Modeling of the electro-mechanical behavior of ITER Nb3Sn cable in conduit conductors

The coupling of electrical and mechanical modeling of superconducting cable in conduit conductors (CICCs) can be useful for the understanding of the complex phenomena occurring in cables based on strain-sensitive materials like Nb3Sn. The MULTIFIL model is a detailed mechanical model aimed at computing the strain distribution at the strand level in multistrand superconducting cables. MULTIFIL is a finite element code dealing with the contacts between beam assemblies that includes plasticity and it allows modeling both transverse and longitudinal loads applied to the cable. The electromagnetic part of the THELMA code is based on a distributed parameter electrical circuit model of CICCs and is aimed at computing the current distribution and electrical losses at an arbitrary cabling stage. In this work, the maps of strain have been computed with MULTIFIL for a petal of a relevant conductor for the central solenoid of the ITER magnet system. These strain maps were computed in three main cases, namely applying only a thermal compressive strain due to cable contraction in cool-down, considering thermal strain plus the strain due to Lorentz force in a virgin state, and thermal strain plus Lorentz force after a given number of electromagnetic cycles. These strain maps have been implemented in the THELMA code, in order to compute the E–T curves corresponding to the different cases, such deriving important parameters to be compared with experimental results (Tcs, n value, effective strain). A methodology to account in the electrical model for the inhomogeneous strain on the strand cross section due to bending is described and the corresponding results are compared to the case of a homogeneous strain on the wire cross section. The model is able to explain the experimental difference between the physical strain that can be determined with Tc measurements and the effective strain that can be derived from the Tcs measurements. (paper)