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A comparative study of in situ MgB 2 wire and MgB 2 bulk was carried out to clarify the mechanism for the high critical current density, J c , in the practical in situ MgB 2 wires. The in situ MgB 2 wire was manufactured with an area-reduction ratio of 99.93%, which was one of the highest values in MgB 2 superconducting wires previously reported. The electrical connectivity, K , and the flux pinning strength, F p , which are important factors in explaining the behavior of J c , could be determined in the same manner as those for the bulk sample; K was well understood with the three-dimensional percolation model, and F p was effectively explained by the electron scattering mechanism by grain boundaries. On the other hand, the area-reduction process dramatically enhanced the value of K , leading to an increase in the value of J c . The respective values of K and J c (20 K, ∼0 T) reached 0.24-0.34 and 6.0 × 10 3 -8.4 × 10 3 A mm −2 , which were twice or three times higher than those of typical in situ bulks. This is because the plastic deformation of magnesium particles increased the packing factor of raw powders through a repetitive drawing process.

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AC losses in superconductors are essential for the design of cooling system for large scale power applications. Magnesium diboride (MgB2) superconducting wires have been investigated and manufactured over the last decade due to cheap raw materials and flexibility for coil design. In addition, multifilamentary MgB2 wires have been manufactured to reduce AC losses. In this paper, self-field AC losses of multifilamentary MgB2 wires with magnetic barrier were investigated using both experimental and numerical methods. A short straight wire sample and a coil sample were tested under various temperatures and frequencies between 16 Hz and 128 Hz. The test results show that the transportation loss is independent of the operating temperature. On basis of both theoretical and numerical study, it is found that hysteresis loss in superconductor accounts only for a small fraction of the transportation losses, ferromagnetic hysteresis loss in the magnetic barrier dominates when the transport current is low, whereas eddy current loss dominates when the transport current is close to the critical current.