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In this paper, we discuss two points: (a) the accuracy of contact resistance measurement using the low-frequency AC (LFAC) method and (b) the relation between the contact resistance and the contact resistivity. The LFAC method is used to measure the turn-to-turn contact resistance of a no-insulation (NI) rare-earth barium copper oxide (REBCO) pancake coil. We have previously shown the validity of the LFAC method by experiment. In this paper, the current distribution of aNI REBCO pancake coil on the applyication of the LFAC method is investigated using numerical simulation. The simulation results show the possibility of improving the measurement accuracy. Hence, a correction method is also proposed using the phase difference of the coil voltage and current. The LFAC simulation results also indicate the inconsistency of the well-known relation between contact resistance and contact resistivity. Therefore, we investigated the relation through the LFAC and sudden-discharging test simulations.

This paper overviews the combined effects of winding, cool-down, and screening current-induced stresses in REBCO coils. First, a simulation method to model the circumferential stress modification effect due to the screening-current is overviewed. The simulation includes coil winding, cooling down, and coil charge up to the operating current. Second, we will compare the numerical simulation results with the experimental results. The numerical simulations for a dry coil and an epoxy impregnated coil agree well with the experimental results. Third, the enhanced circumferential stress did not degrade the performance of a dry winding REBCO coil, but the improved increased compressive stress buckled the coil structure. Finally, it is demonstrated that epoxy impregnation has beneficial effects in reducing the stress modification effect. However, the circumferential stress is enormously enhanced at the coil ends, sometimes resulting in degradation of the coil performance.

In recent years, the applications of high-field REBa2CuOy (REBCO) coils have remarkably progressed towards NMR MRIs, accelerators, and other such devices. In a REBCO coil, the magnetic field due to the screening current is generated in the direction opposite to the field by the transport current, thus reducing the magnetic field, deteriorating the field homogeneity, and affecting the time stability of the magnetic field. Recently, the additional force and stress due to the screening current (referred to as screening-current-induced stress) has become a topic of concern. The screening current results in non-uniform current distributions in the REBCO tape. Therefore, the distribution of electromagnetic force in REBCO coils differs from that during the designing of the coil, assuming that the current flow in the tape is uniform. Thus, there is the possibility that the existence of screening current is a serious problem in the mechanical design of REBCO coils. In this study, we evaluate the electromagnetic force and stress due to the screening current by using the developed numerical simulation code for the electromagnetics and stress. We discuss the winding tension, thermal and electromagnetic stresses, and mechanical strength structure of the REBCO coil.

Biomedicine is indispensable for the treatment of various diseases such as cancer and diabetes mellitus. In particular, antibodies utilized by the immune system are believed to represent the future of medical supplies as they offer an effective treatment for diseases with minor side effects. The development of antibody drugs requires a large-scale, sequential, fast separation, and refinement process for the medical proteins. Thus, in the previous study, we proposed a medical protein screening system that is based on high-gradient magnetic separation using a superconducting magnet. In the proposed system, the protein is specifically attached to the magnetic beads and captured by the magnetic force around a magnetic filter generated by a high magnetic field. In this paper, the behaviour of the magnetic beads was investigated using particle trace simulation coupled with electromagnetic and fluid analyses for three types of commercially available magnetic filters. Simulations were performed using the filter stack intervals and number of filters as design parameters. Finally, we designed the suitable filter for capturing the magnetic beads.

The development of NMR/MRI REBa2Cu3O y (REBCO, RE = rare earth) magnets is happening all over the world. It has been found that screening current-induced magnetic fields (SCMFs) are a serious problem for NMR/MRI magnets wound with REBCO tapes. The reduction of SCMFs is strongly desired, and the estimation of SCMFs is also desired at the design stage of REBCO magnets. In order to evaluate a SCMF, a finite element method (FEM) or a boundary integration method is needed thus far, and a high-level simulation technique is required. In this paper, we develop an easy method to estimate a SCMF without a high-level simulation technique. In this method, the inductance of a winding turn is calculated and then a screening current is estimated according to the magnetic field penetrating into the winding turn. The SCMF is obtained from the estimated screening current. The SCMFs computed by the proposed method were compared with measurements and simulation results of the FEM. The results agreed well, but we can see a large difference. However, the SCMFs found by the proposed method are accurate enough to know they are at the design stage without the use of a high-level simulation technique.

We have been developing an active magnetic levitation system composed of field-cooled disk-shaped YBCO bulk and multiple ring-shaped electromagnets which are vertically piled up. We have demonstrated and reported that the levitation height, as well as stability, can be remarkably improved by adjusting the operating current of each electromagnet individually. In this study, we constructed a noncontact position control system in the vertical direction based on the feedback control theory and the electromagnetic field analyses considering superconducting characteristics of the YBCO bulk. Depending on levitation height, the operating current in the electromagnets is controlled automatically. The system consists of two ring-shaped copper-winding electromagnets without iron core and a ring-shaped YBCO bulk magnetized by the field-cooling process. We carried out experiments to verify the feasibility of noncontact position control system. In the experiment for position control, the levitation height maintained a target position accurately, and responded smoothly to changing target position. In addition, we aimed at the construction of the position control system with higher accuracy using the numerical simulation based on the finite element method analyses. And, the position control system that piled up three electromagnets was designed and constructed. Concretely, accuracies of the position control within 28µm were obtained. Final goal of our study is to apply the levitation system to the inertial nuclear fusion in which the accuracy as high as several µm would be required.

It is expected that electric devices would be high efficiency, high output and downsized by using high-temperature superconducting (HTS) bulk materials. In the previous study, we have fabricated and tested a linear synchronous actuator with double-sided primary and YBCO bulk secondary; the linear actuator consists of a field-cooled YBCO bulk for secondary (mover) and copper windings with an iron core as a primary. The primary, which was excited by a three-phase VVVF (variable voltage variable frequency) power source to generate sinusoidal traveling magnetic fields, was divided into two sections: 1) the starting section in which the secondary is accelerated as an induction machine; and 2) the synchronous section in which the secondary moves with a specified synchronous speed. And we have evaluated both of the static and dynamic characteristics on this linear synchronous actuator. However, for practical use of linear actuator with field-cooled HTS bulk secondary, it is required to examine the distribution of magnetic field within the HTS bulk which is exposed to the traveling magnetic field in a realistic operational environment of linear actuator. Therefore, in this study, we repeated the mode of acceleration, constant-speed, deceleration and stop with the control circuit in the synchronous section. The direction of the mover can be turned automatically by the switching device and it made round-trip running possible. Then, we carried out many times round-trip operations and evaluated the change of the magnetic field on the surface of the YBCO bulk in various conditions with trapped field of the secondary and traveling magnetic field generated by the primary windings.

We have been investigating the electromagnetic behaviour of HTS bulks for realizing an X–Y (two-dimensional) magnetic levitating transporter without any fixed guides. The transporter is mainly composed of disc-shaped YBCO bulks and Nd–Fe–B permanent magnets or field-cooled YBCO bulks. The lower part and the upper part (levitating part) are magnetically connected. In such a 'levitating X–Y transporter', it is necessary to comprehend the characteristics, such as the lift, the levitation height and the stability against mechanical disturbances. In this paper, we evaluated the lift and the restoring force characteristics experimentally in the case where a field-cooled YBCO bulk is adopted instead of permanent magnets and also developed a numerical simulation program based on the finite element method to clarify the electromagnetic phenomenon in the HTS bulks.

We have been developing a new type of active-maglev system composed of field-cooled disc-shaped YBCO bulk and multiple ring-shaped electromagnets, which are vertically piled up. One of the useful features of the active-maglev system is that the levitation height is adjustable by varying the operating current through an electromagnet. Maximum levitation height in stable levitation, however, is restricted by the magnetic field distribution produced by the electromagnet. To enhance the levitation height, we have fabricated and tested an active-maglev system with multiple ring-shaped electromagnets instead of using a larger single electromagnet. Up to now, we have reported the experimental results in a model active-maglev system composed of five electromagnets and showed that the levitation height, as well as stability, can be remarkably improved by adjusting the operating current of each electromagnet individually. In this study, we constructed a model system operated as a tubular linear synchronous actuator in the vertical direction in expectation of it being applied in factory automation upgrading. The primary consists of six ring-shaped copper-winding coils without iron core and is supplied with three-phase sinusoidal excitation. A disc-shaped YBCO bulk, which was magnetized by a field-cooling process, was adopted as a secondary (mover). We carried out preliminary experiments to confirm the feasibility of synchronous operation and also numerically investigated the electromagnetic phenomenon within the bulk superconductor by a developed computer program based on the finite element method.

We have been developing a magnetic levitating device with two-dimensional movement, namely a 'levitating X–Y transporter'. For the real design of a levitating X–Y transporter, it is necessary to clarify the levitation characteristics, such as the lift, the levitation height and the stability against mechanical disturbances. Furthermore various kinds of force may be applied to the levitating part and cause mechanical oscillation. Therefore the characteristics of oscillation are also important factors in the dynamic stability of such a levitation system. In this paper, we examine experimentally the lift and the restoring force and develop a new simulation code based on the three-dimensional hybrid finite and boundary element method to analyse the dynamic electromagnetic behaviour of the HTS bulk. We have investigated a suitable permanent-magnet arrangement to enhance the levitation characteristics through experiment and numerical simulation. We can then determine the suitable conditions for stable levitation from those results.

The magnetic drug delivery system (MDDS) is a key technology to reduce the side effects in the medical applications, and the magnetic force control is very important issue in MDDS. In this application, the strength of magnetic field and gradient required to MDDS devices are 54 mT and 5.5 T/m, respectively. We proposed the new magnetic force control system that consists of the multiple racetrack HTS magnets. We can control the magnetic field gradient along the longitudinal direction by the arrangement of the multiple racetrack HTS magnets and operating current of each magnet. When the racetrack HTS magnets were used, the critical current was reduced by the self-magnetic field. Therefore, the shape design of HTS magnet to reduce the magnet field into the surface of HTS tapes was required. Therefore, the electromagnetic analysis based on finite element method (FEM) was carried out to design and optimize the shape of multiple racetrack HTS magnet. We were able to suppress the reduction of critical current by placing the magnetic substance at upper and lower side of the HTS magnets. It was confirmed that obtained maximum values of magnetic field strength and field gradient were 33 mT and 0.18 T/m, respectively.

High temperature superconducting (HTS) magnets wound with REBCO wires are used in many applications, and the superconducting magnet which is operated in persistent current mode has many advantages. Therefore, we have been developing compact NMR relaxometry devices using HTS coils operated at liquid nitrogen temperature. The required strength and homogeneity of magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm3 respectively. The proposed HTS magnet for NMR relaxometry device consists with stacked HTS double pancake coils wound with REBCO wires and operated by persistent current mode (PCM) using superconducting joint between REBCO wires. In PCM operation, the ability of persistent current switch (PCS) is very important, so, in this study, the thermal properties of YBCO wire against the various thermal inputs by heater were investigated experimentally to design the PCS for PCM HTS magnets. The thermal behaviors of the YBCO wires were measured as functions of amount of heat input using two types of epoxy resin. The current bypassing properties on the YBCO loop coil with developed PCS were studied experimentally.

A project on the development of REBa2Cu3O7−δ (REBCO) magnets for ultra-high-field magnetic resonance imaging (MRI) was started in 2013. Since REBCO-coated conductors feature high mechanical strength under tensile stress and high critical current density, use of REBCO coils would allow superconducting magnets to be made smaller and lighter than conventional ones. In addition, a conduction-cooled superconducting magnet is simpler to use than one cooled by a liquid helium bath because the operation and maintenance of the cryogenic system become simpler, without the need to handle cryogenic fluid. Superconducting magnets for MRI require homogeneous, stable magnetic fields. The homogeneity of the magnetic field is highly dependent on the coil shape and position. Moreover, in REBCO magnets, the screening-current-induced magnetic field, which changes the magnetic field distribution of the magnet, is one of the critical issues. In order to evaluate the magnetic field homogeneity and the screening-current-induced magnetic field, a 1 T model magnet and some test coils were fabricated. From an evaluation of the 1 T model magnet, it was found that the main reason for magnetic field inhomogeneity was the tolerances in the z-axis positions of the coils, and therefore, it is important to control the gap between the single pancakes. In addition, we have already demonstrated the generation of an 8.27 T central magnetic field at 10 K with a small test coil. The screening-current-induced magnetic field was 0.43 T and was predictable by using an electromagnetic field simulation program. These results were reflected in the design of a conduction-cooled 9.4 T REBCO magnet for whole-body MRI systems. The magnet was composed of six main coils and two active shield coils. The total conductor length was 581 km, and the stored energy was 293 kJ. The field inhomogeneity was 24 ppm peak to peak and 3 ppm volume-root-mean-square (VRMS) for a 500 mm diameter spherical volume (DSV). The axial and radial 5 gauss line locations were less than 5 m and 4.2 m respectively.

In applications to power transmission cables, a number of YBCO tapes would be assembled and wound spirally on a Cu former. The YBCO tapes and the Cu former would be connected in parallel and could be subjected to short-circuit fault currents 10 to 30 times the operating current. The fault currents would drive the YBCO tapes into the normal state and thus generate Joule heating for the duration of the fault. Therefore, in order to examine the stability and feasibility of YBCO power transmission cables, it is important to demonstrate the redistribution of the transport current and the electromagnetic coupling between the YBCO tapes and the Cu former during a short-circuit fault current. We constructed a 1 m long superconducting model cable and subjected it to an overcurrent with a peak of 31.5 kArms for a duration of 2.0 s, as established by JEC (Japanese Electrotechnical Committee), in a liquid nitrogen bath. We examined the redistribution of the transport current between the YBCO sample tapes and the Cu former by using Hall probes. In addition, we developed a novel computer code based on the finite element method and an equivalent circuit in order to clarify the characteristics of the redistribution of the transport current and the thermal behaviour within the cable. Finally, we designed a 10 m long cable that would reach the degradation temperature of the YBCO tape due to the fault currents.

To establish a stability criterion for Yttrium barium copper oxide (YBCO)-coated conductors, it is necessary to clarify their transient thermal characteristics. Thermocouple thermometers, resistance thermometers, or the relation between the temperature and resistance of the stabilizer used for the conductors are generally employed for measuring their temperatures. However, these methods can only be employed to observe fixed-point data. Hence, it is difficult to observe the longitudinal and transversal distributions of temperatures in coated conductors. In this study, we adopt a method for the visualization of the temperature distribution of coated conductors by using fluorescent paints whose color changes with temperature. By adopting this method, we can measure the two-dimensional distribution of temperature in magnetic fields and at cryogenic temperatures. Therefore, local heat generation in the YBCO tapes can be observed with a higher spatial resolution and the uniformity in the critical current density of YBCO tapes can be identified. In this paper, we describe the developed thermal visualization system and the preliminary experimental results of the visualization of normal-zone initiation and propagation in YBCO sample strips. Further, we also report a part of the normal-zone initiation and propagation mechanism by electromagnetic field and heat transfer analyses by employing a three-dimensional finite element method (FEM).

We have developed an active magnetic levitation system that comprises a field-cooled disk-shaped or sphere-shaped HTS bulk and multiple ring-shaped electromagnets. In this system, the levitation height of HTS bulk can be controlled by adjusting the operating current of each electromagnet individually. Further, the application of the vertical noncontact levitation system is expected due to its levitation stability without mechanical supports. We assume that this system is applied to inertial nuclear fusion. However, one of the important issues is to achieve position control with high accuracy of the fusion fuel in order to illuminate the target evenly over the entire surface. Therefore, this system is applied to the levitation and position control of a sphere-shaped superconducting capsule containing nuclear fusion fuel. In this study, we designed and constructed a position control system for the sphere-shaped HTS bulk with a diameter of 5 mm by using numerical simulation based on hybrid finite element and boundary element analysis. We then carried out the experiment of levitation height and position control characteristics of the HTS bulk in this system. With regard to position control, accuracies within 59 ?m are obtained.

YBCO tapes are expected to be used in future high temperature superconductor (HTS) applications because they have excellent Jc characteristics at high temperatures and in high magnetic fields. For application to electric power devices such as transmission cables, transformers, and fault current limiters, the YBCO tapes might be subjected to a short-circuit fault current that is 10–30 times the normal operating current. In a worst-case scenario, YBCO tapes may degrade and burn. Therefore, it is important to clarify the mechanism of the degradation caused by an over-current pulse. This paper describes the experimental results of the degradation of the YBCO tapes through a series of over-current pulse tests. We focussed on the degradation temperature and carried out the experiments with different bending strains. Measurements were performed as a function of the amplitude of an over-current pulse for an operating temperature of 80 K (Gifford-McMahon cryocooler was adopted) in a self-field. We also examined a tape after the experiments with magneto-optical (MO) imaging, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron diffraction patterns.

In this paper, we report the fatigue characteristics of IBAD/PLD YBCO coated conductors. A YBCO coated conductor used in the superconducting coil of a SMES system is repeatedly subjected to mechanical tensile or compressive strain due to the Lorentz force during electrical charging or discharging. The superconducting characteristic of this conductor may deteriorate because of this cyclic strain. Therefore, it is necessary to investigate the effect of cyclic strain on the superconducting characteristics of YBCO coated conductors that have a laminated structure. We developed an experimental apparatus with a U-shaped sample holder in order to apply cyclic strain to the sample tape. This apparatus was used to perform the fatigue tests on YBCO coated conductors in liquid nitrogen in the absence of an external magnetic field. The strain cycles with the maximum strain max (zero external strain → max → zero external strain) were applied and repeated up to 5000 times, and the Ic measurements were performed at max. Therefore, the application of cyclic strain with max ranging from 0.3% to 0.5% did not result in any significant deterioration of the superconducting characteristics of the conductor.

A transversely excited atmospheric-pressure (TEA) CO2laser beam has been focused on a Zn plate at reduced ambient gas pressure. It is demonstrated that reflection and diffraction phenomena take place in the laser plasma when the free expansion of the laser plasma is limited by an obstructing material placed in front of the target. These findings strongly support the shock wave model for explaining the mechanism in forming a laser plasma.