Implications of the strain irreversibility cliff on the fabrication of particle-accelerator magnets made of restacked-rod-process Nb3Sn wires.

The strain irreversibility cliff (SIC), marking the abrupt change of the intrinsic irreversible strain limit εirr,0 as a function of heat-treatment (HT) temperature θ in Nb3Sn superconducting wires made by the restacked-rod process (RRP®), is confirmed in various wire designs. It adds to the complexity of reconciling conflicting requirements on conductors for fabricating magnets. Those intended for the high-luminosity upgrade of the Large Hardon Collider (LHC) at the European Organization for Nuclear Research (CERN) facility require maintaining the residual resistivity ratio RRR of conductors above 150 to ensure stability of magnets against quenching. This benchmark may compromise the conductors' mechanical integrity if their εirr,0 is within or at the bottom of SIC. In this coupled investigation of strain and RRR properties to fully assess the implications of SIC, we introduce an electro-mechanical stability criterion that takes into account both aspects. For standard-Sn billets, this requires a strikingly narrow HT temperature window that is impractical. On the other hand, reduced-Sn billets offer a significantly wider choice of θ, not only for ensuring that εirr,0 is located at the SIC plateau while RRR ≥ 150, but also for containing the strain-induced irreversible degradation of the conductor's critical-current beyond εirr,0. This study suggests that HT of LHC magnets, made of reduced-Sn wires having a Nb/Sn ratio of 3.6 and 108/127 restacking architecture, be operated at θ in the range of 680 to 695 °C (when the dwell time is 48 hours).