A comparison of the E-J characteristics of four NbTi superconducting composites at equivalent electric field levels from 10-4 to 10-7 V m-1

The four-terminal short-sample test used to measure the critical currents of commercial superconducting composites typically employ equivalent electric field levels of 10^-5 and 10^-4 V m^-1 to specify the onset of the superconducting-resistive transition. The authors describe refinements to this technique that enable resolution of the transition at the 10^-7 V m^-1 level. The techniques allowing access to the very lowest electric field regimes are based on the measurement of the decay rate of current in a continuous loop of a superconducting composite in background field. Data on four samples of NbTi conductor are compared over the full electric field range available using the above method, illustrating the influence of composite geometry and matrix material on the detailed form of the E-J characteristic