Effect of the Sub-Elements Layout on the Electro-Mechanical Properties of High Jc Nb3Sn Wires Under Transverse Load: Numerical Simulations

The brittle intermetallic Nb 3 Sn superconductor is currently being used to develop high field magnets in the framework of the Hi-Luminosity upgrade of the Large Hadron Collider at CERN. Despite its excellent superconductive properties,Nb 3 Sn wires suffer from significant critical current Ic reduction due to the transverse load applied during the magnets’ assembly and energization. In high critical current density ( Jc >1200 A/mm 2 at 15 T, 4.22 K) RRP and PIT Rutherford cables when applying a transverse load of 150 MPa, the Ic (12 T, 4.22 K) is reduced respectively by about 10-15% and 20%. At this level of transverse load, the Ic reduction is practically reversible, and it is due to the strain induced in the superconductor, which reduces its upper critical field Bc2. Because of the Bc2 reduction, at 19 T and 4.22 K, with a load of 150 MPa, the same cables are expected to experience an Ic reduction up to 40%. Further increasing the transverse loads, cracks in the superconductor also start to reduce (irreversibly) the Ic . A dedicated FEM 3D numerical model coupled with a Jc scaling law has been developed to predict the electro-mechanical behaviour of RRP and PIT wires under transverse loads in the reversible regime. By using this model, the effects of different geometrical factors have been studied to identify the key parameters that allow limiting the effect of transverse loads on the Ic reduction under transverse load. In particular, this paper deals with the role of the: production technologies diameter, sub-elements layout, heat treatment and precompression.