The Large Hadron Collider (LHC) has more than 1600 superconducting electrical circuits. They power the magnets that guide the beams around the 27 km ring. Every time the LHC stops for a major shutdown, all these circuits must be carefully checked following the switch-off and then again before switching them on. This is the job of the Electrical Quality Assurance (ELQA) team from the Technology Department (TE-MPE). During Long Shutdown 3 (LS3), the team is strongly engaged to prepare the LHC for its next phase: the High-Luminosity LHC.

ELQA carries out a wide range of electrical tests, and all of them must be finished and approved before the machine can be safely powered. Without them, the accelerator cannot restart.

The work follows three phases. First, the circuits are tested at cryogenic conditions as they are switched off to see if any changes have occurred after many years of operation. Then, after reaching room temperature, the circuits are again verified to ensure circuit integrity following the mechanical movements linked to the warm-up. ELQA will also support the replacement of four magnets in the arcs, the repair of 42 cold diode stacks used for magnet protection, the installation of the new HL-LHC inner triplet magnets, and many other repairs throughout the machine. Finally, each sector is tested again once the LHC is closed and cooled down, to make sure that we are ready for beam operation.

This is a big team effort involving colleagues from CERN and the Institute of Nuclear Physics in Krakow (IFJ PAN), Poland, with between ten and twenty people down in the tunnel every day for these activities.

Contributors: Marta Vargas Jorba & Jaromir Ludwin.

On behalf of the LS3 coordination.

The LHC is warming up for a major overhauling. For only the third time since it began operating in 2008, the whole machine is slowly returning to room temperature after running at close to absolute zero. Warming up more than 36,000 tonnes of cryogenic equipment across 27 kilometres of tunnel requires meticulous planning at every step. The cryogenics team has been running the warm-up sequence since the start of Long Shutdown 3 on 29 June 2026, working through two LHC sectors at a time over campaigns of about five weeks each, until the whole machine reaches room temperature on 11 September 2026.

The first step is removing the liquid helium stored inside the magnets and the cryogenic line. Heaters make it evaporate, and the gas is sent to the surface to be reliquefied and stored. CERN cannot keep all its helium inventory on site. Around 150 tonnes are shipped in specialised containers to external suppliers, who hold it until it is needed again. The transfers run for several months and are adjusted week by week to follow the warm-up. This challenging logistical operation ensures the helium is preserved and will be back before the LHC restarts.

With the helium removed, large heaters in the caverns take over, gradually raising the temperature of the magnets while a control system keeps the process safe. Operators work in shifts at the CERN Control Centre, monitoring everything continuously.

The final stage requires extra care. As materials expand with temperature, tunnel access is restricted, except under strict safety protocols. To increase the heat transfer efficiency, the insulation vacuum is deliberately degraded at two specific moments. Once the entire system is warm enough to prevent moisture damage, the job is done and the cryogenic circuits can be locked out and safely opened.

Figure 1. Liquid helium storage.                                                                                                          Figure 2. 600kW warm-up helium heaters.

Figure 3. LHC average temperature in each sector since the LS3 start.

Contributors: Marta Vargas Jorba, Frederic Ferrand, Benjamin Bradu, Laurent Delprat.

On behalf of the LS3 coordination.