A basic study on critical field and critical current density in multifilamentary superconducting wires

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In order to develop multifilamentary(MF) superconducting wires which promise to be in use in future electrical machines, cables and electrical power devices, relations between wire structure and superconducting properties such as upper critical field and critical current density must be clarified. Much attention of this work has been directed to the understanding of factors which influence the critical current density and upper critical field of MF wires. To carry out this program, the apparatus for the measurement of temperature dependences of critical current density in MF wires was designed and tested. Further, the upper critical field of layer superconductors similar to MF wires in structure was analyzed theoretically. Obtained results are summarized as follows.1. The field dependent critical current density J_c(H) in MF wires was measured at 4.2 K by the method of sweeping the electric current in short periods. Results of J_c(H) measurement in sweep periods higher than 0.2 s agree well with that obtained in an usual static measurement,2. The cryostat where coil samples of MF wires are set for the measurement of temperature dependences was designed and tested. The exact temperature control from 4.2 to 20 K was confirmed, but the temperature rise was observed when an electric current of 1 A flowed through a MF wire.3. The angular dependences of the critical field exhibit a cusp if the nucleation of the order parameter takes place at the definite position of the sample.