IOP : Irreversible degradation of Nb$_3$Sn Rutherford cables due to transverse compressive stress at room temperature

In the framework of the Future Circular Collider design study for a 100 TeV circular collider, 16 T superconducting bending magnets based on Nb3Sn technology are being developed. A pre-stress on the conductor during magnet assembly at room temperature (RT) is needed to counteract the Lorentz forces during operation. The superconducting properties of the brittle Nb3Sn superconductor are strain sensitive and excessive pre-stress leads to an irreversible degradation of the superconductor. In order to determine the level of acceptable pre-stress during the magnet assembly process, reacted and impregnated Nb3Sn cables were exposed to increasing transverse compressive stress up to a maximum stress level of 200 MPa at RT. After each stress cycle, the critical current of the cable specimens were characterized at 4.3 K in the FRESCA cable test station. No significant critical current degradation was observed up to 150 MPa, followed by degradation less than 4% after a nominal stress of 175 MPa. A dramatic permanent critical current degradation occurred after applying a nominal stress of 200 MPa. A comprehensive post analysis consisting of non-destructive micro-tomography followed by microscopic characterization of metallographic cable cross sections was carried out after the critical current test to reveal cracks in the Nb3Sn sub-elements of the loaded specimen.