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7.2-m-long niobium–tin quadrupole magnet manufactured at CERN reaches nominal current for the first time

The 7.2-metre-long version of this vital HL-LHC component reached nominal current plus an operational margin corresponding to a coil peak field of 11.5 T at 1.9 K during a test in SM18

MQXFBP3 test (2)
The MQXFBP3 magnet after the test, during assembly with the nested dipole orbit corrector. (Image: CERN)

Another success for the HL-LHC magnet programme: after the successful endurance test of a 4.2-metre-long niobium–tin quadrupole magnet in the United States in spring 2022, the HL-LHC quadrupole’s longer version proved its worth later in the year. “MQXFBP3”, the third full-length quadrupole prototype to be tested at SM18, reached nominal current plus an operational margin in September–October 2022, confirming the success of the niobium–tin technology for superconducting magnets.

MQXFBP3 is the third in the series of HL-LHC triplet quadrupoles that have been produced and tested at CERN in recent years. These 7.2-metre-long superconducting magnets, along with their shorter counterparts currently being produced in the United States, will focus proton beams more tightly around the ATLAS and CMS collision points to allow the tenfold increase in integrated luminosity (the number of collisions) targeted by the HL-LHC.

The first two magnets tested at CERN fell short of reaching nominal current, which prompted the Accelerator Technology department’s magnet group to improve the design and the assembly processes of its prototypes as part of the so-called “three-leg strategy”. The magnet cold mass was reworked to reduce the coupling between the welded outer stainless-steel shell and the aluminium structure of the magnet.

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The MQXFBP3 magnet on its way to reaching nominal current in SM18. (Image: CERN)

This updated version – the third prototype – was able to reach nominal current (corresponding to 7 TeV in operation) plus 300 A of operational margin with only one training quench at 1.9 K. This is the first MQXF cold mass assembly, tested horizontally with a welded outer shell (as in the final configuration), to achieve this performance, which corresponds to a peak field in the coil of 11.5 T. The magnet has been subjected to two warm-up–cooldown cycles, showing no performance degradation. Even though the magnet satisfies the acceptance criteria for operation in HL-LHC, the magnet was limited 3% below nominal current at 4.5 K. The localisation and phenomenology of these quenches is very similar to those of the limiting quenches of the first and second MQXFB prototypes.

After the test, the magnet was removed from its stainless steel shell and is now being assembled with the nested dipole orbit corrector, which was provided by the Spanish institution CIEMAT. A new test in this configuration will be carried out in mid-2023. Should the test confirm its performance, MQXFBP3 will be the second Q2 cryomagnet to be installed in the IT (inner triplet) STRING.

The positive outcome of the recent test is cause for satisfaction and relief, especially as niobium–tin technologies, known to be more brittle than niobium–copper components, have come under particular scrutiny. Even so, engineers in the magnet group have more tricks up their sleeves to bring the performance of the 7.2-m-long MQXFB to the same levels obtained in the short models and in the 4.2-m-long magnets manufactured in the US: MQXFB02, the stage-two magnet of the three-leg strategy, will include further technical improvements in the magnet assembly to eliminate the coil overstress during keying and bladdering operations that was observed on the first three prototypes. The magnet community is eagerly awaiting the outcome of the magnet’s powering tests, which will continue throughout the first months of 2023 at SM18 – stay tuned!

Timelapse insertion of a High-Luminosity Third Nb3Sn quadrupole prototype. (Video: CERN)