AC losses in superconducting Nb3Sn and NbTi CIC conductors

AC losses measurements and calculation on large scale Cable-in-Conduit (CIC) conductors have been carried out at NHMFL In particular samples of the Nb₃Sn and NbTi conductors developed for the NHMFL 45-T Hybrid Magnet superconducting outsert have been tested. A calorimetric technique involving superfluid helium (He II) has been used. The samples have been tested with applied field variation rates varying in the range between 0.01 T/s to 30 T/s. From results it appears that for most of the samples, at high field variation rates, the coupling currents are so intense to locally saturate the superconducting strands. As no theories describing the saturation regime in CIC conductors exist, we tried to extend the theories developed for monolithic conductors to the more complicated CICC geometry. It emerges by defining an effective coupling time constant and an effective radius for the conductor that it is possible to fit the data with good accuracy. Effective coupling time constants between 3 ms and 200 ms, depending on the strands surface condition and the cable void fraction, have been measured for the Nb₃Sn conductors, while a time constant of 16 ms has been determined for the NbTi sample. The experimental results and the theoretical analysis are reported in the paper