If experimental physicists had their way, CERN’s accelerator complex would run nonstop, 365 days a year, without a winter break. However, the machines in the accelerator complex, along with their many subsystems, require regular maintenance to stay in topnotch condition, especially as some of them are getting quite old.

Take the Proton Synchrotron (PS), for example, the oldest operational machine at CERN. On 24 November, it celebrated its 65th anniversary, an age at which many people retire, but not the PS! Instead, it will continue to play a central role in the production of nearly all the beams within the accelerator complex for years to come, thanks to the care and attention of the many people who carry out preventive and corrective maintenance to maintain good availability as well as upgrades to further enhance its performance.

This winter stop began gradually on 25 November, when the LHC was stopped. The same day the beam from the PS Booster (PSB) to ISOLDE was stopped, leaving the Proton Synchrotron (PS) as the PSB’s only client. However, the ISOLDE team and some of the experiments continued running for another two weeks without the PSB’s proton supply.

During this ISOLDE winter physics run, a pre-irradiated target was installed in the location where targets usually receive the proton beam from the PSB. This target has already been exposed to a substantial amount of proton beam in the past and continues to produce isotopes. These isotopes were separated and directed as usual down the transfer lines to some of the experiments, allowing ISOLDE physics to continue even without fresh protons. This 2024 winter physics run was also successfully accomplished. In 2024 the PSB delivered 1.25x10²⁰ protons, which means that 66% of the total amount of protons accelerated by the PSB (1.9x10²⁰) was consumed by ISOLDE.

Meanwhile, the Neutron Time-of-Flight facility (n_TOF) also concluded its physics run on 25 November, having received an impressive 1.39x10¹⁹ protons on its target in 2024. With this, the PS focused on delivering protons and lead ions to the remaining facilities – the antiproton decelerators (AD and ELENA), the East Area and the SPS – before stopping operations entirely on 2 December.

Although CERN’s accelerator complex shut down on 2 December, some systems remain active. The ELENA machine, which usually decelerates antiprotons, will continue to deliver negatively charged hydrogen ions (H⁻) to the GBAR experiment until the end of the week. Meanwhile, the BASE experiment still has its trap filled with antiprotons collected over a year ago and will carry out physics experiments during the winter shutdown. For BASE, the YETS is the ideal time for measurements, as the absence of pulsing equipment means no electromagnetic disturbances, improving the precision of their data.

While many staff will work in the tunnels during the YETS and the accelerators take their winter break, the CERN Control Centre (CCC) will remain staffed. The Technical Infrastructure team will operate 24/7 to monitor the technical systems and quickly address any issues that arise, ensuring smooth operation also throughout the annual closure.

As this time of year is perfect for sharing wishes, here are mine: if I could make a Christmas wish, it would be for another incredible run in 2025, with beam performance and availability at least matching, and preferably surpassing, the successes of 2024.

Best wishes to everyone, and don’t forget to reconnect with us in 2025 to stay updated on all the exciting developments in the accelerator complex!