宮城 大輔

No-insulation coil (NIC) has been proposed for improvement of thermal stability. NIC can reduce heat generation due to a current distribution that bypass the defect when a local hot-spot is formed. However, a charge delay because of no turn-to-turn insulation is one of the important characteristics of NICs. To decrease the charge delay of NIC, a winding technique with parallel HTS tapes without insulation between layers has been proposed. However, the increase of the number of parallel layers causes the significant current imbalance between parallel tapes. In this paper, the relationship between the number of parallel tapes and the effect on charge delay reduction is investigated. In addition, the effect on the thermal stability and magnitude of central magnetic field and thermal stability due to the current imbalance is also verified. Both analyses are carried out for both NIC with and without defects. The analysis results show that charge delay of NICs is reduced according to increase of number of parallel tapes. Current imbalance between layers increases in accordance with the increase of the number of parallel tapes. However, the analysis results also show that thermal stability and central magnetic field are not degraded by this imbalance current. In terms of NIC with the defect, thermal stability and fluctuation of central magnetic field are most improved by the effect of current sharing between parallel tapes when the number of parallel tapes is largest.

Three-phase coaxial high-temperature superconducting (HTS) cables have three layers, and HTS conductors are arranged circularly in each layer. Since each arranged conductor transports current, the conductor arrangement determines the magnetic field distribution inside the cable. In addition, the AC loss varies depending on the conductor arrangement because the magnitude of the perpendicular magnetic field on each conductor determines the AC loss of the cable. Therefore, it is essential to consider the optimal conductor arrangement in cable design. In this study, we investigated the influence of the relative position of the conductors constituting each phase on the AC losses. The conductors constituting the cable were Y-based superconducting conductors, and the AC losses were calculated by one-dimension finite element analysis of the T-method. The calculation results show that shortening the gap between conductors in the radial direction reduces the AC loss. Furthermore, we have shown that simultaneously shortening the circumferential and radial gap between conductors can significantly reduce AC losses. In addition, the cost estimation shows that it is essential to consider a conductor arrangement with low AC losses in cable design.

AC loss is one of the key issues need to be considered in designing high temperature superconducting (HTS) devices. An effective method for AC loss reduction is applying magnetic flux diverters (MFDs) in close proximity of the HTS coil windings. In this work, two types of low-loss powder-core MFDs -high flux (HF) and molypermalloy-powder (MPP) MFDs -were applied to a REBCO coil assembly for reducing the AC loss. The measured and simulated results show that both types of MFDs can significantly reduce the total loss (the summation of the losses in the HTS windings and the MFDs) in the coil assembly. At 77 K and 40 A coil current, compared with the coil assembly without MFDs, the total loss with HF and MPP MFDs was reduced by 77 % and 81 %, respectively. Although MPP MFDs show slightly better performance in reducing total loss in the coil assembly than HF MFDs at low coil current, HF MFDs are more obvious option for AC loss reduction in high magnetic field applications owing to their relatively high saturation magnetic field.

We have investigated the effectiveness of the insertion of a low thermal conductive layer (LTCL) in a triaxial high temperature superconducting (HTS) cable to realize a long-length triaxial HTS cable. This article analyzed the relationship among intrusion heat from outside of the cable, ac loss, temperature distribution, and the maximum temperature within a triaxial HTS cable with the LTCL to clarify the suitable insertion position and thickness of the LTCL for a long-length triaxial HTS cable. Analysis results showed that inserting the LTCL inside the HTS layers was effective for the long-length triaxial HTS cable. In addition, there was a tradeoff relationship between the effects of suppressing the thermal conduction from the outer refrigerant to the inner refrigerant and increasing the intrusion heat from outside of the cable by inserting the LTCL. The suitable thickness of the LTCL can be obtained by considering this tradeoff relationship.

This study investigated the axial compressive stress dependence of the critical current of Rare Earth Barium-Copper-Oxide (REBCO) double-pancake coils. We measured the critical currents of REBCO coils cooled by liquid nitrogen while various axial compressive stresses from 0 MPa to 130 MPa were applied. After each stress was applied, the critical currents of the coils were also measured without the stress application to examine if the REBCO coils were irreversibly deteriorated. The experimental results showed that the REBCO coils were deteriorated at more than 100 MPa and the critical currents of the coils decreased by about 30% by applying 120 MPa to the coils. Moreover, reversible reduction of the critical currents of the coils were observed while the stresses of less than 100 MPa were applied. The reversible critical current decrease occurred even at a small stress, and the critical currents of the coils were reversibly reduced by about 15% while the stress of 100 MPa was applied. This study quantitatively clarified the dependence of the critical current of the REBCO coil on the axial compressive stress and the limitation of the axial compressive stress to prevent the deterioration of the REBCO coil.

AC loss in high temperature superconducting (HTS) coils affects the performance of HTS devices. Using magnetic flux diverters (MFDs) is an effective way to reduce AC loss in HTS coils. In this paper, measurement and finite element method simulation of AC loss results in a REBCO coil assembly comprising four double pancake coils with two molypermalloy-powder MFDs are presented. Both experimental and numerical results show that MFDs can significantly reduce the AC loss in the REBCO coil assembly while generating negligible loss in themselves. Further, the influence of the distance between the coil assembly and the diverters on AC loss reduction is explored. Compared with the AC loss data in the coil assembly without MFDs, over 80% AC loss reduction is achieved when the distance between the coil assembly and the diverters is at its minimum value, 2 mm. The simulation results reveal that the AC loss reduction in the coil assembly is mainly due to the reduction of the radial (perpendicular) magnetic field component to the surface of REBCO wires in the end windings of the coil assembly.

© 2002-2011 IEEE. This article investigated a suitable excitation method for a conduction-cooled REBCO coil to reduce the temporal attenuation of the screening current induced field (screening field) for magnetic resonance imaging. We investigated the effect of the preexcitation of the coil and high temperature magnetization method on the screening field attenuation by measuring the variation rate of the magnetic field at the side surface of a conduction-cooled rare-earth barium copper oxide double pancake coil. Experimental results showed that the preexcitation of the coil and high temperature magnetization increased the temporal stability of the coil magnetic field when the preexcitation current is higher than the main operational current, or the temperature change of the high temperature magnetization is over 5 K, respectively. We also clarified that the combination of the preexcitation, overshooting, and high temperature magnetization method greatly suppressed the attenuation of the screening field and also the load factor of the coil than in the case when only the high temperature magnetization is applied.

© 2002-2011 IEEE. Research and development for the practical application of a medical-use magnetic resonance imaging system (MRI) superconducting magnet that does not require liquid helium started as the New Energy and Industrial Technology Development Organization's (NEDO) supported project in fiscal 2016. Development of a liquid helium-free medical MRI superconducting magnet has been desired. Another purpose is to reduce the size and weight of high magnetic field magnets. By using the high temperature superconducting coil, it is possible to make the 3T magnet of the same shape, weight, leakage magnetic field as 1.5T magnet. In this project, we are developing a half size active shield-type 3T REBCO coil for MRI. This magnet has active shield coils with a maximum diameter of 1200 mm, and its room bore diameter is 480 mm. This magnet is one of the world's largest magnets using a REBCO wire with an accumulated energy of 1.6 MJ at the rated magnetic field. It generates a magnetic field of high homogeneity and stability necessary for imaging. In this paper, we report the half-size active shield-type 3T coil and its cooling system that can reduce the initial cooling time by thermal switches.

© 2020 The Institute of Electrical Engineers of Japan. This paper proposed a magnetic levitation type seismic isolation device composed of radial arrangements of High Temperature Superconducting (HTS) bulks and Permanent Magnets (PM). We first analyzed and compared the levitation force of an axial arrangement and radial arrangement of the HTS bulk and PM rail for the seismic isolation device, respectively. Due to Halbach arrangement of the PM rail, the radial type HTS seismic isolation device can provide a comparable levitation force to the conventional axial type device. We also investigated the dependence of the levitation force on the shape of the HTS bulk and PM rail to clarify the design guidelines of the radial type device. The analysis results showed that a thinner HTS bulk generated a larger levitation force per unit volume of the HTS bulk. The analysis also clarified the proper bulk width by considering the magnetic flux density distribution of the Halbach PM rail. We also found that increasing the volume of the PM rail does not greatly improve the levitation force of the radial type device. These results suggest that the levitation force of the radial type HTS seismic isolation device can be most effectively increased by arranging several thin HTS bulks in the longitudinal direction of the PM rails.

The MgB2 coil production technology obtaining 30 kJ stored energy the investigation about the SMES coil consists of 600 A, 1.7-T Rutherford-type conductors made of commercially-available MgB2 wires. Due to strain sensitivity before/after heat treatment for MgB2 production, the proper designs of the large-scale twisted conductors both in wind and react, react and wind methods are needed, choosing optimized twist pitches and cable compaction factors. To demonstrate the SMES coil performance, we have been carried out the test campaign of conductors and small prototype coils in various temperature and background field conditions. These results are used for a computer simulation for estimating full size double pancake coil performance of the system, based on the non-steady state heat conduction analysis. The calculated result seems to be a good tool for predicting coil performance for the large capacity energy storage operation.

It has been often reported that a superconducting property of a coil wound by a coated conductor is degraded by epoxy impregnation. The degradation is considered that a superconducting layer or a buffer layer is peeled from a metal substrate by thermal stress. The steady thermal stress in the winding at the operating temperature is examined frequently. However, the transient thermal stress in a cooling process becomes larger than the steady thermal stress due to the difference of heat contraction of each material and temperature distribution, since heat contraction is dependent on a temperature. In this study, by using finite-element analysis, we examined the transient temperature distribution and the transient thermal stress distribution inside a coated conductor's coil winding with epoxy impregnation, when the cooling process with liquid nitrogen was considered. The transient one at the edge of the winding with a glass fiber rein-forced plastic bobbin is more than twice as large as the steady thermal tensile stress and is greater than the cleavage stress strength. Moreover, we have found that it is desirable to use a material with a smaller thermal expansion coefficient and larger heat transfer as the coil bobbin in order to reduce the transient thermal stress in the winding. The effectiveness of the stainless steel bobbin to reduce the transient thermal stress in the winding was showed. Furthermore, it was shown that sufficient precooling was required for reduction of transient thermal stress when a bobbin material with smaller thermal conductivity is used.

A high temperature superconducting (HTS) triaxial cable fits long-distance power transmission compared to the three-in-one superconducting (SC) cable, since the HTS triaxial cable has lower ac loss and larger cooling channel, and the HTS tape length used for the HTS triaxial cable is much shorter than that of the three-in-one SC cable. We investigated a suitable cable structure of the HTS triaxial cable, which has the limitation of the outer diameter (150 mm), cooled by counter-flow cooling for long-distance power transmission using the numerical analysis that considered the heat transfer from the cable outside and the heat generation caused by the electrical loss and the frictional loss of a coolant inside the cable. Our results show that it is very important to expand an outside channel and increase the flow rate in order to operate the HTS triaxial cable for a long-distance. The suitable flow rate for increasing the cable length that each cooling station can cool the HTS cable at 77 K or lower is about 50 L/min. The maximum cable length is 1.7 km at the conductor diameter of 88 mm. This cable length of 1.7 km is larger than that of the three-in-one cable the maximum cable length of the three-in-one cable is less than 1 km. Therefore, the number of cooling stations can be reduced by adopting HTS triaxial cable.

We have investigated the application of a high-temperature superconducting (HTS) coil to a high-power wireless power transmission system operated in the frequency region less than 10 kHz. In order to reduce the ac loss of the HTS coil, it is necessary to clarify the characteristics of magnetization loss in the HTS tape with a copper layer in the kHz frequency band. Therefore, we prepared GdBCO tapes with and without the copper layer and measured the magnetization loss of the GdBCO tape exposed to a vertical magnetic field in the kHz frequency band. We analyzed the ac losses of the GdBCO layer, the copper layer, and the other layers within the tape by a finite element method analysis. The influence of the copper layer thickness on the ac loss in each layer was investigated by a finite element method (FEM) analysis. The magnetization loss per cycle of the GdBCO tape slightly decreased with the frequency. The magnetization loss, however, can be approximately estimated by Brandt's formula with a certain accuracy when the n value of the tape is large. In the GdBCO tape with the copper layer, when the external magnetic field was large, the magnetization loss per cycle of the GdBCO layer was larger than the copper layer loss. The magnetization loss, however, decreased and the copper layer loss increased with the thickness of the copper layer. The total loss of the GdBCO tape can be reduced by placing the coppers layer near the GdBCO layer.

Large hoop stress causes the deterioration of the transport properties of an high-temperature superconducting (HTS) coil winding. A high-strength pancake coil structure called “Yoroi coil” has been suggested. Yoroi-coil structure is expected to decrease the electromagnetic force applied to the winding and the volume of the coil frame at the outside of the winding. The effect of Yoroi coil on the hoop stress reduction has been investigated in an ideal “Yoroi-coil” structure without bolts and electrodes. In a practical “Yoroi-coil” structure, however, there are electrodes and bolts for fixing frames and reinforcing outer plates to the coil. In this study, the effect of the stress reduction of an HTS coil winding using the practical “Yoroi-coil” structure with the bolts and the electrodes was numerically investigated by 3-D-finite element method (FEM) analysis. Numerical results showed that the circumferential strain and hoop stress generated in the winding depended on the bolt location. Local large stress was applied to the winding, especially around the electrode. In order not to apply the local large stress to the winding, the mechanical strength of the electrode was very important. Adding bolts to the electrode was effective as the counterplan of stress concentration at the electrode. The optimal number of bolts at the electrode, however, largely depended on the thermal contraction during the cooling process of the coil.

We experimentally investigated the influence of local critical current degradation on the thermal and electromagnetic behavior around the local degradation region and the time-dependent characteristics of the voltage across it. The local critical current degradation was produced by cutting a part of the width of a piece of GdBCO tape. The current that caused the tape to burn out and the voltages across the degradation region were measured in short tape samples and pancake coils made using GdBCO tape. In a sample without copper stabilizer covered with Styrofoam, the voltage rapidly increased over a short time period and the sample burned out soon after the operating current exceeded the local critical current. On the other hand, the tape with the copper stabilizer cooled by an LN 2 was able to stably maintain the voltage even when the operating current largely exceeded the local critical current. The current sharing effect of the copper stabilizer greatly contributed to an improvement in stability because the current sharing effect was much larger than the LN2 cooling effect. The cooling effect from the top and bottom surfaces of the coil was also important and should be improved upon as much as possible to suppress the temperature rise during coil excitation. The heat transfer effect in the radial direction between coil layers was smaller than the LN2 cooling effect.

Superconducting magnetic energy storage (SMES) devices of several tens of kJ class are generally suitable for voltage compensation for microgrids, which produce and distribute electric power to restricted areas. MgB2 material has been developed with superconducting properties by decreasing the production cost. Since hydrogen energy would be widely utilized to realize society with low carbon emission and stored in liquid state for reducing its volume, the power distribution system consisting of MgB2 SMES for compensation of voltage fluctuations cooled by the liquid hydrogen would be effective by synergy effect. However, the MgB2 introduction to large-scale devices is still not enough and under investigation. Our group carried out the investigations to develop MgB2 cable and pancake coil for the SMES device with specific capacity. The bending strain-sensitive characteristic of MgB2 material forces us to design the twisted conductors and pancake coils with various parameters properly within its tolerable bending strains of both before/after heat treatment. The conductor design for small pancake coils and large SMES coils is shown in this research, as well as the demonstration results of a small test coil fabricated as a prototype of SMES coil.

We investigated a suitable excitation method to suppress the shielding magnetic flux density attenuation using only the one-time excitation of a REBCO coil, which was cooled by a refrigerator. We investigated the operating temperature and current load factor dependences of the variation rate of the shielding magnetic flux density immediately after an overshooting process. Regardless of the operating temperature and current load factor, the variation rate of the shielding magnetic flux density decreases with the magnitude of overshooting current and changes from a positive value to a negative value. In each current load factor, the ratio of the optimal overshooting current to the critical current of the REBCO coil becomes almost identical regardless of the operating temperature. Based on these results, we devised a suitable overshooting current waveform to suppress the shielding magnetic flux density attenuation in the REBCO coil cooled by the refrigerator. The overshooting current was repeated to the coil in a one-time excitation the amount of overshooting current was gradually increased until the variation rate of magnetic flux density was changed from positive to negative. Using this current waveform, we can suppress the shielding magnetic flux density attenuation in the conduction-cooled REBCO coil using only a one-time excitation.

The miniaturization of a synchrotron accelerator can be realized by applying a conduction-cooled high-temperature superconducting (HTS) bending magnet with a cryocooler. A variation of an exciting current through the HTS magnet generates the ac loss, and the ac loss causes a temperature rise of the HTS magnet. AC loss in HTS coils is typically measured using the electromagnetic method or thermal measurement method. However, the measurement method of the ac loss of the conductioncooled HTS coil under non-sinusoidal current excited has not been established because of the very large inductive voltage of the coil, the very low power factor, and no refrigerant liquid or gas. This study reports a novel thermal measurement method of ac loss in a conduction-cooled HTS coil using a heater and a cryogenic temperature controller. The validity of the proposed thermal measurement method was verified by measuring the loss in a copper coil which was cooled by conduction cooling with a 4-K Gifford-McMahon cryocooler. The measured electrical loss in the coil was good agreement with analytical one. Moreover, the validity of our method was verified by measuring the ac loss in a conduction-cooled small double-pancake HTS coil wounded by a rare-earth barium copper oxide (REBCO) tape conductor at 20 K. The measurement results of ac losses in the HTS coil show that our proposed measuring method is very useful for evaluation of the ac loss in the conduction-cooled HTS coil.

The superconducting magnet is effective to get a high stable and high magnetic field for magnetic resonance imaging (MRI). The current MRI superconducting magnet needed cooling in the liquid helium (4.2 K) to use NbTi superconducting wire. In the past few years, price increase and low availability of liquid helium has become a serious problem. Under such circumstances, the development of a high-temperature superconducting (HTS) coil dispensing with liquid helium cooling is greatly desired. The research and development project of the high stable magnetic field ReBCO coil system fundamental technology that started from the latter half of 2013 develops a ReBCO coil for 3 T MRI superconducting magnets. It gets a prospect of the practical use as the final aim. In this project, we will produce an HTS test coil of 300 mm bore experimentally and evaluate the magnetic field. This coil is cooled in less than 20 K by a GM refrigerator. We are going to make MRI used by the ReBCO coil field to evaluate the uniformity and stability of the magnetic field.

In Y-based coated conductor coils, a shielding current is induced by the magnetic field component perpendicular to the broad face of the coated conductor. Since the magnetic field induced by the shielding current gradually attenuates with time, the temporally stable magnetic field required for high-resolution imaging in magnetic resonance imaging (MRI) applications is difficult to obtain. We investigated the suitable excitation conditions in an overshooting process required for suppressing the magnetic field attenuation. We fabricated a double-pancake coil composed of GdBCO tapes and measured the shielding magnetic flux density and its variation rate using Hall probes attached to the outermost layer of the coil. The variation rate of magnetic flux density decreased with the maximum coil current. The overshooting current required for zero variation rate of magnetic flux density increased with target coil current in the small coil current region, and decreased with target coil current in the large coil current region. The suitable powering conditions for the overshooting process could be obtained by considering the relationship between the final operating current of the coil, the shielding magnetic flux density, and the required overshooting current for zero field variation rates at the end of the current ramping.

A critical component of the 1.3-GHz nuclear magnetic resonance magnet (1.3 G) program, currently ongoing at the Francis Bitter Magnet Laboratory, Plasma Science and Fusion Center, Massachusetts Institute of Technology, and now approaching its final stage, is the all high-temperature superconductor 800-MHz insert (H800). The insert consists of three nested double-pancake (DP) coils fabricated with 6-mm-wide REBCO conductor. Coil 1, the innermost coil of H800, has already been fabricated and tested at 77 and 4.2 K. In addition, one third of the DPs for Coil 2 have been wound and each DP individually fully tested. Work described here includes details of Coil 1 fabrication: DP winding, DP testing, assembling, joint performance, overbanding, and coil testing; winding details of DPs for Coil 2 and their testing are also included.

In the laminated core of transformers, motors, and so on, each electrical steel sheet is usually insulated in order to reduce the eddy current loss. It has been reported that insulation is not necessary in such a laminated core under some conditions. However, few studies have been performed in the form of a quantitative and systematic examination of the relationship between the insulation and eddy current. In this study, the eddy current losses of a core made of SPCC (cold rolled steel sheets) of different widths with and without insulation under various conditions are analyzed using the finite element method (FEM) and with the contact resistance taken into consideration. The equivalent circuit of a laminated core without insulation is presented, and can be used for determining the necessity of insulation. It is shown that the increase in the eddy current is affected by the ratio of the resistance of the steel to the contact resistance. An experimental investigation is also carried out.

Because of the development of power electronics technology, pulse width modulation (PWM) inverters are now being used to drive motors in order to achieve precise and energy-efficient control. Precise estimation of the increase in iron loss due to the high harmonic components of flux, including the carrier frequency, is important in the design of motors excited by a PWM inverter. We measured the iron losses of non-oriented electrical steel sheets that were excited using a single-phase full-bridge PWM inverter, and examined the influence of the carrier frequency and circuit resistance on the iron loss. We showed that the iron loss increased because of the generation of minor loops when the circuit resistance was high. Therefore, the circuit resistance should be decreased in an actual motor system. (C) 2015 Wiley Periodicals, Inc.

Two types of shaking coils are focused on reducing screening currents induced in solenoid coils wound with high-temperature superconducting (HTS) tapes. One is a pair of copper shaking coils coaxially located inside and outside the HTS coil to apply an ac magnetic field in the axial direction. The other is an HTS shaking coil with notch located only outside the HTS coil to minimize the radial components of local ac fields applied to windings of the HTS coil as small as possible. It is found that the copper shaking coils yield the allowable amount of power dissipation in liquid helium. The effectiveness of the HTS shaking coil to reduce screening-current-induced fields generated by another magnetized HTS coil is also experimentally validated in liquid nitrogen using a commercially available coated conductor with narrow width.

Power generation by renewable resources that are environment friendly has been attractive lately. However, it is a big problem that the output fluctuation from the renewable energy sources causes instability to the power network when they are directly connected to the utility grid. Although a power storage system is effective for compensating for the output fluctuation, this electric power storage system must have large capacity and quick response. We proposed an advanced superconducting power conditioning system (ASPCS) composed of two kinds of power storage systems. In this paper, we experimentally verified the validity of the compensating system for fluctuating output power using a superconducting magnetic energy storage and an electronic load instead of an electrolyzer. It was demonstrated that the ASPCS was effective for compensating for the output fluctuation of the renewable energy.

One of the reasons for degradation of the critical current in a Cable-in-conduit conductor is unbalanced current distribution due to inhomogeneous resistance distribution in a lap joint. As a way of solving this problem, a sleeve longer than the highest order sub-cable twist pitch (cable twist pitch) and soldering a joint are actually effective to reduce the inhomogeneity in the resistance distribution between individual strands and the copper sleeve. However, the influence of individual sub-cable twist pitch on contact condition between individual strands and the copper sleeve in a CICC has not been investigated. In this study, the influence of the individual sub-cable twist pitch on contact condition between individual strands and the copper sleeve in a CICC lap joint was investigated using calculated strand paths. Our calculated results show that contact condition between strands and the copper sleeve in a CICC lap joint are improved regardless of the cable configuration when the n-th order sub-cable twist pitch is a multiple of all the previous sub-cables twist pitch. Moreover, it is shown that the difference in the characteristic associated with the contact condition and the combination of sub-cable twist pitch is caused by the cable outline.

In this paper, we present results, experimental and numerical, of the electromagnetic interaction forces between pairs of racetrack coils under time-varying conditions. Three turn-to-turn insulation designs were applied to wind three racetrack coils with GdBCO coated conductor: 1) no insulation (NI); 2) partial insulation (PI) of a polyimide layer every eight turns; and 3) insulation (INS) of a polyimide layer between each, i.e., NI, PI, and INS racetracks. Two racetrack pairs, namely, NI-INS and PI-INS, were tested for their interaction forces, measured with load cell under current-ramping conditions in a bath of liquid nitrogen at 77 K. Good experimental and simulation results validate our equivalent circuit model to compute interaction forces of PI-INS racetrack pair. Overcurrent test of NI and PI coils, where each racetrack coil was charged above critical current (I-c), was also performed to compare coil stability. This result implies that, although the PI winding technique improves the dynamic response, stability will be somewhat compromised.