With the pure whiteness of its walls, its impeccable cleanliness and its alternating white and blue lights, the new High-Luminosity LHC (HL-LHC) cavern, situated at Point 1 near the ATLAS detector at around 80 metres below ground, could easily pass for a medical facility. However, “it won’t stay that way for long,” smiles Oliver Brüning, HL-LHC project leader: the empty cavern and 300-metre-long service tunnel will soon experience the clutter and bustle of the installation of its technical infrastructure and, further in the future, of the equipment fitting out the upgraded LHC.

Along with a similar underground complex at Point 5 (close to CMS), which is still under construction, this cavern and gallery are the keystone upon which much of the HL-LHC strategy rests. Indeed, the structure presents a comprehensive solution to the challenges posed by the future accelerator – namely, increased radiation damage to components due to the higher number of collisions, greater losses in magnet refrigeration and storage issues in the tight LHC tunnel, which cannot house all of the cutting-edge equipment needed to improve its performance. These pieces include superconducting links, cold compressors for the triplet quadrupole magnets, a variety of cooling magnet protection systems and the power generation for the new crab cavity SRF system. They will be moved to the new cavern, alongside all of the new and old power converters of the LHC, during Long Shutdown 3.

Ever since the decision to build the caverns was made in 2015, the teams responsible for the civil engineering works (primarily the Civil Engineering group within SCE, although the project involves groups from all across the Organization) haven’t lost a second: concerned by the disturbances that the heavy drilling works could cause in the LHC beam during operation, the HL-LHC leadership advanced the start of the works to 2018 and LS2, with the aim of finalising the civil engineering works before Run 3. Designers settled for a double-decker solution, with the new gallery resting parallel to the LHC tunnel, six metres above it, and connected to the old tunnel at four different points. “This elegant double-decker design allows us to bore the connections through the roof of the LHC tunnel, so as to not lose a single square metre of ground floor in the already jam-packed tunnel,” explains Oliver Brüning.

Laurent Tavian, work package leader for the construction of the underground structure, cannot hide his satisfaction with the unfolding of the civil engineering works: “Our main concern was water, as the flooding of the caverns had complicated the digging of different caverns in the past. But this never crystallised into a real issue here, as we were lucky enough to dig during two exceptionally dry years. What we did not expect, however, were the hydrocarbons.” Despite the minor inconvenience of finding small pockets of natural gas and oil and the few weeks lost because of the 2020 lockdown, civil engineering is now finishing smoothly and on schedule, thanks in part to the trusting relationship built with the main contractor company, The Joint Venture Marti Meyrin. The construction of surface buildings – for cooling systems and other services – is well under way and should be completed by autumn 2022.

“The purpose of the HL-LHC upgrade is not limited to maxing out the luminosity of the accelerator, but also to make the machine more reliable. We want it working like a Swiss clock,” states Oliver Brüning. With the new equipment stored separately from the accelerator tunnel in the new underground structures, interventions will be carried out while the machine is still operating, ensuring continuous data collection – unlike in previous runs, when the machine required frequent breaks for technicians to access the equipment. Though the road to the HL-LHC is still long, the idea of a more luminous, sturdier and more reliable accelerator is now one step closer to completion thanks to the new underground structures.