Investigation of the ac loss properties of superconducting parallel conductors

我们推翻了用低tc超导导线形成超导体的传统理论，提出了转置并联导线的结构是最适合氧化物超导导线的结构。本文研究了超导并联导体的交流损耗特性。我们首先介绍了磁场的基本方程，该方程是基于这样一个假设，即在均匀磁场中，由一种仅转置一次的矩形截面超导多丝导线组成的两股平行导体。通过求解，我们推导出了由于并联导体形成而产生的额外交流损耗的理论表达式。我们还研究了超导并联导体在薄饼线圈和螺线管线圈中的适用性。所得结论如下。1)将导线调换到最佳位置，抵消联动磁通，不产生屏蔽电流。当换位偏离最佳状态时，会产生额外的交流损耗。2)当感应屏蔽电流小于一条线的临界电流Ic时，额外的交流损耗与场幅值的平方Bm^2和偏差长度的平方Δ1^2成正比，并具有德拜型频率依赖关系。3)当感应屏蔽电流达到Ic时，额外的交流损耗与Bm和Δ1成正比，与频率无关。4)明确了2 ~ 4股并联导线绕线线圈的最佳转位。我们得出结论，在通常情况下，超导并联导体中的交流损耗非常小，以至于交流损耗密度几乎等于组成股中的交流损耗密度。验证了超导并联导体的可行性。

We overthrew the conventional theory for the formation of superconductors with low-Tc superconducting wires and proposed that the configuration of transposed parallel conductors is the most suitable for oxide superconducting wires. In this paper, we investigated the ac loss properties of superconducting parallel conductors. We first introduced the basic equation of magnetic field on the supposition that a 2-strand parallel conductor composed of superconducting multifilamentary wires with a rectangular cross section which are transposed only once, is exposed to a uniform magnetic field. By solving it, we deduced the theoretical expressions of the additional ac losses due the formation of parallel conductors. We also studied the applicability of superconducting parallel conductors to pancake-coils and solenoidal coils. The obtained conclusion is as follows.1) In the case that the strands are transposed at the optimum position and the interlinkage magnetic flux is cancelled out, the shielding current is not induced. The additional ac loss is produced in case that the transposition deviates from the optimum condition.2) In the case that the induced shielding current is less than the critical current of a strand, Ic, the additional ac loss is proportional to the square of field amplitude, Bm^2, and the square of the deviation length, Δ1^2, and has a debye-type frequency dependence.3) In the case that the induced shielding current reaches Ic, the additional ac loss is proportional to Bm and Δ1 and has no frequency dependence.4) The optimum transposition in the coils which are wound with 2- to 4-strand parallel conductors were clarified.We concluded that ac losses in superconducting parallel conductors in the usual situation are so small that the ac loss density is nearly equal to that in the constituent strands. The availability of superconducting parallel conductors was verified.