The principle of a homopolar machine (acyclic machine or unipolar machine) has been known since the days of Faraday, and the machine features a direct current machine of low voltage and large current. The construction of this machine is simple and solid, and its application was long awaited. In the 1970s, the practical large superconducting magnet became available on the market, and the merit of homopolar machines it provided was reconsidered, bringing about many developments in various fields. The technology of an efficient and stable large current collection from the high-speed rotating armature was pursued with a carbon brush or liquid metal. Today, homopolar machines are worth being again reconsidered, by a result of the advent of the easy handling conduction cooling magnet and the technological development of the high temperature superconducting magnet. In this paper, the development of a 3, 000kW superconducting homopolar generator, the principle of the homopolar machine, an equivalent circuit, a collector, characteristics, development, and application are described.
The influence of tensile stress on superconducting properties in Ag/Bi2223 monocore tapes has been investigated, especially at the initial stage of fracture. The fracture of oxide core was found to start at the stress lower than the macroscopic fracture strength of oxide observed as the drop in the stress. Stress and electric voltage were measured simultaneously during the tensile test at 77K. A gradual increase of voltage was observed at the stress fairly lower than that of macroscopic fracture as a stress-strain curve deviated from elastic behavior and its slope began to decrease. These results suggested that the initiation of microcracks occurs in the oxide before the macroscopic fracture. Both Ic and n-value decreased with increasing stress, but the change of n-value was more drastic than that of the Ic. The V-I curves before and after loading were analyzed by assuming that the generation of voltage was dominated by the nonsuperconducting transition of grain boundaries.
An Nb3Al conductor with stainless steel (SS) conduit has been developed in the framework of ITER Engineering Design Activity. The critical current performance of this conductor was experimentally investigated by using a short conductor sample. The results show that the critical current achieves the expected, resulting in demonstrating the validity of the fabrication method. The thermal strain is generally applied to the strands if SS conduit is used because of the different thermal expansion between the strands and SS conduit. However, it was not applied in our sample, since the conductor length, a few meters, is too short to keep it by friction force between the cable and conduit. The thermal strain applied to the strands in a long conductor is therefore estimated by using the solid model, in which the strain on the strands and conduit are calculated from the equilibrium of their thermal forces, ignoring the effect of cabling on relaxation of the strain on the strands. The results indicate that the Nb3Al strands are subjected to a compressive thermal strain of less than 0.4%. The critical current degradation by this is less than 10%, even though the SS conduit is used. Consequently, its critical current is estimated to be 90kA, which is sufficiently high compared with the nominal, 46kA, at 13T and 4.5K. Furthermore, it should be noted that the large thermal force, 80kN, appears after the heat treatment at room temperature.
Artificial flux-pinning centers have been introduced into NbTi filaments through the drawing process for the achievement of high critical current density. We discussed periodic patterns fabricated by a lithography instead of the usual drawing process as effective artificial flux-pinning centers for practical superconductors. Periodic grooves and also lattice like holes with various depths were formed on Nb films for evaluation of the flux-pinning effect. The Nb films with 0.22μm thickness were developed on MgO (100) by electron-beam evaporation. The flux-pinning properties of the fabricated Nb films were measured by using a SQUID magnetometer. It is found that the magnetization of the fabricated Nb films increased; thus the patterns work effectually as artificial flux-pinning centers. The increase of magnetization of the film with a groove pattern was two times as large as that of the film with a hole pattern. It is explained from the effectiveness of the groove pattern for the Lorentz force. The magnetization of films with both patterns increased until fabrication depth reach half the film thickness. It is considered that this increase of magnetization is caused from an increase of elementary pinning force for each flux line.