Winter therapy for the accelerators

Hundreds of people are hard at work during the year-end technical stop as all the accelerators are undergoing maintenance, renovation and upgrade operations in parallel.


The new beam absorber on its way to Point 2 before being lowered into the LHC tunnel for installation.

The accelerator teams didn’t waste any time before starting their annual winter rejuvenation programme over the winter. At the end of November, as the LHC ion run was beginning, work got under way on the PS Booster, where operation had already stopped. On 14 December, once the whole complex had been shut down, the technical teams turned their attention to the other injectors and the LHC. The year-end technical stop (YETS) provides an opportunity to carry out maintenance work on equipment and repair any damage as well as to upgrade the machines for the upcoming runs. Numerous work projects are carried out simultaneously, so good coordination is crucial. Marzia Bernardini's team in the Engineering department is responsible for organising and planning the technical stops. “We started planning this technical stop as early as June 2015,” she says. “Hundreds of people from all the technical departments were involved over the course of the 12 weeks.”

In the PS Booster, a painstaking campaign was launched to identify obsolete cables. This might seem like a simple enough task, but it’s actually very complex. Over the years, hundreds of kilometres of cables have been added to this accelerator, which began operation in 1972, and the associated documentation is sometimes incomplete. “We’ve had to check around 3000 cables, totalling 150 km in length,” says Sébastien Evrard, who was in charge of the cable identification project. “It took sixty people eight weeks.” The obsolete cables identified will be taken out during the next YETS to make room for the new cables that will be installed for the LHC Injectors Upgrade (LIU) project. At the same time, new structures will be installed to house future cable racks. A similar campaign was carried out in the Super Proton Synchrotron (SPS), where approximately 7000 obsolete cables were identified at two points of the accelerator, and the false floors covering the cables were consolidated.

In the Proton Synchrotron (PS), in addition to the usual maintenance and consolidation work, two new pieces of beam instrumentation equipment – new wall current monitors to replace the one installed in 1993 – were installed as part of the LIU project: “Tests will begin at the end of January once the cabling has been completed, ahead of the arrival of the first beams in the ring on 8 March,” says Simon Mataguez, PS Facility Coordinator.

The wall current monitors are installed in the PS ring.

Continuing along the accelerator chain, 16 magnets need replacing in the SPS: 14 of the 960 main magnets in the ring and two auxiliary magnets in the transfer line. Since each magnet weighs 16 tonnes, it’s a big job. “But it’s routine for us,” David McFarlane, SPS Facility Coordinator, reassures us. “We usually change one or two magnets during each technical stop, even during short stops.” The SPS is an ageing accelerator – it will turn 40 this year – and its magnets need to be renovated regularly. “The lamination and the cooling circuit both deteriorate. That’s why we have to replace the magnets,” David continues.

In the Large Hadron Collider (LHC), several major operations are ongoing. Two beam absorbers, at Points 2 and 8 of the machine, have been replaced (see box below). Two sectors (7-8 and 8-1) have been emptied of helium to ensure that no losses occur while one of the coldboxes is being repaired. Twelve collimators, on both sides of ATLAS and CMS, have been dismantled. The vacuum chambers, which restricted the movement of the collimators, are being modified. Finally, a piece of instrumentation equipment at Point 4, a beam synchrotron radiation telescope (BSRT), has been upgraded. Cabling campaigns are also taking place throughout the LHC machine – four teams are working in parallel to install 25 km of signal cabling (copper and fibre-optic) for new equipment and upgrades. “There are lots of other jobs to be done, especially maintenance work,” Marzia says. “The Engineering department’s Cooling and Ventilation (CV) and Electricity (EL) groups were busy performing work during the end-of-year closure, for example.” The experiments are also working on their detectors at the same time, and sometimes have to coordinate their activities with the work on the accelerator.

All the work in the injectors has to be completed by early February and in the LHC by early March. The injectors will then be progressively restarted while powering tests begin in the LHC. The first beams in the LHC are scheduled for the end of March.

Absorbing without disturbing

Beam absorbers for injection (TDIs) are used when beams are ejected from the SPS into the LHC. They absorb the SPS beam if a problem occurs, providing vital protection for the LHC. They consist of two jaws that close around the bunches of particles injected, and the beam is gradually absorbed by the jaw materials: boron nitride, aluminium and copper. These absorbers work in a similar way to collimators, but their jaws are much longer – four metres compared with one metre in the case of the collimators. The existing TDIs, situated at Points 2 and 8 of the LHC, showed signs of weakness during the 2015 run, sometimes disrupting injection.

“We observed abnormal increases in the temperature of these absorbers and outgassing that compromised the LHC vacuum,” explains Antonio Perillo Marcone, project leader in the Targets, Collimators and Dumps section of the Engineering department (EN-STI-TCD). Two new absorbers were therefore developed, using a different material in the jaws. “We replaced the boron nitride with graphite, which is more stable over time and won’t disrupt the beam so much," says Antonio. The final assembly has already taken place in CERN’s workshops and the two new components will now be tested.


by Corinne Pralavorio