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

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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