EFFECTS OF FILAMENT SIZE ON SUPERCONDUCTIVITY IN MULTIFILAMENTARY WIRES

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To realize superconducting devices operated by AC power sources of 50-60Hz, the development of multifilamentary superconducting wires with finer filaments and CuNi barrier of high resistivity is in progress. In this work, we have investigated the effects of filament size on superconductivity from experimental and theoretical standpoints, especially, in ultra-fine multifilament NbTi wires consisting of filaments of 0.1 mum or below in diameter, which are promising wires for AC use. Obtained results are summarized as follows.1. Ultra-fine multifilament wires show high critical current density j_c compared to conventionally fabricated NbTi wires with filaments of 1 mum or above in diameter. Their field dependence, however, is highly strong and the critical current density is appreciably lowered with increasing temperature. These results suggest that the performance of applications being constructed of these wires is considerably affected by temperature and magnetic field.2. Most of investigated wires show anomalous high-J_c nature in temperatures near critical temperature T_c suggesting that the mechanism of flux-pinning varies with temperature.3. In superconducting superlattices where two layrs with different Debye frequencies alternately pile up, values of upper critical field B_<c2> parallel and perpendicular to the layr plane are appreciably different. Similar results are seen in superlattices with different BCS coupling constants.The pinning mechanism in multifiamentary superconducting wires must be clarified and their temperature and field dependence should be improved.