Superconductors for fusion: Achievements, open issues, roadmap to future

The need of superconducting magnets for fusion reactors has been obvious since over 30 years. In last century, a dozen of fusion devices have been built with superconducting magnets. In the last years the Chinese and Korean tokamaks started operation. Four devices are under construction (SST1, W7-X, ITER, JT60SA). The size, i.e. the energy stored in the magnetic field, has driven the R and D for conductors, from the multi-filamentary composite of Tore Supra, to the monolithic conductors of T15 and the Large Helical Device (LHD), to the cable-in-conduit conductors, which dominate the fusion magnets of the last 15 years. The large electro-magnetic forces on the windings also drove the selection of cooling, from the bath cooling of Tore Supra and LHD to the force flow of supercritical helium in all the other devices. The state of the art on conductor design and performance is reviewed and three open issues in the superconducting magnet technology for fusion are highlighted (performance degradation in Nb3Sn, self field limitation in large NbTi cable-in-conduit conductor (CICC), change of length upon heat treatment of ITER conductors). A projection in the future of superconductors for fusion is attempted, including the role of HTS.