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We have investigated the effect of aspect ratio and current on the self-field critical current of Ag-sheathed Bi-2223 composite tapes. We have fabricated assemblies of monofilament tapes with different aspect ratio and current capacity. In such a manner, we could study the effect of the composite geometry on the self-field suppression of the critical current while maintaining the quality of the superconducting filaments constant. We find that a large aspect ratio is favorable for achieving higher self-field critical currents which is consistent with a reduction of the self-field. We have also used a modified configuration of several tapes with opposing currents to reduce the perpendicular component of the self field. As a consequence, we have observed large enhancements of the critical current at zero applied magnetic field, and have also been able to reveal the magnetic field dependence of the critical current at low magnitudes of the external field where the self-field usually dominates the externally applied field. We have studied these effects in several tapes with different levels of current capacity.
A vital requirement for commercial application of Bi-2223 tapes is that the high critical current densities currently obtainable in short or intermediate lengths can be maintained when piece-lengths are scaled up by more than an order of magnitude. In this paper the effects of statistical variations in local I-V characteristics on the critical current of long superconducting tapes are investigated computationally. The local I-V characteristic is parameterized by the standard power law parameters I, and index n. Analytic solutions and simulated data are presented for a number of statistical distributions in local I-V parameters, including Gaussian, Weibull and polynomial distributions. The study concludes that for typical parameters, given local statistical variations in I,, there should be no significant loss in performance as tape lengths are scaled up by factors of 10 to 100.
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