The days are getting longer, the trees are getting greener and it is time, too, for the CERN accelerator complex to reawaken. Following the year-end technical stop (YETS) - a 17-week period in which the accelerators undergo maintenance and minor upgrades - each accelerator in the chain has, in turn, restarted for the 2023 data-taking season. Each restart is an important step in the process of conveying protons from their source to their final destination in the Large Hadron Collider (LHC) on 28 March. While most major upgrades of the machines were carried out during the long shutdown before the start of LHC Run 3, the accelerator upgrades performed during the YETS will improve physics data taking for the second year of the run.
It all began on 13 February 2023, when Linac4 started beam commissioning: a short period of adjusting the machine before operation began on 17 February. This linear accelerator is responsible for supplying protons to the whole accelerator chain. It consists of a hydrogen ion source and several accelerating structures (known as radiofrequency cavities) that forward the ions to the PSB (Proton Synchrotron Booster). During the YETS, the Linac4 source was upgraded. Alessandra Lombardi, senior accelerator physicist at CERN, explains: "The novelty is in the extraction. While the new source is similar to the previous one, it allows a 30% increase in the peak current of the beam." An increase in peak current means that the beams running through the accelerators have the potential for higher intensity.
On 3 March, the PSB began beam commissioning. Here, the hydrogen ions accelerated by Linac4 are stripped of their electrons, leaving only protons. These beams of protons are further accelerated in four synchrotron rings, supplying a small number of experiments and the next accelerator in the chain: the Proton Synchrotron (PS).
The PS, which began its 2023 operation on 10 March, is responsible for supplying proton beams to a variety of experiments, including n_TOF, the Antiproton Decelerator and those in the East Area, as well as to the Super Proton Synchrotron (SPS), the next accelerator in the chain. During the YETS, the beamlines between the PS and SPS underwent upgrades to make beam transfer more efficient. Until 2016, when a process called "multi-turn extraction" was introduced, the process of transferring beams from the PS to the SPS resulted in beam loss. Because the circumference of the SPS is eleven times that of the PS, the transfer involved circulating a beam five times around the PS, filling ten of the SPS's eleven sections. This left some space for the machine to direct the beam. For 2023, this method has been improved upon even further, with the introduction of barrier buckets. These help synchronise the beam's injection into the next accelerator with its gap, preventing further beam loss and improving the efficiency of the whole complex.
On 17 March, the SPS began beam commissioning. Adjusting and preparing this accelerator for physics is the final step before proton beams can circulate in the LHC again. The SPS was also upgraded for 2023. Mike Barnes, one of CERN's senior engineers, explains: "We have kicker systems in all the accelerators, many of which have had work carried out. All these systems must be recommissioned and tested before being handed back to the CERN Control Centre for operations." Kicker systems act like switches on train tracks: they change the direction of the beam to transfer it to the next accelerator. He continues: "In the SPS kicker system, four modules were upgraded to reduce the amount of heat that the beam deposits in them. These four modules previously limited high-intensity beam operation in the SPS."
Furthermore, in the LHC, one of the injection kicker magnets was replaced with a new design during the YETS. This, like much equipment installed in the LHC tunnel, will be tested as part of the preparations for the LHC's next run: the High-Luminosity LHC.
The whole process of restarting the machines is extremely complex, involving many people across multiple groups at CERN. The LHC restart is different every year due to the continuous machine upgrades and, at every stage of the restart, each individual machine needs to be adjusted to make the physics run as efficient and productive as possible. Following a commissioning period from 28 March, collisions in the LHC are expected to commence on 22 April, supplying its experiments with more efficiency and a higher beam intensity than ever before.
