Irreversible Degradation in Nb3Sn Cable in Conduit Conductors

One of the biggest goals for the international thermonuclear experimental reactor (ITER) is the steady state operation. For this reason all its coils will be superconducting and Nb3Sn material is used for both the toroidal field (TF) and the central solenoid (CS) coils. The primary current carrying unit in a coil is the cable, which contains hundreds of superconducting strands. The ITER coils will be made using a kind of cable called cable in conduit conductor (CICC). Both CS and TF magnets have to withstand several electromagnetic (EM) cycles in their lifetime. This has been discovered to cause a severe reduction of the magnet performances. Nb3Sn is a brittle material, therefore it is very strain sensitive. The knowledge of the strain state in a CICC allows the explanation of the performances of the cable itself, providing also information regarding the permanent damaging of the Nb3Sn structure. The measurement of the Nb3Sn strain state in a CICC is a very challenging task because it is not possible to make it directly. An indirect technique that allows the measurement of the thermal strain through the measurement of the magnetic susceptibility vs. temperature (χ(T )) for a CICC has been developed. A CICC is a complex system and as such it behaves in a way which is not directly predictable studying the single components. The elastic deformation of the superconducting layers produces reversible phenomena which interact with irreversible phenomena due to Nb3Sn fractures and to plastic deformations of the matrix surrounding the superconducting layers. Reversible and irreversible phenomena occurs together and are deeply shuffled such that it is not straightforward the explanation of the cable performance. The magnetic susceptibility measurements, via an appropriate data analysis, allow to quantify and to separate the thermal strain from the jumble of parameters necessary to describe the cable performance. Several TF and CS samples have been measured with this technique in the SULTAN test facility. Each cable behaves differently from the others but at least it is possible to identify common features. For some samples, the evolution of the performance with the EM cycles is dominated by reversible rearrangements of the strand position in the cable and consequently by the change of the strain distribution during the EM load, for others it is dominated by breakages and irreversible phenomena.