The year-end technical stop (YETS) activities in the accelerator complex’s first machines are nearing completion, but in the LHC about six weeks of work still lie ahead of us before the machine is handed over to the Operations group on 13 March for recommissioning.
While having a well-defined and approved schedule is essential, an equally critical element for the 2025 run is establishing clear operational configurations and scenarios. These are developed and refined throughout the year in various meetings, working groups and committees. They are then consolidated in two key workshops, which are vital not only to ensure the immediate operation of the accelerator complex but also to prepare its long-term future.
The first of these is the Joint Accelerator Performance Workshop (JAPW), which took place from 10 to 12 December 2024. The workshop brought together accelerator and operations teams from across the complex, along with teams from the experiments, to review the performance achievements, challenges and lessons from 2024. The focus was on defining the configuration and performance goals for the 2025 run.
The second, more strategic, workshop is the Chamonix Workshop, which goes beyond operational aspects to address broader topics. These include planning for the remainder of Run 3 and for Run 4, as well as looking ahead to Long Shutdown 3 (LS3), the High-Luminosity LHC (HL-LHC) and the Future Circular Collider (FCC) Feasibility Study. Importantly, the Chamonix Workshop integrates input and recommendations from the JAPW and makes key decisions that will guide the operation of the accelerator complex in the years leading up to LS3.
As I write, the Chamonix Workshop is in full swing, and the initial outlines of the 2025 LHC configuration and operational scenarios are beginning to take shape.
For the 2025 LHC run, the majority of the schedule is dedicated to proton collisions, complemented by shorter runs for lead and oxygen ions. Based on the approved 2025 LHC schedule and the beam parameters discussed, preliminary target figures have been proposed, pending final validation and, of course, assuming equal or even better beam availability throughout the whole accelerator complex than the excellent availability in 2024.
In 2024, the proton run was 147 days long and delivered an integrated luminosity of 124 fb-1. For 2025, 138 days have been allocated for proton collisions – 9 days fewer than in 2024. Despite the shorter runtime, the proposed targets for integrated luminosity are ambitious: 120 fb-1 for ATLAS and CMS, 12 fb-1 for LHCb and 50 pb-1 for ALICE. Achieving these goals will be challenging, but realistic, thanks to the potential to slightly increase luminosity production by exploiting small improvements in electron cloud limitations (see box below).
For the 21-day lead-ion run at the end of 2025, the proposed integrated luminosity targets are 2.4 nb-1 for ATLAS, CMS and ALICE, and 0.8 nb-1 for LHCb. While these targets are also demanding, lessons learned from the successful 2024 lead-ion run could help achieve further optimisations, making these goals challenging but realistic.
How can we slightly increase luminosity production? The LHC beam consists of “bunch trains”, which are groups of closely spaced particle bunches, each of which is separated by 25 nanoseconds (about 7.5 m) from the next one. These bunch trains vary in length, depending on the number of consecutive bunches. As the beam circulates through the LHC’s vacuum chambers and beam screens (tubes that shield the magnets’ coils and cryogenic system from the heat loads, radiation and other damage), it releases free electrons, which are accelerated by the passing bunches. These electrons then collide with the beam screen, releasing more electrons in a chain reaction known as “electron cloud” production. This electron cloud can cause beam instabilities. Furthermore, the collisions of electrons with the beam screen generate heat, which must be removed by the cryogenic cooling system. The heat produced limits both the number of bunches in a train and the intensity of each bunch. Shorter bunch trains and larger gaps between them reduce the formation of electron clouds and, in turn, the heat deposited on the beam screen. In 2024, the LHC operated with bunch trains composed of 3 batches of 36 bunches per injection, with each bunch containing an intensity of 1.6 x 1011 protons. Over the 147 days of operation in 2024, the bombardment of electrons further “conditioned” the beam screens, gradually reducing the number of electrons released (a bit like scrubbing). As a result, by the end of the run, the heat deposited in the beam screens was slightly lower than at the start, even though the bunch train configuration and bunch intensity remained unchanged.  This reduction in heat deposition creates a small margin to adjust the bunch train pattern (e.g. 4 x 36 bunches or perhaps even 5 x 36 bunches) and/or slightly increase the bunch intensity (e.g. 1.7 x 1011 protons per bunch or even 1.8 x 1011 protons per bunch). These modifications would allow a modest increase in luminosity production, making the target of 120 fb-1 for 2025 a realistic goal. |