I concluded my previous Accelerator Report with: "…if the two-week delay holds, the 2025 commissioning may once again fall during the Easter weekend. Have we discovered a new constant of nature?", referring to the delay caused by a leak in the water-cooling system of the ATLAS argon calorimeter.
But, as is often the case at CERN when challenges arise, everyone stepped up. ATLAS mobilised its resources and the other experiments offered their support to resolve the issue as quickly as possible. Thanks to this efficient coordination and dedicated work, the expected two-week delay was reduced to just four days.
So, the start of the LHC beam commissioning during Easter is not a new constant of nature after all.
In the early hours of 8 April, the final preparations were under way: the ATLAS access shaft was being closed, the final checks were being carried out on the LHC machine and experts were gathering in the CERN Control Centre (CCC) to contribute to beam commissioning and to witness protons circulating in the LHC for the first time in 2025.
As the LHC team gathered for the 9 o'clock briefing, the teams working on the injectors (PS Booster, PS and SPS) performed their final checks on the meticulously prepared single-bunch LHC beam - the result of weeks of careful tuning. At 9.30 a.m., the LHC engineer in charge gave the SPS team a heads-up that the first beam injection into the LHC was just minutes away. At 9.39 a.m., protons were transferred from the SPS and injected into the LHC at Point 2, in front of the ALICE detector. This marked the start of the beam 1 (clockwise) threading process.
Threading the beam involves injecting a single low-intensity bunch into the machine and guiding it through a sector of the LHC. Horizontal and vertical beam positions are measured and corrected and the process is repeated, allowing the beam to travel further each time until it completes a full circumference and circulates.
Not much later, at 11.00 a.m., the same process was initiated for beam 2 (anti-clockwise), with the first protons injected at Point 8 in front of the LHCb detector. Once each beam had been made to circulate individually, both beams were successfully injected and were circulating simultaneously by midday.
These first injections are always an exciting moment. To an outsider, they might seem like the main event of the LHC beam commissioning. But while they are indeed an essential milestone, they are just the beginning, a small part of the complex process required to prepare beams for physics.
Since 8 April, teams of experts, together with the LHC engineers in charge, have been working around the clock to tune the machine and address issues that only become visible once beam is circulating.
Despite the four-day delay to the start of beam commissioning, the schedule remains on track. The first collisions with stable beams are still scheduled for 2 May, and the start of meaningful physics with around 1200 bunches for 19 May.
Meanwhile, physics at ISOLDE, n_TOF and the East Area is well under way and the SPS beam commissioning was completed successfully. On 4 April, the beamline physicists took over the SPS beam to commission the nearly 6 km of beamlines between the SPS and the various experiment zones in the SPS North Area, where the first experiments made use of the beam on 14 April.
Further upstream in the accelerator chain, the Antiproton Decelerator (AD) completed its hardware commissioning on 3 April, several days ahead of schedule, and the first antiprotons were injected shortly after.
In the antiproton complex, antiprotons are produced by bombarding the AD target with a high-energy proton beam from the PS. These antiprotons are first decelerated down to 5 MeV by the AD, and then further slowed down to 100 keV by ELENA. Thanks to its dedicated H-⁻ ion source, ELENA was able to get a head start. The AD/ELENA team began beam commissioning using H- ions well before antiprotons were available, allowing significant progress to be made in advance.
Now that antiprotons are available from the AD, both the AD and ELENA have successfully completed deceleration cycles. Notably, ELENA has already extracted bunches containing 10 million antiprotons, a number that, in 2024, was only reached by mid-summer and constituted record bunch intensities at that time. Physics in the antiproton complex is officially scheduled to begin on 5 May but, with the rapid progress made so far, an earlier start is likely (but remains to be confirmed).
