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The LHC leak repair: a short photostory

Scroll through the photo diary to relive the successful repair of the LHC

smiling group picture

The intervention teams prepare to reclose the affected magnet after successful repairs (Image: CERN)

At 1 a.m. on Monday, 17 July, the LHC beams were dumped due to an electrical perturbation. Approximately 300 milliseconds later, several magnets lost their superconducting state (“quenched”). During a quench, the magnet warms up, which in turn warms and pressurises the liquid helium that surrounds it.

While not common, this sequence of events is a normal occurrence that protects the superconducting cable of the magnet when an electrical glitch occurs; the mechanical stress exerted on different parts of the magnet can be quite strong. 

Among the magnets that quenched on 17 July were the inner triplet magnets located to the left of Point 8 of the LHC, which play a crucial role in focusing the beams for the LHCb experiment. Unfortunately, on this occasion, the quenches led to a helium leak in these magnets and put a halt to regular LHC operations.

Scroll through the photo diary below to relive the ten-day race against the clock to repair the leak.

tree falling on power cables, LHC page 1 showing quenches
Monday, 17 July, 1 a.m.: ROOT CAUSE
The reason for the electrical glitch that caused the safety systems in the LHC to dump the beam and several magnets to quench was found: a tree on the Swiss side (about 55 km from CERN in the Canton of Vaud) fell on the power lines and disrupted the power system.
ice on an LHC dipole
Monday, 17 July, 11 a.m.: A CHILLING DISCOVERY
Ten hours later, on entering the tunnel, the investigating team found that the cryostats* of the triplet magnets near Point 8 were partly covered in ice. Tests quickly confirmed that a small amount of helium had escaped through a leak and filled the insulation vacuum.
Action was taken immediately: the adjacent magnets were electrically isolated, circuits were locked off and grounded, and the quench heaters for this sector were switched off. Additionally, to allow work to be done on the triplet, the 3 km of superconducting magnets in the affected sector were stabilised at a temperature of 20 K, instead of their usual 2 K (-271°C).

*All LHC superconducting magnets are housed in cryostats. During normal operation the external wall of the cryostat is at room temperature, while the magnet operates at 2 K. The cryostat is designed to maintain the magnet at such a low temperature by minimising the in-flow of heat – and insulation vacuum is essential to achieve that.
bellows in the LHC
Tuesday, 18 July – Wednesday,  19 July: LOOKING FOR THE LEAK
The exact position of the helium leak in the 50-m-long cryostat was still unknown. By Tuesday, 18 July, vibration and acoustic tests had been performed. Attaching accelerometers and microphones, the intervening team detected a clear signal in the interconnection zone between the first quadrupole magnet (Q1) and the second  (Q2). Additional X-ray scans showed that the spacing of the bellows ridges on one of the pipes in the superconducting magnets appeared to be stretched. Bellows are employed in the physical connections between two magnets, giving flexibility. In this case the stretched bellows were on the M2 pipe, which contains the instrumentation connections.
different teams underground
Thursday, 20 July - Sunday 23 July: PREPARING TO OPEN
The intervening teams agreed that the Q1-Q2 interconnection between the two quadrupole magnets would have to be opened for further investigation and repairs. To make a safe intervention possible, the sector around the magnets was emptied of liquid helium. In parallel, an electrical quality assessment showed that the electrical circuits of the triplet were fine – the problem was thus elsewhere.
Teams of experts from different CERN groups (safety, vacuum, cryogenics, magnets, engineering, powering, magnet protection, survey, beam instrumentation, operations) discussed how to tackle a problem that had never been encountered before on a 15-year-old string of magnets and established a procedure on the spot.
opening the triplets
The whole triplet cryostat reached room temperature. The external bellows and inner thermal shields at the Q1-Q2 interconnection were removed so that the inner helium lines could be inspected.
the leaky bellows
Monday 24 July: GOTCHA!
The leaking bellows in the M2 pipe.
cracks in the bellows
Monday, 24 July, afternoon: SMALL BUT MIGHTY
The teams located the M2 bellows that were suspected to be at the root of the problem and, indeed, found that there was a 1.6-mm-long crack on it, which was the source of the helium leak. An action plan was put in place: remove the broken bellows, replace them, do all the necessary tests, close up again and start the cooldown… 

…all in under 10 days. Otherwise, a complete warm-up of the affected LHC sector could not be avoided, and this would put an end to the whole LHC physics programme for 2023.
working the bellows
While the broken bellows were cut out, the vacuum team conducted pressure and leak tests on spare bellows to test their resilience and provide a replacement unit for the tunnel repair.
Thursday, 27 July: TEAMWORK...
Experts led by Sandrine Le Naour and Said Atieh discussed the possible repair solutions on site.   
The new bellows were installed. On the far left: threading the instrumentation through the new bellows. In the middle: many hands make light work! On the right: skilled welders do their magic.
beampipe, m1, m2 and m4
Graeme Barlow looking at the open interconnection, with the various pipes inside visible. The M lines allow the helium to be transported between magnets (M1 contains the busbar for the electrical connection, M2 contains the instrumentation connections, and M4 has a cryogenic function). In the middle sits the beampipe where the particles circulate. The M2 bellows are just visible between the M1 and the beampipe.
the repair zone
discussing repairs
The vacuum and mechanical teams discussed the action plan while the repairs were in progress.
work in progress
Two teams were often at work at the same time: on the left, reinstalling beam position monitor (BPM) cables, on the right, starting the leak test on the new bellows.
leak test tooling
The vacuum team installing the leak-test tooling.
leak test machine installation
Graeme Barlow of the vacuum team installing the leak-test machine with Paul Cruikshank.
leak test
Paul Cruikshank, leader of the LHC vacuum intervention, together with his team, starting the leak test on the newly installed bellows.
cable reinstallation
During the opening of the Q1-Q2 interconnection, the beam position monitoring (BPM) cables had to be removed. Here, the cable reinstallation is under way.
assessing progress
The reinstalled bellows required several new welds. Each required a dedicated leak test to avoid any bad surprises once the interconnection was reclosed.
Sandrine Le Naour (far right) assessing the progress. She coordinated the mechanical interventions to open the magnet interconnections and then had to prepare the careful reclosure. 
leak testing
Paul Cruikshank with the vacuum team and expert welder Didier Lombard. On the right, a close-up view of a clamshell-leak testing tool (a technology developed at CERN to leak-test the entire LHC during its installation) used by the team to leak-testnew welds. This CERN innovation (also used by industry today) enables the vacuum quality of a tube to be checked from the outside, which is a great advantage when the objects to be tested are very long and difficult to remove: something that is very typical in the 27-km-long LHC vacuum systems.
final leak test
Wim Maan and Marcel Knoch checking the tightness of the final weld.
fully repaired
The M2 bellows fully repaired. The bellows are surrounded by external shells to support and guide them when the helium is pressurised during different operational phases of the LHC.
The bellows were repaired and the leak test was successfully completedwithin the ten day deadline. Although there’s still plenty to do to reclose the interconnection, the light at the end of the tunnel is in sight! After the teams have repumped the vacuum and cooled down the magnets, the LHC can restart.
The LHC operations team is confident of seeing the first beam back in early September.

For more information on this story, watch a video interview with Paul Cruikshank, one of the coordinators of the repair operation: