Late in the afternoon of Friday, 21 April, another important milestone was achieved in the LHC beam commissioning. At 6.58 p.m., one day ahead of schedule, the LHC Engineer in Charge declared “stable beams” at 6.8 TeV, the first time in 2023 that the two beams had been collided at that energy with the experiments in data-taking mode. Although just three bunches were circulated in clockwise and three bunches in anti-clockwise direction, it signalled the start of the data-taking season and the “intensity ramp-up” phase for the LHC.
During the past three weeks of beam commissioning, many checks and adjustments have been made to get the LHC ready for stable beam. One of the many important steps was the validation of the “loss maps”. The LHC collimation system protects the machine and its components, notably the superconducting magnets, against any possible damage induced by the beam. Therefore, the collimation and operations team must make sure that all the collimator positions and openings are well adjusted for all phases of the cycle, from injection until the last collision of the fill. To do so, they inject a few bunches, deliberately create controlled losses, and observe that these losses occur where expected. If not, the collimator hierarchy needs to be addressed. When the LHC was first started, this was a manual task that took many days and machine refills. Nowadays, the loss map process is highly automated and takes only three to four fills and a handful of shifts, even though the number of loss maps has increased quite a bit with the luminosity levelling that was introduced.
As soon as the first stable beams were achieved, the intensity ramp-up phase started, meaning that the machine will gradually be filled with more and more bunches until the full circumference is filled, leaving only a gap for the dump kicker to rise. This should be achieved with about 2400 bunches per beam. Initially, there will be three bunches per beam, followed by 75, then 400, 900, 1200, 1800 and, finally, 2400. Each step will require at least two fills and more than 15 hours in stable beams. At the end of each intensity step, equipment experts will be asked to validate the correct functioning of their equipment and machine protection, using a checklist. The next intensity step will be initiated only once the checklist is fully completed and validated.
As scheduled, the intensity ramp-up was interrupted this week to scrub the machine, with the aim of reducing the formation of electron clouds in the vacuum chamber. These clouds negatively affect the quality of the beams and also put a strain on the cryogenics system, which must keep the beam screen cold by evacuating the heat induced by the electron bombardment. This is a necessary step to prepare the LHC for receiving longer bunch trains, which are needed in order to reach the requisite ~2400 bunches per beam.
By mid-May, the LHC is expected to achieve stable beams with 1200 bunches per beam, which will provide a significant level of collisions for meaningful physics data taking. Two to three weeks later, physics with a full machine should be established.