CERN Yellow Reports: Monographs, Vol. 3 (2025): Status and challenges of Nb3Sn accelerator magnets at CERN: Lessons learned from ITER to HL-LHC

The High-Luminosity Large Hadron Collider (HL-LHC) project at CERN has offered the opportunity to promote and develop various types of enabling accelerator technologies, such as MgB$_2$ superconducting links for cold powering and Nb$_3$Sn accelerator magnets for the interaction regions where the two high-luminosity experimental areas of the machine are located.A prototype superconducting link system has been successfully tested in the second quarter of 2024 and series production is now ongoing. The Nb$_3$Sn magnet development has encountered some difficulties characterized by performance limitation or degradation, which have now been overcome. We report on the status and challenges of HL-LHC Nb$_3$Sn accelerator magnets at CERN, with a primary focus on the root-cause analyses and recovery actions implemented for the 11 T dipole magnet program and for the final-focusing quadrupole magnet program. The symptoms have similarities with those encountered over a decade ago on the Nb$_3$SnCable-in-Conduit conductors for the magnets of the International Thermonuclear Experimental Reactor (ITER) and the methodology to address them was inspired from that developed to resolve the ITER crisis. Thanks to the successful efforts of ITER and HL- LHC, Nb$_3$Sn has been demonstrated to be a viable technology for fusion and accelerator magnet applications and is now reaching maturity.