Magnetic field profiles of NbTi tapes

Magnetic field profile measurements were performed on stabilized NbTi tapes in zero applied field carrying large transport current densities. Field profile data were taken on large diameter superconducting sample loops at temperatures between 2 K and Tc(about 9.2 K) and several persistent currents up to Ic(T), the maximum. The field profiles scaled with current, independent of temperature, at temperatures below 8.5 K and were well described by an elliptical shell approximation which left a current-free, field-free central reqion. Critical state models applied to this self-field situation proved inadequate. Complex profiles produced by trapped flux in the tape loop were described quantitatively by assuming concentric elliptical shells each with a different current density. The maximum current which could be persisted in the loop at each temperature was linear from 4 to 8.5 K. The effective resistance of the superconducting loop including its joint was less than 10^-13Ω.