Abstract. We propose an improved model for the strain dependence of the superconducting properties of Nb 3 Sn.The model is based on the three dimensional strain tensor and derived in terms of the first, second and third invariants, and improves an existing model that only includes the second invariant. The axial form of the new model accurately accounts for the experimentally observed dependence of the effective upper critical magnetic field (H * c2 ) on axial strain, i.e. a quasi-parabolic strain dependence, asymmetry, and an upturn at large compressive axial strain. An accurate model that accounts for the three dimensional nature of strain is important for scaling relations for the critical current that are used to model magnet performance based on wire measurements.

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International audienceA next step of energy increase of hadron colliders beyond the LHC requires high-field superconducting magnets capable of providing a dipolar field in the range of 16 T in a 50-mm aperture with accelerator quality. These characteristics could meet the requirements for an upgrade of the LHC to twice the present beam energy or for a 100-TeV center of mass energy future circular collider. This paper summarizes the activities and plans for the development of these magnets, in particular within the 16 T Magnet Technology Program, the WP5 of the EuroCirCol, and the U.S. Magnet Development Program

Although the high-temperature superconducting (HTS) REBa 2 Cu 3 O x (REBCO, RE = rare earth elements) material has a strong potential to enable dipole magnetic fields above 20 T in future circular particle colliders, the magnet and conductor technology needs to be developed. As part of an ongoing development to address this need, here we report on our CORC ® canted cos θ magnet called C2 with a target dipole field of 3 T in a 65 mm aperture. The magnet was wound with 70 m of 3.8 mm diameter CORC ® wire on machined metal mandrels. The wire had 30 commercial REBCO tapes from SuperPower Inc., each 2 mm wide with a 30 µm thick substrate. The magnet generated a peak dipole field of 2.91 T at 6.290 kA, 4.2 K. The magnet could be consistently driven into the flux-flow regime with reproducible voltage rise at an engineering current density between 400 -550 A mm −2 , allowing reliable quench detection and magnet protection. The C2 magnet represents another successful step towards the development of high-field accelerator magnet and CORC ® conductor technologies. The test results highlighted two development needs: continue improving the performance and flexibility of CORC ® wires and develop the capability to identify locations of first onset of flux-flow voltage.

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