The SM18 test facility, located in France, near the Meyrin site, was originally built to test and validate the magnets and radiofrequency (RF) cavities of LEP (the Large Electron–Positron Collider). It has since seen over 1700 such components pass through it – including, of course, those of the LHC – and is now preparing to welcome the magnets and RF cavities of the future HL-LHC.

To test the LHC magnets, the SM18 hall was equipped with 12 similar test benches, allowing the cryoassemblies to be cooled down to 1.9 K and then powered in nominal conditions for validation. All magnets were tested individually in the different test benches before being installed in the accelerator.

Between 2009 and 2020, the configuration of the 12 benches was progressively changed: in the R&D phase of the HL-LHC, five vertical cryostats have been installed in SM18 to test the model magnets. Three of them were recovered from an existing test stand and two have been newly designed, specially manufactured and installed. These imposing new cryostats needed more space, resulting in the dismantling of two of the 12 benches to make space for a vertical test stand.

The major upgrade campaign that has been under way at SM18 since 2014 was justified by the fact that the new magnets employ new technologies, different from the LHC ones. In particular, the HL-LHC inner-triplet (IT) quadrupole magnets, which will focus proton beams more tightly around the ATLAS and CMS collision points, are built with niobium–tin (Nb₃Sn) coils, instead of the niobium–titanium (Nb-Ti) alloy currently used for LHC magnets.

The R&D phase for the HL-LHC magnets has been successfully closed; magnets are now being produced in their final configuration. These long magnets (up to 12 m) are now to be tested in their nominal (horizontal) position. Thus, four of the remaining 10 benches have also been modified with a powering system of up to 20 kA, instead of the existing 15 kA. This also comes with an innovative protection scheme, adapted to the magnets’ higher energy.

In addition, a new test benches will host the superconducting link system – a versatile powering scheme infused with helium gas. “In the coming years, no fewer than 50 cryoassemblies and superconducting link systems will pass through these test benches,” says Marco Buzio, a test engineer in the TE-MSC-TM section. “It will be a challenging job for all teams involved.”

The inaugural run of the new cryoassemblies and of the prototype superconducting link system in a combined mode will take place within the HL‑LHC IT string. To test the collective behaviour of these cryoassemblies with all warm and cold powering systems, just as it will be in the HL-LHC, the HL-LHC IT string has been created – an integrated test facility located at SM18.

Installed on a new metallic structure, the HL-LHC IT string is fully representative of the HL-LHC inner triplet that will be installed at Point 5. “It will allow for early development and validation of the installation procedures and individual systems tests. It also facilitates the execution of the entire hardware commissioning programme for many of the key technologies of the HL-LHC project,” says Marta Bajko, leader of the IT string facility section (TE-MPE group). The installation and interconnection of the IT magnet chain is planned for the second quarter of 2023, while the operation period spans from 2024 to 2025. The validation of the systems, procedures and hardware commissioning steps will be one of the most important milestones for the HL-LHC.