The significant increase in luminosity targeted by the high-luminosity LHC (HL-LHC) demands large-aperture quadrupole magnets that are able to focus the proton beams more tightly as they collide. A total of 24 such magnets are to be installed on either side of the ATLAS and CMS experiments in time for HL-LHC operations in 2027, marking the first time niobium-tin (Nb3Sn) magnet technology is used in an accelerator.
Nb3Sn is a superconducting material with a critical magnetic field that far exceeds that of the niobium-titanium presently used in the LHC magnets, but once formed it becomes brittle and strain-sensitive, which makes it much more challenging to process and use.
The milestone signals the end of the prototyping phase for the HL-LHC quadrupolesGiorgio Apollinari
Following the first successful test of a US-built HL-LHC quadrupole magnet at Brookhaven National Laboratory (BNL) in January last year—attaining a conductor peak field of 11.4 T and exceeding the required integrated gradient of 556 T in a 150 mm-aperture bore—a second quadrupole magnet has now been tested at BNL at nominal performance. Since the US-built quadrupole magnets must be connected in pairs before they can constitute fully operational accelerator magnets, the milestone signals the end of the prototyping phase for the HL-LHC quadrupoles, explains Giorgio Apollinari of Fermilab, who is head of the US Accelerator Upgrade Projects (AUP). “The primary importance is that we have entered the ‘production’ period that will make installation viable in early 2025. It also means we have satisfied the requirements from our funding agency and now the US Department of Energy has authorised the full construction for the US contribution to HL-LHC.”
The design and production of the HL-LHC quadrupole magnets are the result of a joint venture between CERN, BNL, Fermilab and Lawrence Berkeley National Laboratory, preceded by the 15 year-long US LHC Accelerator Research Program (LARP). The US labs are to provide a total of ten 9 m-long helium-tight vessels (eight for installation and two as spares) for the HL-LHC, each containing two 4.2 m-long magnets. CERN is also producing ten 9 m-long vessels, each containing a 7.5 m-long magnet. The six magnets to be placed on each side of ATLAS and CMS – four from the US and two from CERN – will be powered in series on the same electrical circuit.
The synergy between CERN and the US laboratories allowed us to considerably reduce the risksEzio Todesco
“The synergy between CERN and the US laboratories allowed us to considerably reduce the risks, have a faster schedule and a better optimisation of resources,” says Ezio Todesco of CERN’s superconductors and cryostats group. The quadrupole magnet programme at CERN is also making significant progress, he adds, with a short-model quadrupole having recently reached a record 13.4 T peak field in the coil, which is 2 T more than the project requirements. “The full series of magnets, sharing the same design and built on three sites, will also give very relevant information about the viability of future hadron colliders, which are expected to rely on massive, industrial production of Nb3Sn magnets with fields up to 16 T.”
Since the second US quadrupole magnet was tested in October, the AUP teams have completed the assembly of a third magnet and are close to completing the assembly of a fourth. Next, the first two magnets will be assembled in a single cold mass before being tested in a horizontal configuration and then shipped to CERN in time for the “string test” planned in 2023.
“In all activities at the forefront of technology, like in the case for these focusing Nb3Sn quadrupoles, the major challenge is probably the transition from an ‘R&D mentality’, where minor improvements can be a daily business, to a ‘production mentality’, where there is a need to build to specific procedures and criteria, with all deviations being formally treated and corrected or addressed,” says Apollinari. “And let’s not forget that the success of this second magnet test came with a pandemic raging across the world.”