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

为了开发有望用于未来电机、电缆和电力设备的多丝超导线材，必须弄清线材结构与超导性能之间的关系，如上临界场和临界电流密度。这项工作的重点是了解影响磁流变丝的临界电流密度和上临界场的因素。为实现这一计划，设计并测试了测量中频焊丝临界电流密度随温度变化的装置。在此基础上，从理论上分析了结构类似于中子线的层状超导体的上临界场。所取得的成果总结如下：1.采用短周期扫流的方法，在4.2K下测量了磁化丝的场变临界电流密度J_c(H)。扫描周期大于0.2时的J_c(H)测量结果与通常的静态测量结果符合得很好。2.设计并测试了用于测量温度依赖关系的中频线圈样品的低温恒温器。确定了在4.2~20K温度范围内的精确控制，但当电流为1A通过中频导线时，观察到了温升。如果有序参数的成核发生在样品的特定位置，则临界场的角度依赖关系表现为尖点。

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.