Superconducting magnet technology for the outer coils of resistive-superconducting hybrid magnets

The world's highest-field dc magnets have, for more than 50 years, consisted of a combination of resistive and superconducting (SC) coils that we refer to as a 'hybrid'. These magnets use SC technology for the outer coils, where the magnetic field is moderate, and resistive-magnet technology for the inner coils, where the field is highest. In such a configuration, higher fields have been attained than was possible with purely SC magnet technology, and lower lifecycle costs are attained than with a purely resistive magnet. The peak field available has been 45 T for over 20 years in Tallahassee, Florida, USA. There is presently a 'revolution' underway in hybrid magnet development. A second 45 T hybrid was completed in 2022 in Hefei, China that might be upgraded to 48 T in a few years. The high field lab in Grenoble, France is also testing a hybrid magnet intended to reach 43.5 T but which also might be upgraded to 46 T in a few years. In addition, the lab in Nijmegen, The Netherlands is presently assembling a hybrid magnet intended to operate at 46 T. Papers have been presented and published with conceptual designs of hybrid magnets with fields up to 60 T. Given the developments underway, this is an appropriate time to review the history of such systems, with a particular focus on the larger, more expensive part of the magnets: the SC outsert coils. The demands placed on the SC coils of these magnet systems are unique due to their coupling with resistive coils that are operated at very high stress and wear out regularly, resulting in large field transients and fault forces. The evolution of the technology used for the SC coils of these hybrid systems is presented, evolving from ventilated windings to cable-in-conduit to cryogen-free.