The 2023 LHC run ended with a quench in the early morning of 30 October, two hours ahead of the official start of the year-end technical stop (YETS). Later that same morning, bells started to ring on the LHC access console in the CERN Control Centre: the technical teams were ready to enter the LHC tunnel, kicking off the 2023–2024 YETS. In the coming months, many preventive and corrective maintenance activities will take place in different locations in the LHC machine, as well as in the injector chain. The schedule is tight – the machines have to be ready for the injection of the first 2024 LHC beam on Monday, 11 March.

In the injector chain, the beams to the majority of the experimental facilities were also stopped – but not to all of them. Linac4, the PS Booster and the PS will still produce beams for the antimatter factory until 13 November, to partially compensate for the 50 days of lost beam time earlier this year.

Pauses in beam production before the start of the maintenance activities are a good opportunity for dedicated machine development studies and tests: on 30 and 31 October, Linac4 and the PS Booster were able to study beam production with a high beam current out of the Linac4 H⁻ ion source.

In the framework of the LHC Injectors Upgrade project, Linac2 was replaced by Linac4, which, in combination with the PS Booster, is able to produce particle beams of higher brightness – more protons in the same beam size. The Linac4 source provides a beam current of 35 mA, which is sufficient to produce the operational beams currently required, but further developments to its source will enable an extracted beam with a higher current to be produced. In order to deploy such a beam in future operations, tests must be performed, in particular to explore the capacities of the PS Booster with a high beam current.

On 30 October, the source experts adjusted the Linac4 source parameters and managed, in less than one hour, to extract a beam current of 48 mA – almost 40% higher than usual. The next step was to accelerate and transport this high-current beam through Linac4 to the PS Booster. The operational cycles used to deliver beams to the LHC, AD, n_TOF, ISOLDE and the SPS North Area were successfully adjusted to match these new beam parameters, proving that with the high-current beam from Linac4 the PS booster can produce the operational beams that are required today.

In a second stage, the intensity reach was explored, with the cycle used to send beams to ISOLDE. The nominal beam intensity to ISOLDE is usually 3.2 x 10¹³ protons per cycle, which come from the four PS Booster rings (each ring accelerates 8 x 10¹² protons per cycle). During the test, the experts managed to double the beam intensity in three of the four rings, without significantly increasing the beam losses. This performance was achieved during a relatively short period, and its stability remains to be proven over longer periods of time. Nonetheless, in the future, such a beam intensity could allow more than 6 x 10¹³ protons to be provided per cycle, which is an unprecedented amount. This potential is a great asset for future upgrades and consolidation of downstream experimental facilities and will benefit future fixed-target physics.