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Accelerator Report: The accelerator complex gears up for action after the yearly winter maintenance break


The Linac4 fixed display showing the beam’s electrical current along the Linac. The first bar indicates the beam current coming out of the Linac4 source; the last bar indicates the beam current knocking on the PS Booster door. The change in the height of the bars indicates the transmission efficiency. The aim is to minimise the beam losses between the beam current measurement points in order to increase the overall transmission efficiency. (Image: CERN)

The symbolic key to resume LHC operations will be handed over from the ACE (Accelerator Coordination and Engineering) group in the Engineering department to the Operations group on Friday, 16 February, kicking off the 2024 “particle season”.

As winter bids farewell, the recommissioning of the accelerator complex gathers pace, with the scientific community eagerly awaiting particle beams in their experiments. Following the year-end technical stop (YETS), Linac4 is the first machine to resume beam operation, followed by the downstream machines: the PS Booster, PS, SPS and LHC.

Beam entered Linac4 on 5 February, two days ahead of schedule – extra time welcomed by the Linac team. During the YETS, work was done on the chain of accelerating cavities, requiring a re-phasing – a challenging and often time-consuming task. To do so, the acceleration of the particle beam is optimised as the beam goes down the Linac: the voltage waves in the cavities are timed correctly as the beam passes by, ensuring optimum acceleration in each of the cavities and bringing the energy to 160 MeV at the end of the Linac.

This week, the beam was then sent to the PS Booster. The operations team has one week to prepare for the first beam to be injected into the PS on 21 February. The PS will then have to prepare the first beam for the SPS beam commissioning, scheduled to start on 1 March. The first particle beams will reach the LHC on 11 March, initially with one to a few bunches at most.

Before injecting particle beams, the hardware recommissioning coordinators of each machine and the many equipment experts have the task of meticulously recommissioning and validating all the subsystems. They run the machine “as if” particle beams were being accelerated, but without particles. They go through checklists, validating and ticking off thousands of tests, to give the green light for beam commissioning.

The expectations for 2024 are high. Firstly, in the LHC, the focus is on luminosity production with proton–proton collisions, aiming at an unprecedented accumulation of luminosity of up to 90 fb-1. This, together with the accumulation of luminosity forecast for the 2025 run, should provide a sizeable analysis data set to keep physicists busy during Long Shutdown 3. The 2024 LHC run will conclude with lead–lead collisions; the first lead ions will be injected into the LHC on 6 October. The 2024 run is scheduled to end on 28 October.

The injector chain has an ambitious year ahead as well: the injectors have a busy fixed-target programme and will provide beams to all the experimental facilities. The first fixed-target physics will start in the PS East Area on 22 March, followed by the PS n_TOF facility on 25 March. Physics in ISOLDE, downstream of the PS Booster, will start on 8 April, followed by the SPS North Area on 10 April. The antimatter factory is set to start delivering antiprotons to its experiments on 22 April. The AWAKE facility, behind the SPS, will run for ten weeks in total (in blocks of two or three weeks) until the middle of September, when the dismantling of the no-longer-used CERN Neutrinos to Gran Sasso (CNGS) target facility will start, to allow for a future extension of the AWAKE facility. The SPS HiRadMat facility will see four 1-week runs.

Beyond this busy physics programme, many machine development studies and tests are planned in all the machines. One of these tests will take place between mid-March and early June to configure the Linac3 source to produce magnesium ions, which will be accelerated in Linac3, injected into LEIR, and possibly even into the PS. This test will help assess the feasibility and performance of magnesium beams in the accelerator complex, for potential future applications in the LHC and the SPS North Area.

The resumption of operation of the accelerator complex heralds a new year of physics, surely leading to important physics results. As the countdown to 11 March continues, the operations and expert teams are working diligently to prepare the machines and the beams for another successful physics run.