests performed during the same run have provided very useful input.

The planning for LS2 is already under way. Aside from the LIU implementation discussed above, ALICE and LHCb will perform major upgrades of their detectors and read-out systems. An impressive amount of consolidation work is foreseen across the complex. One particularly noteworthy aspect is the significant maintenance and consolidation work planned by the non-LHC experiment facilities.

by Mike Lamont