On 8 March, three days ahead of schedule, the first proton beam was injected into the LHC; 20 minutes later, the second beam was injected, circulating in the opposite direction.

Since the last Accelerator Report, the hardware tests and subsequent cold check-out were successfully completed, both ahead of schedule. Once the usual remaining wrinkles were ironed out, everything was ready to start the 2024 LHC beam commissioning. The single bunch low intensity probe beam, meticulously prepared in the injector chain in the past weeks, came knocking at the LHC's door.

Many of the LHC engineers in charge and system experts gathered in the CERN Control Centre (CCC) on 8 March, alongside members of the Management, to witness the process, eagerly waiting for the first beams to circulate again in the LHC.

The LHC Operations team started the injection and threading process for beam 2 (circulating counter clockwise): they injected the beam at LHC Point 8, just in front of the LHCb experiment, and let it circulate up to Point 7, where a set of collimators was fully closed to intercept it. The measurements performed by the beam position monitors indicated that the beam trajectory could be improved. This was quickly done using an automated beam steering tool that powers corrector magnets to smoothen the trajectories of the particles.

Confident in this correction, the Operations team opened up the collimators at Point 7 and closed the ones further along the ring at Point 6, before injecting the beam again. This process was repeated until the last collimators, at Point 1 (ATLAS experiment), were opened, leaving the way clear for the beam to make a second, third, fourth… and millionth turn.

Another small correction to adjust the orbit of the circulating particles was made before attention switched to beam 1, which ended up circulating in the machine less than 20 minutes after beam 2 and was welcomed by many happy faces in the CCC. The next step – accelerating both beams up to 6.8 TeV – was also accomplished during the weekend. Witnessing both beams in circulation is something of a relief for everyone involved, although the real beam commissioning work starts at that point.

For the 2024 run, it was decided to modify the optics of the accelerator and to replace them by reverse polarity optics (RP-optics). The objective is to mitigate the radiation suffered by some of the magnets of the inner triplet region on both sides of the ATLAS experiment. The inner triplet is a set of quadrupole magnets that focus the beam to very small dimensions at the centre of the experiments.

Some of the collision debris – particles produced by the collisions and travelling  parallel to the beams, outside the experiment – is intercepted by the magnets in the inner triplet regions, inducing radiation damage to their insulation. With different optics, the debris is deposited in other places in these magnets, so that the burden of the radiation damage is distributed more widely. This helps to extend the magnets' lifetimes, even with an increased number of collisions.

The commissioning and validation of the RP-optics are among the many beam commissioning steps that have to be taken in the coming weeks before beams enter into collision at 6.8 TeV, hopefully on 8 April. Depending on how work progresses, this milestone may shift forwards or backwards by a few days.