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Accelerator Report: SPS and LHC lead-ion physics: navigating technical hurdles for success


The LHC fixed display showing the last verification and validation step before establishing collisions, initially with 119 bunches, which will gradually be increased to up to 1248 bunches in the coming days. (Image: CERN)

On Thursday, 28 September, at 8.00 a.m., as planned, the Operations team at the SPS stopped producing proton beams for the experiments in the SPS North Area (NA). Instead, they switched over to lead-ion beams, in preparation for a month-long physics run that begins on Monday, 2 October, and will continue until 6.00 a.m. on Monday, 30 October.

The end of a physics run is an excellent time to look back and reflect. The proton physics run for the North Area started on 1 May, 150 days ago. One important criterion for both the accelerators and the experiments is the “beam availability” – i.e. the amount of time the beam was ready for the experiments. This year, the beam was available for the NA experiments 85.8% of the time it was requested, which is more than our target (85%). This represents a significant improvement over the 2022 proton physics run, which reached only 72% of beam availability due to numerous major issues.

The 14.2% of non-availability was caused by a total of 1859 faults, amounting to 495 hours, as documented in the Accelerator Fault Tracking (AFT) system. This system is used across all CERN accelerators to log faults, along with their start and end times. Experts then review these faults to ensure consistency throughout the accelerator complex. The information is used by the equipment groups to address the issues and make decisions on system and equipment consolidation. Approximately half of the beam non-availability (6.7%) was due to issues in the SPS injector chain (Linac 4, PS Booster and PS). The other half (7.5%) resulted from faults within the SPS itself.

The longest fault we experienced this year was the malfunction of a dipole magnet in the SPS on 6 September. While the actual replacement took only a few hours, the time required to restore the vacuum to a level suitable for beam circulation resulted in a 24-hour downtime.

A more comprehensive analysis of the unavailability of each of the accelerators will be conducted at the end of 2023.

In the meantime, the LHC kicked off its lead-ion commissioning achieving the first lead-ion collisions on Tuesday, 26 September. This was earlier than originally planned for 2023 (although still later than anticipated, following the helium circuit and insulation vacuum leak that occurred during the summer). Indeed, despite a swift restart at the end of August, various unrelated technical problems arose in the injectors, the LHC machine and the experiments, leading to additional delays. These issues prevented the planned proton–proton reference run from occurring before the start of the lead-ion collisions.

One significant problem emerged on Thursday, 31 August, when a vacuum leak was detected at Point 8 in a TDIS (target dump injection system), which plays a crucial role in safeguarding the machine against beam losses during the injection process. On Tuesday, 8 September, another leak occurred in a different but identical component of the same system at Point 8. In both cases, the vacuum team took swift action, identified the leaks and successfully sealed them.

Due to these repairs, the affected parts of the system could no longer be repositioned into the machine, limiting the level of protection for beam injection at Point 8, but still guaranteeing a sufficient level of protection for injecting lead ions. If no further issues arise, both the LHC and the experiments are looking forward to a productive and successful lead-ion physics run.

Meanwhile, protons continue to be the primary focus for the rest of the accelerator complex. They are supplied to various areas, including ISOLDE, the PS East Area and n_TOF and the AD and ELENA. AWAKE, situated behind the SPS, will also continue to rely on proton beams in the upcoming weeks.