Magnetic properties of Ni-based substrates for HTS tape

Ni-W alloys are used as the substrate in the manufacture of HTS tape via the RABiTS (TM) process. Theoretical work has shown that the ferromagnetism in a substrate can affect the ac losses in the superconductor. This occurs in part because of the increased magnetic permeability, which alters the magnetic induction in the superconductor and thereby the loss. Magnetic hysteretic loss in the substrate is an additional contribution to overall ac loss. We report measurements of both permeability and magnetic loss as a function of frequency and magnetic induction for the standard alloy Ni-5W as well as composite substrates incorporating non-magnetic Ni-W alloys. The measurements are done using ac susceptibility at 77 K, at frequencies up to 10(4) Hz and inductions in the range 1-150 mT. It is shown how to use these results to predict total ac loss in isolated superconducting tape and wound coils. Some important magnetic parameters cannot be obtained with sufficient accuracy using a SQUID or vibrating sample magnetometer, since the size of a sample is limited in these measurements. This makes the demagnetizing factor large enough to degrade the accuracy of a permeability measurement.