Considerations Towards a High Field Accelerator Magnets Programme Part I: Motivation, Goals, Challenges and Program Drivers

The Large Hadron Collider (LHC) at CERN can be regarded as the ultimate collider built with Nb-Ti magnets. Its main dipoles have reached a field of approximately 8 T, which is very likely close to the highest practical field for this superconductor in accelerators. The next major step is the High Luminosity upgrade of the LHC at CERN, which among the many upgrades of the accelerator calls for a few tens of Nb3Sn dipole and quadrupole magnets, operated at 1.9 K and at conductor peak fields up to about 12 T. HL-LHC magnets are in the production phase, marking an historical milestone in accelerator technology and the culmination of 20 years of worldwide R&D. Here we describe a rationale for high field accelerator magnet R&D beyond HL-LHC, consisting of two complementary axes: (i) development of an ultimate Nb3Sn technology, increasing the field reach and achieving maturity and robustness level required for deployment on a large scale and (ii) demonstrating suitability of high-temperature superconductors for accelerator magnet applications. We start with a review of the state-of-the-art, review the main goals, and identify the drivers for an R&D program responding to the declared priorities of the European Strategy Upgrade. This note contains personal considerations intended as a contribution towards the formation of a structured High Field Accelerator Magnet R&D Program.