Image credit: CERN.
The LHC Performance Workshop took place in Chamonix from 25 to 28 January. Attended by representatives from across the accelerator sector, including members of the CERN Machine Advisory Committee, and the LHC experiments, the programme covered a review of the 2015 performance and a look forward to 2016, as well as the status of both the LHC injector upgrade (LIU) and the High Luminosity LHC (HL-LHC) projects. It finished with a session dedicated to the next long shutdown (LS2), planned for 2019–2020.
For the LHC, 2015 was the first year of operation following the major interventions carried out during the long shutdown (LS1) of 2013–2014. At Chamonix, an analysis of the year’s operations and operation efficiency was performed, with the aim to identify possible improvements for 2016. The performance of key systems (e.g. machine protection, collimation, radio frequency, transverse dampers, magnetic circuits, and beam diagnostics) has been good, but nonetheless a push is still being made for better reliability, improved functionality and more effective monitoring.
The first year of operation also revealed a number of challenges, including the now-famous unidentified falling objects (UFOs), and an unidentified aperture restriction in an arc dipole called the unidentified lying object (ULO). Both problems are under control and there should be no surprises in 2016.
A dominating feature of 2015 was the electron cloud. The worst effects were suppressed by a systematic scrubbing campaign and a strategy that allowed continued scrubbing in physics conditions at 6.5 TeV. This strategy delivered 2244 bunches and encouraging luminosity performance. The electron cloud has side effects such as heat load to the cold sectors of the machine and beam instabilities. These have to be effectively handled to avoid compromising operations. In particular, the heat load to the beam screens that shield the walls of the beam pipes was a major challenge for the cryogenics teams, who were forced to operate their huge system close to its cooling-power limit. Plans for tackling the electron cloud in 2016 were discussed at the Chamonix meeting, including a short scrubbing run that should allow the conditions at the end of 2015 to be re-established. Further staged improvement will be obtained by further scrubbing while delivering luminosity to the experiments.
The machine configuration, planning and potential performance for 2016 and Run 2 were outlined. The LHC has shaken off the after effects of LS1, and the clear hope is to enter into a production phase in the coming years. Besides luminosity production, 2016 will include the usual mix of machine development, technical stops, special physics runs and an ion run. The special runs will include the commissioning of a machine configuration that will allow TOTEM and ALFA to probe very-low-angle elastic scattering.
Machine availability is key to 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 has the job of upgrading the injectors to deliver the extremely challenging beams for the HL-LHC. The status of Linac4 and the necessary upgrades to the Booster, PS and SPS were presented. Besides the completion of Linac4 and its connection to the Booster, the upgrade programme comprises an impressive and extensive number of projects. The energy upgrade to the Booster will involve the replacement of its entire radio-frequency (RF) system with a novel solution based on a new magnetic alloy (Finemet). The PS will have to tackle the increased injection energy from the Booster, as well as upgrades to its RF and damper systems. The SPS foresees a major RF upgrade, a new beam dump, an extensive campaign of impedance reduction, and the deployment of electron-cloud reduction measures. The upgrade programme also targets ions as it plans improvements to Linac3 and LEIR, and looks at implementing new techniques to produce a higher number of intense ion bunches in the PS and SPS.
An in-depth survey of the potential performance limitations of the HL-LHC and means to mitigate or circumvent them were discussed. Although it is clear that the electron cloud will remain an issue, the experts gathered at Chamonix proposed a number of measures including in-situ amorphous-carbon (a-C) coating and in-situ laser-engineered surface structures (LESS) as a way of tackling the electron cloud in the magnets at the insertion regions.
Besides the complete re-working of the high-luminosity insertions, key upgrades to the RF and collimation systems are also required. Here, plans have been base-lined and work is in progress to develop and produce the required hardware. An important novel contribution from RF is the production of crab cavities, which are designed to mitigate the effort of the large crossing angle at the high-luminosity interaction points. The preparation for the installation of test crab cavities in the SPS is well under way.
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.
Although it will only start in 2019, planning for LS2 is already under way, and a dedicated session looked at the considerable amount of work foreseen for the next two-year stop of the accelerator complex. A major part of the effort will be devoted to the deployment of the LIU injector upgrade discussed previously. Looking at the experiments, ALICE and LHCb will perform major upgrades to their detectors and read-out systems. An impressive amount of consolidation work is also foreseen. Of note is major work in the much-solicited PS and SPS experimental areas.
Besides the exploitation of the LHC in the short term, the workshop revealed that there is a huge amount of work going on to anticipate and assure the mid-term future of the laboratory, both at the high-energy frontier and in the extensive non-LHC physics programmes. The LIU upgrade and the consolidation effort will help to guarantee the future for, and offer potential performance improvements to, the extensive fixed-target facilities, including the Antiproton Decelerator and the new Extra Low ENergy Antiproton ring (ELENA), HIE-ISOLDE, nTOF and AWAKE.
