Inter-strand resistance measurements in cored Nb-Ti Rutherford cables

Cored Nb-Ti Rutherford cables with high crossover resistance R_c are being investigated to minimize ac losses in a set of superconducting dipole magnets for the proposed rapid cycling heavy ion synchrotron facility at the Gesellschaft fur Schwerionenforschung (GSI) in Darmstadt, Germany. This ring of magnets is based on the Relativistic Heavy Ion Collider (RHIC) design. R_c in cored cables is significantly larger than the contact resistance between adjacent strands R_a. The latter, however, dominates the voltage profile when the interstrand contact resistance is measured via the usual VI technique. This makes the extraction of the magnitude of R_c out of the voltage profile very difficult. Knowledge of R_c is important to predict ac losses, because if R_c and R_a were equal, perpendicular field induced losses due to R_c would be around 50 times greater than those induced by R_a. Because of the importance of the crossover resistance, we have developed a novel measurement technique for R_c in cored Nb-Ti Rutherford cables. The procedure involves removing the edges of the conductors along the region to be measured. We have measured the cables using the new technique and the usual VI technique. We describe the novel measurement technique and present results for cables made with different core materials. While measuring R_c, we found evidence that R_a is nonuniform over the length of a twist pitch and is in fact significantly lower at the cable edge. We report on this as well.