This paper reviews the recent developments of Nb₃Sn superconductors and proposes prospects for future improvements, with a particular focus on the activities running at UNIGE. Since the last few decades, Nb₃Sn is holding the leading place in the domain of high-field applications, which includes nuclear magnetic resonance (NMR) spectrometers, magnets for plasma confinement in tokamak fusion experiments and laboratory magnets. More recently, the goal of a 100 TeV proton-proton collider set by the high-energy physics community initiated a focused R&D program coordinated by CERN to push Nb₃Sn towards its upper limit of performance. The baseline configuration for this Future Circular Collider (FCC) requires superconducting dipoles generating a magnetic field of 16 T.  The prime challenge to reach this target is the development of conductors that exceeds the performance of state-of-the-art Nb₃Sn wires in terms of both critical current density (J_c) and tolerance to mechanical stresses. Hence, the advancement of Nb₃Sn technology must build on novel processing routes scalable at the industry level and with a full control of the material at the nanoscale dimension, tailoring the vortex-pinning landscape to enhance J_c, and at the microscale dimension, linking the electromechanical behavior to the wire architecture. At UNIGE, we are investigating methods for the inhibition of the grain growth in Nb₃Sn by means of nanoparticles – typically ZrO₂ or HfO₂ – that form through an internal oxidation process. Our prototype rod-type multifilamentary wires exhibit average grain size £ 50 nm, i.e. two to three times smaller compared to the optimized industrial conductors. The resulting increase in grain boundary density combined to the presence of additional point-like pinning centers due to the oxide nanoparticles lead to a significant enhancement of J_c, which extrapolates beyond the target of 1’500 A/mm² at 16 T and 4.2 K set for FCC. Interestingly, our wires exhibit also record-high upper critical fields above 29 T at 4.2 K. The other aspect where we are focusing our research concerns the tolerance to stress of Nb₃Sn wires. It appears clear from various conceptual studies that the design of a 16 T dipole entails electromagnetic stresses in the 200 MPa range and, thus, it becomes crucial to establish precisely the mechanical limits at which Nb₃Sn can operate safely. Permanent reduction of the critical current occurs from the combination of the effects of cracks in the Nb₃Sn filaments and plastic deformation of the Cu matrix and we present here a method to identify the dominant mechanism. The method builds on electromechanical measurements and a novel non-destructive and non-invasive tool to investigate wires’ internal features that combines X-Ray tomography and deep learning networks. The implications of our results towards the development of practical multifilamentary Nb₃Sn wires reaching the FCC specification is also discussed.

This work was done under the auspices of CHART (Swiss Accelerator Research and Technology Collaboration, https://chart.ch. Research supported by the Swiss National Science Foundation (Grant No. 200021_184940) and by the European Organization for Nuclear Research (CERN), Memorandum of Understanding for the FCC Study, Addendum FCC-GOV-CC-0175 (KE 4663/ATS) and Addendum FCC-GOV-CC-0176 (KE 4612/ATS). Research also supported by the European Synchrotron Radiation Facility (Grant No. MA-2767) and by LNCMI-CNRS, a member of the European Magnetic Field Laboratory (EMFL).

Keywords: Nb3Sn wire development, internal oxidation, x-ray tomography, machine learning