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Accelerator Report: Technical stops always hold a few surprises

Restarting the entire accelerator complex after a technical stop is rarely seamless and often comes with unexpected technical challenges


The LHC technical stop started on 10 June, and the injector complex’s technical stop two days later. These stops were scheduled to align with planned work on the Swiss electrical network. The planned work by Swissgrid, the Swiss electricity transmission grid operator, could have caused fluctuations on the network, potentially disrupting or damaging the accelerator subsystems.

Swissgrid scheduled their work from Wednesday, 12 June to Friday, 14 June (Saturday being reserved as a back-up day in case additional time was needed). CERN's technical teams worked hard to complete their activities in the accelerator complex by 2.00 p.m. on the Friday, hoping to restart the complex early if Swissgrid finished ahead of schedule. At around 5.30 p.m., Swissgrid informed CERN that their work was complete. This notification marked the official restart of the accelerator complex.

Restarting the entire accelerator complex after a technical stop is rarely seamless. It can be particularly challenging late on a Friday afternoon… Nevertheless, with excellent support from standby services and experts, the restart went rather well. By Friday evening, beams were circulating in most of the accelerators in the complex, including the LHC.

Following each technical stop, the LHC requires a brief period for revalidation and intensity ramp-up. This time, the revalidation also included corrections to address the collimation hierarchy issue.

Unfortunately, the process was interrupted on Sunday, 16 June, in the morning, when a vacuum leak was discovered in the SPS (see picture). This required the replacement of a magnet, which was scheduled for the following day. Despite the leak, beams could still be provided to the LHC and the SPS North Area experiments during the night.

The vacuum pressure in several parts of the SPS. In royal blue, the vacuum pressure in a magnet with a leaking vacuum chamber. Two things can be observed: 1) the vacuum pressure is increasing over time, and 2) the ripple on the vacuum pressure signal is synchronous with the pulsing of the magnet. This means that, as the magnetic field of the magnet increases, the vacuum leak area opens wider. (Image: CERN)

On 17 June, technical teams replaced the magnet and, by 4.30 p.m., the magnet with the leaking vacuum chamber had been removed from the SPS tunnel. The newly installed spare magnet and vacuum chamber were connected, and vacuum pumping began. By 18 June, at around 10.00 a.m., the vacuum pressure was low enough to resume beam operation. However, a new issue, this time with the accelerating radiofrequency (RF) cavities, prevented beam acceleration in the SPS. After work by the RF experts, beam acceleration was possible by early evening, allowing the LHC revalidation to continue.

The LHC revalidation and intensity ramp-up were completed by the early hours of 20 June, allowing luminosity production to resume. However, when the ATLAS magnets were ramped up to their nominal magnetic field, clogging was discovered in the main refrigerator cold box. This required a warm-up and purging cycle, which began that morning and finished on 25 June.

During the weekend, after completing the intensity ramp-up, luminosity production resumed. However, to prevent a significant difference in integrated luminosity between ATLAS and CMS, it was agreed to bring forward some scheduled machine development (MD) activities and reduce luminosity production until the ATLAS magnet was fully operational again. These MD activities will be deducted from the next MD block, which is scheduled to start on 19 August, and will be replaced by luminosity production.