Accelerator Report: When plans meet reality…

With 100 fb⁻¹ delivered to both ATLAS and CMS, the LHC Machine Committee (LMC) had given the green light to gradually increase the bunch intensity from 1.63×10¹¹ protons per bunch to 1.7×10¹¹, with a possible further step to 1.75×10¹¹ protons per bunch. At the beginning of 2025, there had even been the idea to aim for 1.8×10¹¹ protons per bunch, but it now appears that 1.75×10¹¹ is close to the maximum the cryogenic system can handle. The limiting factor is the electron-cloud-induced heat load, which must be evacuated by the cryogenic system. A change in the filling scheme (i.e. bunch pattern and total number of bunches) could help, but there is, in general, little appetite for such an adjustment.

On 14 October, following what initially appeared to be a successful increase in bunch intensity, three consecutive fills were dumped due to losses at Point 2. These losses were correlated with increased vacuum activity in the non-conforming RF-finger vacuum module located in cell 6 left of Point 2 (6L2).

As a consequence, on the morning of 15 October, the bunch intensity was reduced to 1.5×10¹¹ protons, following which both beams could again be brought into collision. However, a second fill at the same intensity again showed signs of vacuum activity. It was therefore decided to further reduce the bunch intensity to 1.4×10¹¹ protons. This reduction, however, comes with an estimated loss of about 8% in luminosity production.

This naturally raised the question of whether the non-conforming RF-finger vacuum module should be replaced. Experts estimated that such an intervention would require a two-week technical stop, with possible temporary consequences for ALICE background levels once operations resumed. Since this option was not favoured, and as the non-conforming module only affects Beam 1, the decision was taken to keep Beam 1 at 1.4×10¹¹ protons per bunch, while continuing to increase the bunch intensity for Beam 2.

This approach required adaptations in the injector complex to allow each beam to be filled with different bunch intensities, a challenge that was met thanks to the remarkable flexibility and adaptability of the injector complex and its operations teams.

On the morning of 17 October, new X-ray images of the RF-finger module confirmed no significant evolution of the non-conformity. Later that day, Beam 1 was filled with bunches at 1.4×10¹¹ protons per bunch, and Beam 2 with 1.65×10¹¹ protons per bunch, as a first step. A successful two-step increase of the Beam 2 bunch intensity then followed over the weekend, reaching 1.7×10¹¹ protons per bunch.

As the electron-cloud-induced heat load decreases with lower bunch intensity, the reduction in Beam 1 intensity freed up additional cooling capacity in the cryogenic system. This allowed a further increase in the Beam 2 bunch intensity, while remaining within the system's operational limit of 175 W per half-cell.

Since this operating regime had not yet been explored, the proposal was carefully reviewed and approved by the LHC Machine Committee (LMC) on 22 October. The next day, on 23 October, Beam 2 reached 1.8×10¹¹ protons per bunch in collision for the first time, with a fully filled machine and, crucially, without any noticeable vacuum activity.

With this configuration, the potential 8% loss in luminosity caused by running both beams at 1.4×10¹¹ protons per bunch could be mostly compensated for. The luminosity curve recovered nearly its original gradient, putting the LHC back on track to reach the target of 120 fb⁻¹.

Thanks to its excellent performance, and despite the challenges encountered along the way, the LHC has celebrated several major milestones in recent days.

On 22 October, the LHC surpassed 500 fb⁻¹ of total integrated luminosity accumulated over all three runs combined. Then, on the morning of 27 October, CMS reached its target of 120 fb⁻¹. LHCb and ALICE had already achieved their respective goals of 12 fb⁻¹ and 50 nb⁻¹ during the weekend. By the evening of 27 October, ATLAS had also reached its 120 fb⁻¹ target. Therefore, by the end of the day, all luminosity objectives for the 2025 proton-physics run had been successfully achieved, with a full week still remaining before the switch to lead-ion operation.

When one challenge is met, we naturally look forward to the next. In our case, the next one is to beat the 2024 record integrated luminosity of 124.3 fb⁻¹ (average of ATLAS and CMS). Stay tuned - the race is not over yet!