In my last Report, I announced the completion of the first proton physics run of the 2025 LHC schedule and the start of a series of special runs. These included the oxygen and neon ion runs, as well as Van de Meer (VdM) scans to calibrate the luminosity monitors in the experiments. At that time, the integrated luminosity had reached 24 fb-1.
Since then, and despite a tight schedule, all the special runs have been successfully completed thanks to both the good availability of the LHC machine and the excellent, very close collaboration between the machine team and the experiment teams. However, the ion run was extended by one day for neon-neon collisions, and the VdM run took significantly more time to complete than originally planned. The time needed to carry out the many precision scans and measurements had been underestimated. If a scan is interrupted (e.g. by a beam dump), the entire scan has to be redone with a new fill, as beam conditions vary slightly from one fill to another and consistency is required for good calibration results.
As a result, luminosity production has fallen behind the original forecast. Fortunately, the high availability of the LHC and the excellent beam quality delivered by the injectors have enabled the luminosity production rates to be slightly improved, and the trend in the luminosity production curve indicates that we are gradually catching up with the forecast (see graph below). We are counting on the summer period, which is typically calmer in terms of machine studies and special runs, to maintain a high production rate. The goal is to reach around 72 fb⁻¹ by the start of the second machine development (MD) block on 1 September.
Today's LHC performance is based on beams of 2460 bunches per beam, each containing 1.6 × 10¹¹ protons when the beams collide at an energy of 6.8 TeV per beam.
Over recent years, in particular after the LHC Injectors Upgrade (LIU) during Long Shutdown 2 (LS2), the teams responsible for the injector complex have worked intensively to prepare for the HL-LHC beam. This high-intensity and high brightness beam, reaching up to 2.3 × 10¹¹ protons per bunch, is now available in the injectors. However, the current LHC machine cannot safely handle such a bright and intense beam until the HL-LHC project has been completed, after LS3.
That said, there is still some margin with the present LHC set-up to increase the bunch intensity to around 1.8 × 10¹¹ protons per bunch, even if it is physically constrained by electron cloud effects, which limit the total beam intensity (a product of both the number of bunches and the bunch intensity). We must also keep in mind that pushing the beam intensity higher may reveal other - as yet unknown - limitations in the machine that could require a temporary stop of beam operations. This is something we want to avoid, especially during a key luminosity production phase. However, there are significant advantages in gaining operational experience with higher bunch intensities in preparation for the jump to HL-LHC bunch intensity levels after LS3.
To balance these considerations, a conservative strategy was outlined at the beginning of 2025, consisting of achieving a solid integrated luminosity before considering a stepwise increase in bunch intensity.
At a meeting of the LHC Machine Committee (LMC) later in August, experts in the field and the management of the Accelerators and Technology Sector (ATS) will discuss the possibility of increasing the bunch intensity towards a target of 1.8 × 10¹¹ protons per bunch while carefully evaluating input from all the equipment groups and the potential risks.
On the injectors side, the complex is running very well, delivering protons to the fixed-target facilities with high availability. Preparations for the final weeks of the 2025 run are already under way.
Following the intense period of oxygen and neon ion runs, the Linac3 source underwent a period of maintenance, during which it was switched back to lead (Pb) ion production. On 25 July, a week ahead of schedule, the Pb ion beam was successfully accelerated again in Linac3. The next steps include delivery of the Pb ion beam to LEIR on 15 September, followed by injection into the PS on 6 October and into the SPS on 13 October. These steps are all part of the lead-ion preparations for the upcoming Pb-Pb and possibly proton-Pb physics run in the LHC and for the Pb physics run in the PS East Area and SPS North Area, which are scheduled for the final weeks of the 2025 run, which will end at 6.00 a.m. on 8 December.
Until then, a significant amount of proton beam time remains and physics production and MD activities will continue in both the injector complex and the LHC.
