Masataka Iwakuma

Enhanced Jc properties in superconducting NbTi composites by introducing Nb artificial pins with a layered structure

High critical current densities (4250 A/mm2 at 5 T, 4.2 K, and 15 000 A/mm2 at 2 T, 4.2 K) were achieved by using an artificially designed pin structure composed of randomly oriented clusters of Nb/NbTi multilayer in a NbTi composite superconductor (with a round cross section). The maximum values were obtained when the Nb layer thickness was designed to be approximately twice the coherence length in NbTi, where the layer thickness is five to ten times thicker than that of the optimized α‐Ti ribbon thickness in a conventional NbTi system.

Effects of artificial pins on the flux pinning force and other superconducting properties in NbTi superconductors

A marked enhancement of global pinning strength in Nb-50 wt.%Ti alloy was obtained by introducing ribbon-shaped artificial pins. Simultaneously, however, depression of B/sub c2/ was observed. This behavior is analyzed theoretically, and a satisfactory explanation of the experimental results is obtained.<<ETX>>

Fabrication of inner secondary winding of high-T/sub C/ superconducting traction transformer for railway rolling stock

We studied the possible application of high-T/sub C/ superconducting traction transformer to railway rolling stock, and designed an iron core type two-leg 4 MVA superconducting traction transformer for Shinkansen. This traction transformer has a primary winding, four secondary windings and a tertiary winding. The four secondary windings are independent of each other. Two secondary windings are arranged around each leg of the iron core and placed inside and outside the primary and tertiary windings. In this study, we fabricated a high-T/sub C/ superconducting coil whose form and dimension are the same as those of the inner secondary winding. This coil can be used as the inner secondary winding of the experimental high T/sub C/ superconducting traction transformer that will be fabricated in the future. The inner secondary winding has 96 turns and a layer with eight parallel Bi2223 superconducting tapes. Transposition among the superconducting tapes is performed 15 times every six turns. We measured the voltage-current and AC loss characteristics when it was cooled in saturated and sub-cooled liquid nitrogen.

Fabrication of winding model of high-T/sub c/ superconducting transformer for railway rolling stock

We fabricated two high-T/sub c/ superconducting coils that simulated the winding of a traction transformer for railway rolling stock. The multi-layer solenoid coil to simulate the primary winding of the transformer had five layers with a single Bi2223 superconducting tape. The closed solenoid coil to simulate the secondary winding of the transformer had one layer with eight parallel Bi2223 superconducting tapes. We measured the voltage-current, AC loss and current sharing characteristics of these coils cooled in saturated liquid nitrogen at 77 K. As a result, we concluded that the multi-layer solenoid coil is applicable to the primary winding and the closed solenoid coil is also applicable to the secondary winding.

Experimental results of the model coil for cooling design of a 1 T cryocooler-cooled pulse coil for SMES

The authors have been developing high-Tc superconducting coils for SMES applications. Their primary goal is to make a HTS coil which is cooled to 40 K by a single-stage cryocooler and continuously operated at 1 Hz with a field amplitude of 1 T. The coil has heat drains of AlN plates to remove heat because of AC losses. They made a cooling model coil system to study the effective arrangement of the heat drains. The system consisted of a model coil using Cu conductors, current leads and a cryocooled system. The test coil was divided into three sections in different arrangement of heat drains. The model coil was daubed with a high thermal conductivity epoxy resin to improve thermal contact resistance between the conductors and AlN plates. They tested the coil by Joule heating which was equal to AC losses. They measured the temperature distribution in the coil and the temperature difference between Cu conductors and AlN plates. The temperature difference was measured between 0.2 K and 0.7 K. The results will be applied to the 1 T HTS coil design.

Levitation pressure and stiffness of a high Tc superconducting bearing in field‐cooling process

Properties of a high Tc superconducting bearing consisting of a set of alternating‐polarity magnets and a superconductor are investigated. The superconducting bearing is field‐cooled in liquid nitrogen, because it is expected that field‐cooling process will be used for superconducting bearings in the future. To analyze the levitation pressures of the superconducting bearing Bean’s critical state model was applied. The results of the analyses show that it is possible to estimate the levitation pressures to some extent by using the model.