Accelerator Report: Ending the 2024 LHC proton run on a high note

This year's LHC proton run, which has seen protons colliding at an energy of 6.8 TeV, ended on 16 October, surpassing expectations with 11% more collisions than planned. Next up: a four-day machine development (MD) block, before the LHC gears up for its final run of the year, the lead-ion run, scheduled to end at 6.00 a.m. on 25 November.

The end of the proton run and the transition to the lead-ion run is a nice moment to briefly look back at what we have achieved so far. This year's proton run began on 11 March with four weeks of beam commissioning, during which the LHC was successfully set up using a new "reverse polarity" optics scheme. The aim was to reduce the effects of radiation on key magnets near the ATLAS experiment (see the 14 March Accelerator Report). By 5 April, the LHC had already achieved stable beams and collisions with a small number of proton bunches, with experiments starting to collect data three days ahead of schedule.

The intensity ramped up very smoothly to about 1800 bunches but, on 17 April, unexpected beam losses occurred, indicating that the collimator hierarchy was broken (see the 3 May Accelerator Report).

The machine performance was limited while extensive studies and tests followed, which led to the development of a successful solution that was implemented and validated after the scheduled technical stop from 10 to 15 June. Nevertheless, the intensity ramp-up was completed on 29 April, when we managed to inject 2352 bunches per beam.

In the meantime, the LHC injectors switched to another beam production scheme, providing the BCMS (Beam Compression, Merging and Splitting) beam - a brighter beam designed to deliver more collisions (see the 30 May Accelerator Report). Additionally, the LHC Machine Committee approved a four-week extension of the 2024 run on 24 April, raising the luminosity target from 90 fb⁻¹ to 110 fb⁻¹.

After the technical stop, the restart of luminosity production was challenging and progress initially lagged behind predictions. However, by mid-August, the pace picked up and the LHC exceeded expectations, delivering a final integrated luminosity of 124 fb⁻¹ for both ATLAS and CMS - about 11% higher than the 110 fb⁻¹ target. LHCb and ALICE also surpassed their goals, achieving 10.6 fb⁻¹ and 61.6 pb⁻¹ respectively.

From the start of physics operations on 25 April to the conclusion of the proton run on 16 October, the LHC was operational 73% of the time and in collision mode 52% of the time. This remarkable performance wouldn't have been possible without the outstanding availability, performance and flexibility of the injector complex, which continuously adapted to the LHC's needs and pushed the machines' performance whenever possible.

After the four-day LHC MD block, a three-day technical stop will take place to prepare for the upcoming lead-ion run. Interestingly, the lead-ion run isn't just about colliding lead ions - it also includes a brief period of proton-proton collisions at 2.68 TeV. This serves as a sort of calibration for the lead-ion collisions.

Once the proton and lead-ion beams are set up, there will be six days of proton-proton collisions at 2.68 TeV, followed by 19 days of lead-lead collisions. However, these lead-ion collisions present significant challenges. During the 2023 run, the LHC machine experienced beam losses during acceleration that prematurely dumped the beams, and the ALICE experiment observed high background noise, complicating data collection (see the 12 October 2023 Accelerator Report). To address these issues, extensive studies and tests were conducted and several improvements were implemented, resulting in record-breaking data collection for ALICE.

Still, there is a need for further enhancements in beam performance and background reduction to ensure a smooth and successful 2024 lead-ion run. Stay tuned!