Evaluation method of critical current and current sharing temperature for large-current cable-in-conduit conductors

An evaluation method of critical current (I_c) and current sharing temperature (T_cs) is proposed and applied to the experiment of ITER Center Solenoid (ITER-CS) Model Coil Insert, which is a Nb₃Sn superconducting coil. Voltage behavior related to normal state transition of conductors during I_c or T_cs measurement is not yet well understood especially in the case of such a large cable with more than one thousand strands as the ITER-CS Insert, because the magnetic field, which has a large effect on its superconducting property, is not constant inside the cable. From the detailed analysis of the voltage behavior of coils, it is found that the integral of electric field averaged over conductor cross section along strand-longitudinal direction is equal to the voltage which is measured by voltage taps during a coil test. This is because the twist pitch of a cable is less than the range of longitudinal field variation in the case of a large-cable-conductor coil. This evaluation method can estimate voltage behavior and predict I_c and T_cs values, which are important parameters for the design of a large-cable-conductor coil, based on the property of the strands composing the conductor.