The accelerator complex delivered its last beams of 2025 on 8 December before beginning the year-end technical stop (YETS). During a technical stop, the responsibility for accelerator coordination is passed to the Engineering department's Accelerator Coordination and Engineering group (EN-ACE) to oversee the many interventions planned in the accelerator complex.
This YETS has been particularly intense, being both shorter than usual, to maximise the limited time for physics in 2026, and very busy, to make essential preparations for Long Shutdown 3 (LS3), which begins in late June (for the LHC) and late August (for the rest of the complex). At times, more than 500 people per day have been accessing the underground complex.
A large number of activities have been taking place across the accelerator sites in preparation for LS3, ranging from 3D site scans to cabling campaigns to civil engineering works. In this last category, works have been progressing in the HL-LHC galleries, with the performance of some important tests that are necessary to successfully connect the superconducting magnet currents and crab cavities waveguides. A 6-metre-deep test core drilling is currently taking place, which is invaluable as 28 similar cores will be required during LS3 (14 each at Points 1 (ATLAS experiment) and 5 (CMS experiment)), making this test critical for refining procedures and minimising risks to the shutdown schedule. Lessons are being learned as the test drilling encountered some unexpected voids and fissures in the substructure. This allowed some of the water used to lubricate the drilling to escape into the LHC tunnel below - an unpleasant surprise for the technical teams concerned. Fortunately, the consequences are minor, with no equipment damaged, and the techniques for drilling and sealing are being updated to allow current and future works to continue safely and efficiently.
The experimental areas have also been very busy.
AWAKE already started its long shutdown in June 2025 and immediately began the complex choreography of dismantling and de-cabling old equipment in preparation for installation of the next upgrade. This has included the construction of new facilities on the surface of SPS Point 4 to handle the old CERN Neutrinos to Gran Sasso (CNGS) equipment, which now enters the dismantling phase.
The ISOLDE complex started its long shutdown on 8 December. A major activity there is the ISOLDE dump replacement to allow the safe reception of the post-LS3 beam from the Proton Synchrotron Booster (PSB), following the increase from 1.4 GeV to 2 GeV beam energy. The shutdown programme also includes extensive consolidation of the electrical circuits of the PSB-to-ISOLDE transfer line (named the "BTY" line). This began spectacularly with the removal of the first magnets by the Handling and Engineering team (EN-HE), as shown in the images below. In parallel, consolidation and upgrade activities will target several Radioactive Ion Beam delivery systems, in addition to HIE-ISOLDE radiofrequency cryomodule refurbishment work to recover the nominal secondary beam acceleration performance.
And the North Area complex has been preparing the way for the long shutdown with a large number of activities during this YETS. Some examples of the many activities are: adding new fire compartment doors to improve the safety of the complex; adding new electrical transformers and fibre-optic connections to improve the infrastructure's robustness; and archiving many high-resolution pictures of the facility in the Panoramas database to allow remote visits by both CERN staff preparing interventions and visitors curious about the facility.
As you read this, the process of handing the accelerators back to the operations teams has already begun, starting with Linac4 and the PS Booster. The Linac4 source is already pulsing, in preparation for what we hope will be a productive although necessarily short year of protons in the CERN accelerator chain in 2026 before LS3 commences.
