Superconductivity & Particle AcceleratorS 2026

The turn towards compact, high-field magnetic confinement has placed REBCO coated conductors at the centre of fusion magnet design, and in doing so has moved the limiting engineering problems out of the superconductor and into the materials surrounding it. This contribution reviews those problems from a magnet-technology perspective shared with accelerator applications.
After brief remarks on the present status of magnetic confinement devices, the field and strain limits of NbTi and Nb₃Sn are summarised together with the long-lived activation of niobium, and the ITER design fluence of 10²² n/m² is identified as an extrapolation from fission-spectrum data. The comparison between REBCO tape and Bi-2212 round wire is discussed in terms of cabling and jointing rather than critical current, and Mg¹¹B₂ is considered as a conductor selected primarily for its neutron behaviour, for which ¹¹B enrichment is a precondition rather than an optimisation.
Four unresolved problems are then examined: delamination of coated conductors, where the transverse strength is roughly two orders of magnitude below the longitudinal; resin impregnation, which typically fails between 10 and 100 MGy and often limits magnet life before the superconductor does; superconducting joints, which remain unsolved for REBCO and degrade under irradiation through transmutation of the solder; and copper stabilisation, where displacement damage recovers on warm-up while transmutation does not.
Recent in-situ measurements of REBCO during neutron irradiation, which indicate degradation at fluences well below ex-situ expectations together with recovery on warming, are discussed as evidence that these properties must be measured in beam and at cryogenic temperature. Finally, the consequences of a non-deuterium–tritium fuel cycle for magnet design are considered.