Development in design technology of advanced MgB_2 wires for future electric device windings

二硼化镁是一种新的金属超导体，其临界温度比通常的金属超导体Nb Will高约20K，而且由于其制备工艺简单，有可能开发出无液氦冷却的低成本先进超导导线。本研究的目的是评估超导线材的各种性能，并提出改善其电磁和热性能的结构设计和先进结构的制备工艺。主要研究结果如下：1)从改善超导线材特别是高磁场下的输运性能的角度出发，提出了添加杂质的原位方法和控制初始粉末的离体法的改进方法。2)作为长尺寸低交流损耗…线材的候选材料，CuNi护套和Ta阻挡层复合线材获得了成功。用枪钻工艺原位制备了更多的LLY，并用标准化的拾取线圈法测量了它们的交流损耗。在实验研究的基础上，提出了低交流损耗多芯超导线材的结构设计方案。3)通过在液态氦中或传导冷却条件下利用加热器诱导初始热斑的方法，对多芯超导线材中正常区的传播进行了实验研究，并用考虑瞬时冷却效应的理论热模型很好地解释了正常区的响应，为未来的MGB_2线圈失超分析提供了基本方法。通过比较导线和绞线的临界电流，测试了16股组装的大容量导线的稳定性。结果表明，未来的MGB_2线圈可以在失超保护设计中具有很高的稳定性和灵活性。4)采用缠绕和反应法制备了100m级多芯线材的导冷试验线圈。在15K-30K的温度范围内进行了直流过电流和交流电流的特性测试。实验表明，由于其临界温度高于30K，因此具有较高的稳定裕度。较少

Magnesium diboride, MgB_2, has a new metallic superconductor with a critical temperature about 20 K higher than Nb'll, one of usual metallic ones and furthermore a potential to develop low-cost advanced superconducting wires without liquid helium cooing because of the simple fabrication processes. Aim of the present study is to evaluate various properties of superconducting MgB_2 wires and to propose structural design for improving the electromagnetic and thermal properties and fabrication processes for the advanced structures. The main results in the study are summarized in the following.1) From a viewpoint of improving the transport property of superconducting MgB_2wires especially in high magnetic field, advanced methods were given for the in situmethod by addition of impurities and for the ex-situ method by controlling initial powders of MgB_2.2) As a candidate of long-scale low-AC-loss MgB_2 wires, composite wires with a CuNi sheath and Ta barriers for the filaments were successfu … More lly fabricated by the in-situ method with a gun-drill process and their AC losses were measured by a standardized pickup coil method. The results were reproduced by the usual theory of superconducting multifilamentary wires and, on the basis of the empirical study, structural design for the lower AC loss wires was proposed.3) For basic approach to quench analysis in future MgB_2 coils, normal zone propagation in the wires were experimentally studied by inducing an initial hot spot by a heater in liquid helium or under a conduction cooling condition, and the responses of the normal zone were well explained by theoretical thermal model with transient cooling effects. The stability in a large-capacity conductor with assembled16strands was tested by comparing the critical current of the conductor with that of the strands. The results suggest future MgB_2coils can be constructed with high stability and flexibility in the quench protection design.4) A conduction-cooling test coil with a100m-class MgB_2multifilamentary wire was successfully fabricated by a wind and react method. The characteristic tests with direct over current and alternating current were performed in a temperature rangebetween 15K and 30K. It was shown experimentally that MgB_2coils have a high stability margin because of the critical temperature higher than 30K. Less