Core-suppressed AC loss and strand-moderated contact resistance in a Nb-Sn Rutherford cable

Calorimetric measurements of AC loss and hence interstrand contact resistance (ICR), have been performed on two types of Rutherford cable wound with unplated Nb3Sn strand. One of the cable types was furnished with a thin core of AISI 316L stainless steel and the other was left uncored. The cables were subjected to pressures of 5, 10, and 20 MPa, respectively, during reaction heat treatment (RHT), and to 100 MPa during measurement. AC loss was measured at 4.2 K in sinusoidal magnetic fields of amplitudes 200 and 400 mT at frequencies of 5 to 90 mHz both with and without the presence of DC bias fields of up to 1 T. The magnetic fields were applied both parallel and perpendicular to the face of the cable. For the cored cables ICR was relatively high (≈78 μΩ). Loss and contact resistance were relatively independent of RHT pressure within the range measured. The contact resistance of the uncored cables were also RHT-pressure independent, however, the absolute values of ICR were much lower, being about 2.7 μΩ with a 1 T background field, and very much smaller in the absence of a background field. The background-field dependence was shown to be due to the presence of an unreacted Nb barrier which enveloped the strand just below the external Cu stabilizer and thereby provided very low resistance coupling paths at small bias fields. The relative roles of strand and cable designs in the determination of interstrand losses are discussed. It is concluded that a low-loss Nb3Sn Rutherford cable has been achieved through the use of a metallic-alloy core.