CERN and the US Accelerator Upgrade Project (AUP, a consortium of various US institutions including Fermilab, BNL and LBNL laboratories based at Fermilab) are working in tandem to develop, build and test the HL-LHC’s triplet quadrupole magnets, one of the programme’s cornerstones. These magnets, which come in two sizes – a 4.2-metre-long version built in the US and its 7.2-metre-long counterpart built at CERN – will focus the beams into highly luminous regions at the collision points of ATLAS and CMS. The very strong magnetic field required to achieve this focus is made possible by the niobium–tin (Nb₃Sn) superconductor from which the magnet’s coils are made. A test carried out at Brookhaven National Laboratory (BNL) in the US in January 2023 demonstrated that, if the magnet’s performance is limited by one of its four coils, the performance limitation can be overcome by replacing the limiting coil.

It all started in October 2021, when the eighth quadrupole magnet produced by AUP (MQXFA08) showed performance limitations during a cryogenic test, making it impossible to reach the HL-LHC requirements. After the AUP team at LBNL started to disassemble the magnet and precisely identify the region and the reasons for a possible fault in one of the magnet’s four coils, the limiting coil was sent to CERN, where the EN-MME team confirmed the presence of broken ~50-micron-thick Nb₃Sn sub-elements in the five-tonne object. “Finding the precise cause of the magnet’s limitation – introduced while operating under the unfavourable conditions of the COVID-19 pandemic – was more demanding than finding the proverbial needle in the haystack. Locating those broken filaments was therefore an achievement in itself, made possible by the fruitful collaboration between the two teams,” explains Giorgio Apollinari, head of the AUP.

With an incomplete three-coil quadrupole magnet now on their hands, the CERN and AUP groups needed one question answered: could the dysfunctional coil be replaced, or would the whole magnet need to be thrown away, incurring sizeable costs for the programme? “Although the principle of replacing individual coils within niobium–tin magnets had been demonstrated on small-scale prototypes, the process’s scalability to longer magnets was yet to be proven,” says Susana Izquierdo Bermúdez, engineer in charge of the MQXF quadrupoles at CERN.

The coil was replaced in the autumn of 2022 and, following cooldown, the magnet – now called MQXFA08b – reached its nominal current after four quenches. Two additional quenches were then needed to reach nominal current plus a margin of 300 A, and the recovered quadrupole magnet was deemed fit for assembly in the final cold mass. All training quenches occurred in the new coil, demonstrating good memory in the remaining three coils, which had been trained over the previous testing campaign. “What this result tells us is that, when necessary, we will be able to save whole magnets from going to waste when only one of their coils is faulty, thereby limiting extra costs. This is an encouraging result that demonstrates the soundness of Nb₃Sn technology and of the mechanical design. We are just starting to use Nb₃Sn in the context of particle accelerators, and it still hasn’t revealed all its secrets,” adds Giorgio Apollinari.

This significant result brings new momentum to the triplet quadrupole programme, which is in full swing. So far, seven of the required twenty MQXFA quadrupoles have passed the vertical test at BNL, and the first pair of magnets has been assembled within a cold mass. This cold mass is now undergoing powering tests and will be delivered to CERN once it has been proven to meet the criteria for operation in the HL-LHC.