The magnet programme is one of the keystones of the HL-LHC project. At its heart is the development of triplet quadrupole magnets, which will focus the very intense beam around the collision points at ATLAS and CMS. The niobium–tin compound from which the coils are built allows them to reach the 12 T magnetic fields required by the HL-LHC.
Despite the complexity of Nb3Sn coils and magnet manufacturing, as of early 2023, the technology is being validated for use inside particle accelerators. Twenty 4.2-metre-long magnets (MQXFA) are being produced in the United States – seven of which have successfully passed their individual tests and will be assembled two by two in cold masses as an in-kind contribution to the HL-LHC.
The third prototype of the longer version of the magnet developed at CERN (MQXFBP3, 7.2-m-long) was the first to reach nominal current, plus an operational margin, in a test carried out in late 2022. After this success, the results of the months-long test of its successor, MQXFB02, had teams across the HL-LHC project celebrating: not only does this new magnet also reach nominal current plus operational margin, but it does so with a larger temperature margin. Moreover, it demonstrated resilience in an endurance test to simulate its long-term behaviour in the HL-LHC. A similar test was carried out on a US magnet in 2022.
MQXFB02 is the fruit of the second leg of the “three-leg” strategy that was implemented after performance limitations were observed in the first two MQXFB prototypes. This second leg involved technical improvements in the magnet assembly to eliminate the coil overstress during keying and bladdering operations. Powering for the test started in November 2022 and ended at the beginning of March this year.
The quadrupole magnet reached nominal current, plus a 300 A operational margin (16.53 kA), with two quenches at 1.9 K in the first powering cycle. At 4.5 K, it quenched at nominal current plus 200 A, thus proving a temperature margin of ~2.7 K. This performance limitation is similar to that observed in the first three prototypes, but at a higher current level. The magnet’s resilience was assessed through three warm-up and cool-down cycles, which all reached nominal current at 1.9 K without quenches. Over more than three months of testing, a total of 500 powering cycles and 48 high current quenches, both provoked and spontaneous, were performed – none of them caused performance degradation. This combination of performance and resilience is the base of the acceptance criteria for operation in the HL-LHC.
Given these good results, the magnet will be recovered from its cold mass and, in April 2023, a new cold mass will be manufactured, including, this time, a nested corrector from the CIEMAT collaboration. The cold mass will then be tested again, in its final configuration, in SM18 in 2024.