This paper presents a review of the research and development of low loss AC composites with very fine superconducting filaments as well as of their applications at 50/60Hz frequency. At first a review of formulae for calculating AC losses in a superconducting multifilamentary composite (MFC) placed in an AC external transverse magnetic field is given and the energetical feasibility criterion for an AC superconducting winding is formulated. Then it is shown how this criterion has influenced the development of AC MFC. Short description of the technology and of characteristic parameters of actually produced AC MFC, cables and windings is presented. The results of feasibility studies and of R & D works on superconducting AC devices and machines such as superconducting fault current limiter, power transformer, fully superconducting generator, linear induction motor, magnetic energy storage system and thermally controlled superconducting switches are reviewed. The quench current degradation of several kA-class cables as well as of epoxy impregnated windings with non-cylindrical form is the most important issue of the further development of superconductor applications at 50/60Hz frequency.
In order to explain the basic structure of cryostats, several types of cryostats are illustrated and the functions of their parts are also described. The principal paths of heat leak from ambient temperature to a cryostat are described together with a structure to intercept this heat flow. In recent times, cryogenic applications have progressed with the result that they are much demanding of conditions, which are not favorable for cryostat design. One must solve this problem by understanding the heat flow paths in cryostats and tranding off the requirements of applications.
The effects of transverse compressive stress on the critical current, Ic, of two different superconductors, bronze processed Nb3Sn single-core wires and Nb-tube processed Nb3Al multifilamentary wires, have been measured. It has been found that both type of wires show basically the same stress dependence of Ic. Ic's decrease monotonically with externally applied stress and Ic-degradations become larger with increasing magnetic field. The process of Ic-degradations can be divided into 2 stages. In the first stage the Ic-degradation rate with stress is relatively small and lowered Ic's due to applied stress completely recover to the unstressed value when stress is removed, while in the second stage the Ic-degradation rate with stress is large and permanent Ic-degradations remain after removal of load. In the first stage of Ic-degradations due to stress, from an analysis based on the elasticity theory of the stress state of superconducting filaments of the multifilamentary wire under transverse compressive stress, it is suggested that the sensitivity of the critical current to transverse stress is substantially the same as that to axial stress. On the other hand, in the second stage it can be speculated that nonuniform deformation of the matrix may cause local bends or kinks in the superconducting filaments and these damages to the filaments may be chiefly responsible for the Ic-degradations. The stress beyond which the second stage starts are 150MPa and 700MPa for Nb3Sn and Nb3Al wires, respectively. This stress seems to correspond to the yield stress of the matrix material of the wires where superconducting filaments are embedded.
Bronze processed multifilamentary Nb3Sn superconducting wires with the reinforcing stabilizer CuNb in stead of the conventional Cu stabilizer were fabricated. The mechanical properties and the strain dependence of the critical current, Ic, were evaluated at 4.2K and a magnetic field of 15T. A remarkable increase in the yield stress of 70% and the plastic flow stress as compared with those in the wire with Cu stabilizer were observed. The strain for the peak Ic was also increased by 0.2%. Ic on unloading was reversible within the strain range of 1.5%. The strain sensitivity of Ic in the Cu-Nb/Nb3Sn wire was almost the same as that of the Cu/Nb3Sn wire. Decrease in the wire diameter from 0.8 to 0.5mm resulted in a slight increase in the flow stress of the CuNb/Nb3Sn wire, but no change in the strain dependence of Ic. The heat treatment temperature rise from 700 to 750°C resulted in a decrease in the yield stress of 15%, but no change in the strain dependence of Ic. Marked change in morphology of Nb filament in the CuNb reinforcing stabilizer during the heat treatment was evidenced.
We have studied a superconducting bearing system using high-Tc superconductors with a strong pinning force. The superconducting bearing system consists of a superconductor prepared by quench and melt growth (QMG) method and a set of alternating-polarity magnets. This paper describes the dynamic characteristics of the superconducting bearing system with small damped vibrations. An impulse force was applied to the set of alternating-polarity magnets levitated over the superconductor of the system. The vibrations of the set of magnets were measured using a laser displacement meter. Stiffness, damping coefficient, and energy loss of the superconducting bearing system were discussed. The results show that the dynamic characteristics of the bearing system are closely related to the static characteristics. It is necessary to evaluate the dynamic characteristics of the superconducting bearing system in applying the system to a radial bearing with a high rotation speed.
Considering both self-heating of the normal zones propagating in coils wound with large-capacity superconductors and power consumption by external resistors, we have analyzed the temperature-rises in the coils and deduced new analytical formulas by introducing effective decay time constants and by considering energy-balancing in the system. Calculations by using this formula and computer simulations were conducted for small-scaled SMES coils wound with 20kA superconducting cable conductor and comparison was made between these results. It was found that our formula gave a temperature-rise higher by about twenty percents than the computer simulation result, while Wilson's formula gave a temperature-rise much higher than ours and of too safety side. It has been demonstrated that the formula is physically more reasonable and practical than Wilson's. However, the formula is more limited than computer simulations in taking-in of rigorous temperature-dependence of various properties and in dealing with complicated propagations of normal zones, so computer simulations are desirable for us to get more real temperature-rises to be expected in the coils.
A simple automatic supply system for liquid helium has been constructed and tested. This is composed of a 100L liquid container, a cryostat having a vaporizing rate of about 5L/day, a 1m-long flexible transfer-tube, two solenoid valves, a 5W electric heater, a container pressure sensor and controller, and two liquid level sensors and controllers. The system has no bypass and valves for cooldown of the transfer-tube. Since pressurization of the container is performed through vaporization of liquid by the heater, safety apparatuses have been introduced to prevent over-pressure and heating without liquid. A simple and reliable system operation has been proved by the model experiment and ordinary use.