Accelerator Report: Target reached, but compressor off

In the last report, I highlighted that the LHC had successfully delivered 100 fb⁻¹ of integrated luminosity to ATLAS and CMS by 13 September 2024. Only ten days later, on 23 September, an additional 10 fb⁻¹ was integrated, reaching the 110 fb⁻¹ target set for 2024.

Shortly after achieving this milestone, however, one of the LHC’s static VAR compensators (SVC, see box) switched off unexpectedly. Fortunately, this didn’t affect the beams already circulating in the LHC.

SVCs play a crucial role in stabilising the voltage of the electrical grid, particularly when large pulsed systems – like the magnets located in the transfer line between the SPS and the LHC – are used to inject new beams into the LHC. To inject new beams, it was necessary to turn this SVC back on.

Experts were called in and they followed the established procedure, which had worked reliably in the past. This process involves reducing the electrical load by temporarily switching off some equipment as a precaution.

Despite these steps, an electrical perturbation occurred, leading to the shutdown of one of the two cryogenic compressor stations at LHC Point 8, which could not be restarted due to the damage it had sustained. Both of these stations supply liquid helium to cool down an arc of the LHC. With one of the two compressors out of action, the cooling capacity was severely reduced.

As a result, the number of bunches that could circulate in the LHC should have been significantly reduced, from the usual 2352 bunches per beam to about 1500 bunches per beam. This reduction would have affected the luminosity production significantly.

Fortunately, the cryogenics experts evaluated the situation and, miraculously, managed to exchange the broken cold compressor using a procedure that had never been used in a real-life situation before and without completely warming up the compressor station, saving precious time. In the afternoon of 25 September, the LHC's cryogenic system was fully restored, allowing beams to be injected again.

However, a series of unrelated small technical issues followed. This meant that efficient luminosity production could not be resumed until the scheduled stop, on the evening of 26 September, when the fourth Machine Development (MD) block began.

The MD period concluded successfully at midnight on 30 September, just in time to prepare for the VIP visits on the morning of 1 October, organised as part of CERN’s 70th anniversary.

Luminosity production was resumed later that day and will continue up until the end of the proton physics run, scheduled for 17 October. After that, the lead-ion physics period will start.