AC Loss and Contact Resistance in Nb₃Sn Rutherford Cables With and Without a Stainless Steel Core

Abstract

Calorimetric measurements of AC loss and hence interstrand contact resistance (IRC), were measured 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 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 transverse fields were applied both parallel and perpendicular to the face of the cable. For the cored cable IRC was relatively high, typically 70 μΩ at 1 T. On the other hand for the uncored cable at 1 T, IRC was more than an order of magnitude lower; also for the uncored cable IRC was found to decrease still further with decreasing bias field. Since the strand contained a Ta-protected Nb diffusion barrier it was deduced that the application of the bias field altered the way in which eddy currents flowed within the strand, in one case (0 T) via the Nb barrier and the Cu sheath, and in the other (1 T) via the A15 filaments, the bronze, and the sheath, which led to a higher IRC. It is concluded that AC loss in a Nb₃Sn-strand Rutherford cable can be significantly moderated both by paying attention to strand design as well as through the insertion of a core.