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Accelerator report: Beams injected into the LHC by heavy ion collisions

The LHC resumes beam production, after repairing the internal triple-vacuum leak, and prepares for heavy ion collisions

Screenshot of LHC page 1
The beams are injected back into the Large Hadron Collider, with the status observed in real time LHC Page 1.

On August 30, the beams were injected again into the Large Hadron Collider (LHC), just ahead of schedule. A few days would now be needed to restart the machine with the beam, recheck the machine’s safety systems, and adjust all of the machine’s parameters to ensure it was ready to supply beams for physics research again.

After repairing the internal triple vacuum leak on August 1, the LHC completed cooling down on August 22. This allowed for commissioning tests, a series of pre-determined evaluations of the LHC equipment to re-verify its readiness for regular operation. Magnet quenching training is usually part of the power test after a warm-up and cool-down cycle. However, on this occasion, when the nominal cycle was carried out and a current of up to 11,600 Amperes flowed through the dipole magnets, none of them went dormant. This result was not entirely unexpected, since most of the machine remained cool, and the temperature of Arc 7-8 remained below 80 K. This temperature threshold is critical, because it represents the point beyond which the change in mechanical stresses within the magnet becomes significant.

Graph of average temperature in ARC showing high and low
This graph shows the temperature evolution of arc cold cluster 7–8, which has undergone a limited warm-up to repair the vacuum leak in the left inner triplet of LHCb. The leak appeared on 17 July and Arc 7-8 was heated to about 20 K and held at that temperature using gaseous helium, until the interconnection was opened on 24 July. Active cooling of the arc was not possible during the intervention and thus the average arc temperature increased. On August 1, the intervention was complete and the pacification of the arc could begin.

The LHC schedule has been revised following discussions between representatives of the LHC experiments and the LHC instrument teams. We conclude that the appearance of the leak should mark the end of regular proton operation this year, as restarting the high proton beam intensity at 6.8 TeV would entail significant setup and revalidation time. Therefore, the focus for the rest of the year will be on heavy ion physics which was already scheduled for the end of the year. This will be complemented by relatively brief special physics operations with protons, such as van de Meer scans to calibrate luminosity measurements for the experiments and collisions of protons with strongly unfocused beams at interaction points (high beta stars) in the experiments, together with an extensive instrument development session, which It was tentatively scheduled for the second half of July.

The heavy ion program, which begins the week of September 11, consists of two parts. The first is a 2.68 TeV proton-proton reference run, followed by actual runs with lead-ion collisions in all four large LHC experiments. This lead ion run was originally scheduled to last four weeks, and has now been extended for an additional week. The last lead ion beams of 2023 will then be jettisoned on Monday 30 October at 6 a.m. marking the start of the winter hiatus of the entire CERN accelerator complex. Until then, there is great anticipation for a busy, challenging and above all successful physics period.

The resumption of beam operation is rewarding not only for the physics community, but also for all the dedicated people who worked hard and thought outside the box to fix the unprecedented vacuum leak in a much shorter time than standard procedures would have been applied. This achievement confirms the strength, quality and innovative spirit of the scientific and technical teams at CERN.


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