Makoto Tsuda

Characteristics of lift and restoring force in HTS bulk-Application to two-dimensional maglev transporter

One of the advantages of magnetic levitation using a high-temperature superconducting (HTS) bulk is that stable levitation can be achieved without any control systems. The authors have been investigating the electromagnetic behaviors of an HTS bulk to realize a two-dimensional magnetic levitating transporter without any fixed guides. The characteristics of lift and stability are key parameters to design and optimize such a device. They measured the lift and the restoring force of a YBCO bulk, displaced by a distance in lateral direction, for various field-cooling conditions and permanent-magnet arrangements. Both lift and restoring force are closely related to the air gap in the field-cooling process, distance between the permanent magnets, number of permanent magnets, and permanent magnet arrangement, that is, external magnetic field distribution. The most suitable arrangement of permanent magnets depends on the required levitation height and the weight of the levitating part. It can be considered that the size of the levitating part and geometry are also very important to determine the optimal arrangement in the maglev device.

Trial production and experiments of linear actuator with HTS bulk secondary

To investigate characteristics of a linear actuator with high-temperature superconducting (HTS) bulk secondary, a short secondary type linear actuator has been designed and constructed. The actuator is comprised of an YBCO bulk secondary (mover) and copper windings with iron core (primary). A zero-field-cooled bulk located at the center of the actuator plays a role of generating thrust, while four field-cooled (trapped field) bulks are used for levitation and guidance of the secondary. The authors measured the starting thrust force and magnetic field distribution in the air gap. They developed a simulation program based on the finite element method (FEM) taking the voltage-current (E-J) characteristics of the bulk into consideration to investigate electromagnetic behaviors within the bulk exposed to a time-varying magnetic field. Agreement between experiment and simulation is good, and it validates the simulation program and the presented assumptions in the numerical approach, using the simulation program, they investigated the dependency of n-value and critical current density of the bulk material on the magnetic flux density in the air gap and the starting thrust force. Supercurrent density within the bulk is a key factor for the magnetic flux density and the thrust force.

Static and dynamic characteristics in levitating X-Y transporter using HTS bulks

High T/sub c/ superconducting bulk has been used for magnetic levitation systems such as flywheel energy storage system, noncontact transport system and so on. We have been investigating electromagnetic behaviors of HTS bulk to realize a X-Y (two-dimensional) magnetic levitating transporter without any fixed guides. We have investigated qualitatively the characteristics of lift and restoring force of a YBCO bulk for various field-cooling conditions and permanent-magnet arrangements. In this paper, we evaluated the most suitable conditions of air gap in field-cooling process and permanent-magnet arrangement through lift and restoring force measurements and numerical analysis by the finite element method (FEM) based on the current vector potential method. The characteristics of oscillation in the vertical direction of levitating X-Y transporter were investigated by the measurements of free and forced vibrations. The theoretical characteristics of the free and forced vibrations were compared with the experiments.

Homogeneous current distribution in a coaxial superconductor with and without return current path

The authors have developed a theoretical method based on magnetic flux conservation between adjacent layers. One of the advantages of this method is that one can directly obtain twisting pitch and radius for realizing homogeneous current distribution in coaxial multi-layer superconductor. A set of the obtained twisting pitch and radius was employed in a sample three-layer conductor comprised of silver-sheathed multi-filamentary BSCCO-2223 tapes and the current distribution was measured by a Rogowski coil. Agreement between the experiment and the theory on current distribution is quite remarkable. Using this theory, the authors analytically investigated the influence of the manufacturing error of twisting pitch and radius on current distribution. The results revealed that the manufacturing errors of twisting pitch and radius have large effect on current distribution and a suitable set of twisting pitch and radius against manufacturing error can be found. They also investigated the relationship between twisting pitch and current distribution in coaxial six-layer conductor with return current path. The characteristics of twisting pitch in the conductor with return current path are different from those of the conductor without return current path.

Influence of current distribution on conductor performance in coaxial multi-layer HTS conductor

We have developed a simulation method based on magnetic flux conservation between two filaments of adjacent layers to estimate current distribution in coaxial multi-layer HTS conductor. Using this method, we have demonstrated homogeneous current distribution and verified that current distribution was controllable directly by the conductor parameters of layer radius, twisting pitch and twisting direction. Although it has been considered that homogeneous current distribution is effective for reducing AC loss, the most suitable conductor parameters and operating condition have not been investigated sufficiently yet. Therefore, we improved our developing method to estimate current distribution more rigorously considering the nonlinear voltage-current characteristic of HTS tape. To verify the validity of the simulation method, we measured current distribution using coaxial two-layer conductors. Agreement of current distribution between the experiment and the analysis was good.

Analysis of current distribution in a large superconductor

An imbalanced current distribution which is often observed in cable-in-conduit (CIC) superconductors composed of multistaged, triplet type sub-cables, can deteriorate the performance of the coils. It is, hence very important to analyze the current distribution in a superconductor and find out methods to realize a homogeneous current distribution in the conductor. We apply magnetic flux conservation in a loop contoured by electric center lines of filaments in two arbitrary strands located on adjacent layers in a coaxial multilayer superconductor, and thereby analyze the current distribution in the conductor. A generalized formula governing the current distribution can be described as explicit functions of the superconductor construction parameters, such as twist pitch, twist direction and radius of individual layer. We numerically analyze a homogeneous current distribution as a function of the twist pitches of layers, using the fundamental formula. Moreover, it is demonstrated that we can control current distribution in the coaxial superconductor.

Instability of NbTi/CuNi superconducting wires

The quench current degradation in multifilamentary NbTi superconducting wires with a CuNi matrix have been measured and analyzed by the finite-element method. The authors measured the degradation of the quench current as a function of the current sweep rate in the range of 10 A/s to 100 kA/s, and the initial current, which was a transport current just before current changing. Experimental data are compared with analytical results taking the magnetic and the thermal diffusion in the radial direction of wire cross-section into account. From these comparisons, the relation between the rate of current changing and the degradation of the quench current in superconducting wires is discussed.<<ETX>>

A new type of active-Maglev system using YBCO bulk and multiple electromagnets

The authors present a new type of active-maglev system consisting of a disk-shaped superconducting bulk (YBCO) and multiple electromagnets. Using the active-Maglev system composed of five electromagnets, they demonstrated continuous levitation and verified that the levitation height, as well as stability, could be remarkably improved by adjusting operating current of electromagnet individually. Electromagnetic behavior within the bulk was investigated numerically by the finite element method (FEM) adopting the Bean model. Agreements of levitation force and height between experiment and analysis were good. Suitable electromagnet operation for continuous levitation in terms of consumed operating energy was also investigated. It was found in analysis that continuous levitation could be realized efficiently by adopting a three-electromagnet operation and the operating procedure is applicable to multiple-electromagnet system. Therefore, based on the assumption of constant total operating current of three electromagnets, they numerically investigated the relationship between the operating procedure and levitation force in a five-electromagnet system as a function of levitation height. Maximum allowable weight of float (superconducting bulk and load) was evaluated through the estimation of minimum levitation force during continuous levitation as a function of air gap between electromagnets.

Dependence of lateral stiffness on magnetic field distribution at field-cooling process of HTS bulk system

Stiffness is one of the important parameters for HTS bulk applications to motor, magnetic bearing in flywheel system and magnetic levitating transporter and so on. We prepared several types of disk-shaped permanent magnets with different diameters for realizing different magnetic field distribution. Since the lateral stiffness depends on not only magnetic field distribution but also the magnitude of magnetic field, we measured the restoring force against lateral disturbance under the condition of the same magnetic flux density on the top surface of HTS bulk. The maximum restoring force of HTS bulk against single permanent magnet increased with the diameter of the permanent magnet and the displacement at the maximum restoring force was almost equal to the diameter of the permanent magnet. We also performed analytical evaluation of restoring force by calculating stored energy within HTS bulk in each bulk location to investigate the relationship between restoring force and magnetic field distribution. Experimental and analytical results implies that slope of magnetic field around field-cooling location is one of the important parameter for achieving large lateral stiffness.

Influence of current distribution on quench process in noninsulated AC multi-strand superconducting cables

In AC multi-strand superconducting cables, AC quench current is much less than that of DC. This AC quench current degradation is caused by AC loss, mechanical damage by cabling, nonuniform current distribution among strands due to self and mutual inductances between strands, and different electrical resistivity at the joint between superconducting strands and current lead. In this paper, the current distribution and the thermal behavior of noninsulated 7- and (6+1)-strand cables applying AC transport current are analyzed and the influence of contact electrical resistivity on the current redistribution and the thermal diffusion in the quench process are investigated.<<ETX>>