Investigation of the ac loss properties of superconducting parallel conductors

我们推翻了用低T_c超导线形成超导体的传统理论，提出了氧化物超导线最适合的换位平行导线构型。本文研究了超导平行导体的交流损耗特性。我们首先介绍了磁场的基本方程，假设由多根矩形截面的超导导线组成的两股平行导体只换位一次，暴露在均匀的磁场中。通过对该问题的求解，推导出了形成平行导线所产生的附加交流损耗的理论表达式。我们还研究了超导平行导体对薄片线圈和螺线管线圈的适用性。得到的结论如下：1)当磁链在最佳位置换位并抵消互连磁通时，不会产生屏蔽电流。在换位偏离最佳条件的情况下产生附加交流损耗。2)在感应屏蔽电流小于线束的临界电流Ic的情况下，附加交流损耗与场幅的平方Bm^2和偏离长度的平方Δ1^2成正比，并且具有德拜型频率依赖关系。3)在感应屏蔽电流达到Ic的情况下，附加交流损耗与BM和Δ1成正比，与频率无关。4)明确了绕制2-4股平行导线的线圈的最佳换位。我们得出结论：通常情况下，超导平行导线的交流损耗很小，交流损耗密度几乎等于组成线的交流损耗密度。验证了超导平行导线的有效性。

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.