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We have been developing a cyclotron accelerator for radioisotope production of medical use. The features of developed cyclotron system are compactness, light weight, and multi energy outputs. To achieve such features, coils are wound with rare-earth barium copper oxide (REBCO) tapes and the magnet has no iron core to generate an azimuthal varying field (AVF). To install the developed cyclotron accelerator into hospitals, an iron shield is needed for protection of leakage radiation and magnetic field. In this paper, to optimally design main coils with iron shield, a fast field computation method called Model Order Reduction (MOR) is adopted. Using MOR technique, the field computation is accelerated similar to 50 times. The error of MOR field computation is sufficiently small. Hence, the MOR is effective in the optimal configuration design of coils with iron shield. The optimized configuration of a miniaturized cyclotron accelerator magnet is also shown.

We proposed the air-core cyclotron using high-temperature superconducting (HTS) technology, named Skeleton Cyclotron, as high intensity compact, variable-energy and multi-particle cyclotron for RI production. Skeleton Cyclotron consists of HTS split circular main coils generating the isochronous field and HTS non-circular sector coils generating the azimuthally varying field (AVF). Now, we are currently developing the small-scale cryocooler-cooled HTS coil system of Skeleton Cyclotron, which accelerates the proton up to energy of 5 MeV at an extraction radius of 20 cm. Skeleton cyclotron is multi-coil system consists of HTS split circular main NI (no-insulation) coils and HTS non-circular sector NI coils. Therefore, the electromagnetic, thermal, and mechanical behaviors are expected to be very complicated. In this study, the structure analysis based on finite element method using COMSOL Multiphysics is performed for a non-circular sector coil. The coils in Ultra-Baby Cyclotron are reinforced with YOROI-coil structure. Therefore, the effect of YOROI-coil structure was numerically investigated during cool-down process and excitation process. YOROI-coil structure is effective for non-circular sector coil against the electromagnetic force.

We have been developing a new-generation accelerator named “Skeleton Cyclotron”, which is an ultra-compact and high-intensity accelerator for the stable production of 211At used for targeted alpha-particle therapy. Coil system for Skeleton Cyclotron consists of circular and noncircular high-temperature superconducting magnets and it can cause the output energy to vary without being bounded by the non-linearity of the iron core. This will enable not only the production of 211At, but also the production of radioisotopes for positron emission tomography and neutron irradiation for boron neutron capture therapy with a single accelerator. A high-precision magnetic field distribution and a high temporal stability are required for stable beam acceleration, but the screening current, which is induced in the REBa2Cu3Ox (REBCO, RE: Rare Earth) coated conductor, generates an irregular magnetic field and deteriorates the quality of the magnetic field spatially and temporally. In this study, we numerically evaluated the temporal stability and reproducibility of the screening current-induced field considering the variable energy operation for the Ultra-Baby Skeleton Cyclotron, which is a small demonstration model of Skeleton Cyclotron.

In autumn 2017, the project “Quantum Innovation for Safe and Smart Society (QiSS)” was started in Program on Open Innovation Platform with Enterprises, Research Institute and Academia (OPERA) by Japan Science and Technology Agency (JST). One of the key issues of this project is the advancement of compact cyclotron and beam irradiation device for stable supply of short-lived radioisotope (RI). We proposed the air-core cyclotron using high-temperature superconducting (HTS) technology, named Skeleton Cyclotron, as high intensity compact cyclotron. Currently, we are carrying out a feasibility study on a variable-energy multi-particle Skeleton Cyclotron for RI production. In this paper, key issues in HTS magnet technologies for Skeleton Cyclotron and the preliminary conceptual design of variable-energy multi-particle Skeleton Cyclotron are reported.

No-insulation (NI) coils are an important technique for high-field magnetic resonance imaging (MRI). One of the technical issues with NI coils is their thermal and electromagnetic behavior when a local normal transition occurs. Given that multiple stacked coils are used in MRI, the interaction between coils must also be investigated, because a local normal transition may affect the other NI coils through magnetic coupling. In fact, when a local normal transition occurs, there is a possibility that large electromagnetic forces are applied to the other coils, or quench may be induced in some of them. Even though overcurrent tests of stacked NI coils have been conducted, the results are not helpful in actual MRI operation. In this study, we investigate the effects of the occurrence of a local normal transition in multi-stacked NI coil systems under conditions similar to those of actual operation, with a constant transport current below the coil's I c . The occurrence of a local normal transition is simulated by applying current to a heater inserted in manufactured small NI double pancake coils. To evaluate the interaction between NI coils, the voltage and changes in the generated magnetic field are measured at each coil. We also show the current distribution in the coils, as obtained by the partial element equivalent circuit method. The obtained results allow us to evaluate the interaction of the NI coils and evaluate coil protection methods.

DC induction heating with a superconducting magnet has the capability to melt metal in a short time. We have been investigating high-temperature superconductor coils using REBCO-coated conductors to apply to dc induction heating. In order to obtain a large magnetic field for the aluminum melting examinations, we designed, prepared, and evaluated a split coil. The split coil pair was comprised of three rectangle-shaped double pancake coils on each side. The lengths of the outer short and long side and the inductance of each coil winding were about 270, 340 mm, and 200 mH, respectively. A coil winding contains about 700-m REBCO tapes. The arrangement of six coils was determined by their transport properties at liquid nitrogen temperature. The split coil pair was cooled by conduction cooling and evaluated the coil properties evaluated by excitation. It was confirmed that the split coil successfully generated the magnetic field according to design. The temperature of the coil was guessed at about 50 K according to the magnetic field dependence of the critical current at various temperatures for the tapes used in the split coil pair. We observed aluminum melting, although incomplete, after a rotation of 1200 r/min in 100 s, with a coil current of 114 A in the aluminum melting test. These results suggested the effectiveness of the REBCO split coil to the aluminum melting process but there is a room to improve the split coil especially as the conduction cooling.

REBCO coils applied to superconducting magnetic energy storage system, magnetic resonance imaging, medical accelerator, and so on, are expected to result in high efficiency and compactness due to the realization of higher magnetic field and current density. Because these devices require large coil radius, huge hoop stress “BJR” (= magnetic field × current density × coil radius) is produced in the coil winding and the damage to these devices is likely to cause. In our previous study, we proposed a novel coil structure called the Y-based oxide superconductor and reinforcing outer integrated (YOROI) structure to achieve a high-strength REBCO pancake coil. The YOROI structure comprises upper and lower plates and the outer frame of the winding outer periphery. The frames are forcibly expanded by the superconducting coil winding situated inside when the coil winding is subjected to an electromagnetic force. The outer frame is connected to the upper and lower plates and transfers a part of the electromagnetic force from the coil winding to the reinforcing outer plates. The reinforcing outer plates then support the coil to suppress the coil deformation against the electromagnetic forces. The YOROI structure achieved the resistance to a maximum hoop stress of 2 GPa, which was calculated from the BJR in an excitation test at 4.2 K in 8-T backup fields. In this study, we investigated the effect of the stress control structure to realize the HTS coil with a mechanical strength of over 2 GPa using a three-dimensional finite-element structural analysis.

In magnetic resonance imaging and nuclear magnetic resonance applications, it is important that the high-temperature superconducting coils provide a high magnetic field in a miniaturized form factor. Our research group considers these requirements to be compatible with no-insulation (NI) coils in which the thickness of the Cu stabilization layer and the turn-to-turn contact electrical resistance are key parameters that enable both high current densities and high thermal stabilities. When the Cu stabilization layer is thin, the current density is impaired, but the thermal stability is improved. If the turn-to-turn contact electrical resistance is very large, then the characteristics of the NI REBCO coil approach the characteristics of conventional insulated coils, and the thermal stability may be degraded. However, when the turn-to-turn contact electrical resistance is large, the excitation delay time is reduced. In this study, we used a combination of the partial element equivalent circuit method and the finite element method to perform a current distribution and thermal analysis of the behavior of NI REBCO coils when a local normal-state transition occurs. We also considered the appropriate turn-to-turn contact electrical resistance that could shorten the excitation delay time as much as possible while still suppressing the formation of hotspots, which is a disadvantage of the NI REBCO coil.

To achieve a high current density and a compact REBCO coil, a reinforcing structure is essential to prevent coil degradation and deformation caused by the large Lorentz force. A new reinforcing structure for the REBCO coil called the Y-based oxide superconductor and reinforcing outer integrated (YOROI) coil was proposed in the previous study. The YOROI coil, which exhibited no degradation after an excitation test with a maximum hoop stress of 1.7 GPa at 4.2 K in 8-T backup fields, has great ability to reduce the stress and strain acting on the coil winding to maintain the coil shape and prevent degradation in the REBCO wire. In this study, the hoop stress in the winding and the stress shared by the reinforcing structure of a YOROI model coil were measured at 40 K in 10-T backup fields to clarify the reinforcing mechanism of the YOROI coil. A three-dimensional structural analysis, which simulated the experimental condition, was performed on the model coil to determine the distribution of stress and strain in the winding and reinforcing structure.

Recently, no-insulation (NI) winding techniques have been proposed for achieving high current density and high thermal stability. To verify the excellent characteristics achieved using these techniques, we investigated the transient behaviors of NI REBCO pancake coils. In this paper, we evaluate transient electromagnetic and thermal behaviors during a local normal-state transition, based on a partial element equivalent circuit and thermal coupled numerical analysis. Moreover, we investigate temporal and spatial current distributions in the NI coil winding after a local normal transition occurs. We also clarify the reason for the suppression of temperature increase caused by the local normal-state transition in the NI winding. Subsequently, we evaluate the influence of the thickness of a Cu stabilizer on the thermal stability of the NI and conventional insulation windings. Finally, we discuss the mechanism of high thermal stability attained in the NI coil winding technique.

We have been developing a no-insulation (NI) winding technique to be used in both high thermal stability and high current density, which have a tradeoff relationship. This paper presents numerical and experimental studies on the thermal stability of an NI REBCO pancake coil by focusing on a quench protection scheme. In the case of an accident or a local normal-state transition, the power supply for the superconducting coil must be immediately shut down in order to ensure its safety. During the sudden discharging of a conventional insulated winding coil, most of the stored electromagnetic energy is dissipated by an external dump resistor connected across the coil terminals as a quench protection system. However, in an NI pancake coil, current flows in the radial direction in order to bypass the local normal-state area to the adjacent turns. The stored energy in the coil may be dissipated as Joule heat owing to the turn-to-turn contact electrical resistance. In this study, by using experiments and numerical analyses, we evaluated two transient behaviors during sudden discharging: the distribution of energy dissipation and the temperature increase in model NI REBCO pancake coils. We also evaluated the thermal behavior of the NI coil during a local normal transition.

The billet heating technique by rotating conductive material in a magnetic field for aluminum extrusion has been reported, but metal melting technology by the induction heating with the dc magnetic field has not. Further complex geometries of aluminum profiles are formed by the casting that requires highly efficient metal melting technology. In order to verify the capability of induction heating using dc superconductor coils for metal melting in the casting process, we performed a metal melting experiment with the small examination equipment in a magnetic field. The small examination equipment consists of a motor, a heat insulation structure, a rotating shaft, a specimen holder, a frame, and others. An aluminum pipe of about 0.77 kg, where its outer diameter was 180 mm and its inner diameter was 170 mm, was put into the examination equipment. A dc magnetic field, ranging from about 1 T at the nearest point to 0.4 T at the furthest point, was applied to the pipe by the superconductor magnet. The temperature of aluminum exceeded 500 °C within 2 min by the rotation of the pipe at 800 r/min. Aluminum melting in the sample holder was observed after the rotation of 1200 r/min in 90 s, and some amounts of melted aluminum rose over the sample holder and sputtered out by centrifugal force. These results indicate that induction heating with the superconductor coils can supply melting metal of required amount in a short time.

A series of feasibility studies are carried out on the applications of a high-temperature superconducting (HTS) magnet system to a heavy-ion accelerator for particle cancer therapy. A novel HTS cyclotron is expected to be more compact and have higher efficiency than a conventional heavy ion synchrotron accelerator. A high current density is required for the HTS magnet to realize a compact HTS cyclotron. Further, a high-strength reinforcing structure is absolutely essential for the HTS magnet owing to the large Lorentz force caused by the high current density to prevent the magnet deformation and maintain the required high accuracy field. In a previous study, a novel coil structure called the Y-based oxide superconductor and reinforcing outer integrated “YOROI coil” was proposed and tested at 4.2 K in 8-T backup fields. The YOROI model coil exhibited no degradation after the excitation test with a maximum hoop stress of 1.7 GPa determined by the product of the magnetic field, current density, and coil radius. The mechanism of stress sharing between the coil winding and the reinforcing structures of the YOROI coil was numerically determined. In addition, the electromagnetic and mechanical properties of a reinforcing structure based on the YOROI coil for circular REBCO coils assuming part of the HTS cyclotron magnet system was also numerically simulated in the previous paper. In this study, a new reinforcing structure is proposed on the basis of the YOROI coil for a non-circular REBCO coil assuming part of the HTS cyclotron magnet system. The electromagnetic and mechanical properties for the new reinforcing structure are numerically determined. As a result, the proposed reinforcing structure has great ability to reduce the stress and strain acting on the coil winding to maintain the coil shape and prevent degradation in the REBCO wire.

A series of feasibility studies were carried out on the applications of a high-temperature superconducting (HTS) coil system for a medical accelerator system for particle cancer therapy by our group. A new HTS cyclotron is designed to be more compact and have higher efficiency than the conventional medical accelerator. High strength against electromagnetic forces is required for the development of compact HTS coils during the use of an HTS cyclotron. A new stress control structure for a high-strength REBCO pancake coil was proposed in the previous study. No deterioration was observed in the double pancake coil after a charging test of 1.5 kA at 4.2 K in 8-T backup fields. The maximum hoop stress was determined to be greater than 1.7 GPa because of the Lorentz force. The developed coil structure was able to share the hoop stress with not only the coil winding but also the reinforcing structures. In this study, numerical structural analysis based on the finite element method was performed on the new coil structure to clarify the mechanism of the stress sharing between the coil winding and reinforcing structures. In addition, the stress sharing effect of the meter-class REBCO coil assuming an HTS cyclotron system was determined.

In recent years, YBCO high-temperature superconducting (HTS) tapes have been expected to be applied to devices in power systems or in other industrial applications, owing to their rapidly improving quality and productivity. Our objective is to develop an HTS cyclotron with a compact body, high efficiency of output power, and high performance in heavy particle radiotherapy. In such an application, extremely high precision is required in the range 0.01% to 0.1% of magnetic field, both spatially and temporally. An isochronous field along with an azimuthally varying field is generated by an air-core pancake coil system during construction. Therefore, developing a technology for extremely high coil-winding accuracy is very important. In this study, we constructed an experimental model of a YBCO pancake coil using a high-precision coil winding machine. Then, the winding error in the radial and axial directions was experimentally evaluated using a pair of laser displacement meters and a surface roughness tester. We also developed a numerical simulation model to analyze the influence of winding error on the magnetic field distribution. The relationship between the winding error and the magnetic field of the meter-class YBCO coil assuming a real system was also evaluated by numerical simulation.

In the real application of high-temperature superconducting (HTS) coils to a superconducting magnetic energy storage (SMES) system, coated conductors are cyclically subjected to tensile strain due to electrical charging and discharging. A quench is not only caused by failure in the power supply and cooling system but also could be induced by local deterioration of the superconducting characteristics because of the cyclic strain during operation. In such a local deterioration case, the conventional detection method using voltage signal for low-temperature superconducting (LTS) coils is not applicable for the HTS coils because of the significantly slow velocity of normal-zone propagation. Furthermore, the voltage detection method is considered to be extremely difficult because the noise of the converters and other equipment is much larger than the local normal-zone voltage of the HTS coils. Therefore, a new quench-detection method for HTS coils is required. In our previous studies, a current detection method was developed for a cryocooler-cooled SMES coil wound with a kA-class laminated bundle conductor composed of four electrically insulated coated conductors. In the present study, experiments and numerical simulations were carried out on a double pancake model coil that assumes real SMES operation to verify the validity of the current detection method.

Although interlaminar fracture at a YBa2Cu3O7−δ (YBCO)/CeO2 interface was reported for YBCO coated conductors, this has not yet been investigated by a fracture mechanical approach. In the present study, we developed a mode I type fracture toughness test method for a YBCO coated conductor with an additional Cu layer using double cantilever beam (DCB) specimens. Fracture mechanism was investigated by microscopic observation by a scanning electron microscope (SEM), together with composition analysis by an energy dispersive X-ray spectroscope (EDS). A pre-crack introduced at the YBCO/CeO2 interface deviated from the interface, and propagated into the YBCO layer, and sometimes reached the Ag/YBCO interface. The fracture toughness, GR, for YBCO and the Ag/YBCO interface was evaluated to be 7–10 J/m2 and 80–120 J/m2, respectively. The complex stress intensity factor ratio, K2/K1, at YBCO/CeO2 interface was evaluated to be −0.19, and this ratio controlled the formation of microcracks in the YBCO layer. The main crack propagated into the YBCO layer accompanied with the formation of microcracks.

A multi-laminated HTS tape conductor wound into double-pancake coils has recently been used for large SMES. If the HTS tapes are simply laminated to form the conductor, the current distribution in the laminated tape conductor of the coil is not homogeneous because of the differences among all the tape inductances. Transposition of these tapes at the innermost or outermost layer of the coil is effective for homogeneous current distribution. However, this method would requires the same number of single-pancake coils as that of the HTS tape conductor, and hence the number of HTS tape conductors is restricted. In this paper, we propose a new method to control the current distribution in the laminated conductor using transposition at the innermost layer of a double-pancake coil and adjusting the gaps between the laminated tapes. We analyze the current distributions for multi-laminated HTS tape conductors for a double-pancake coil, and show the homogeneous current distribution of the laminated tape conductor. In order to verify the theory, we designed homogeneous current distribution coil wound with the multi-laminated (four times) HTS tape conductors by transposing them at the innermost layer and by adjusting the additional thickness 0.075 mm between the parallel conductors. We obtained the homogeneous current distribution in the tape conductor. The experimental data were in good agreement with the theory.

One of the most important properties of coated conductors for Superconducting Magnetic Energy Storage (SMES) is the relaxation property of persistent superconducting current. This property can be quantitatively characterized by the apparent pinning potential U 0 ∗ . In this paper, the dependence of U 0 ∗ on the thickness of superconducting layer d is investigated in the range of 0.33–1.43 μm at the temperature range of 20–30 K and in magnetic fields up to 6.5 T for Y 0.7 Gd 0.3 Ba 2 Cu 3 O 7− δ coated conductors. It was found that the value of critical current density did not appreciably depend on d at 20 K. This indicates that no structural deterioration of superconducting layer occurs during the process of increasing thickness. U 0 ∗ increases and then tends to decrease with an increasing magnetic field. The magnetic field at which U 0 ∗ starts to decrease increases with increasing thickness. This property was analyzed using the flux creep–flow model. Application of scaling law is examined for the dependence of U 0 ∗ on magnetic field and temperature. It was found that the dependence could be expressed using scaling parameters B peak , U 0 peak ∗ in the temperature range 20–30 K.

A multi-laminated HTS tape conductor has been recently developed to fabricate large double pancake coils. If the HTS tapes are simply laminated to form the conductor, the current distribution in the laminated tape conductor of the coil is unbalanced because of different inductances of all tapes. It is very important to analyze current distributions in the multi-laminated tape conductor used for the double pancake coil for SMES. In this paper, we analyze the current distribution in the tape conductor by using electrical circuit model, and then discuss how to obtain the homogeneous current distribution. One way is to transpose the tape position at both ends of pancake coil so as to arrange the tapes symmetrically. However, this method is not perfectly geometrical symmetry for more than 3 laminated tapes in the conductor. We propose new method to obtain homogeneous current distribution by adjusting gaps between HTS tapes in the conductor. Finally we numerically demonstrate the homogeneous current distribution in the 4-laminated tapes with inserting additional thickness among tapes.

Oscillating heat pipes (OHP) for cryogenic use are being developed to improve the heat removal characteristics of high-temperature superconducting (HTS) magnets. It is generally difficult to remove the heat generated in HTS windings, because the thermal diffusivities of component materials decrease with an increase of the operating temperature. Therefore, a local hot-spot can be rather easily generated in HTS magnets, and there are possibilities of observing degradation of superconducting properties and/or mechanical damages by thermal stresses. As a new cooling technology to enhance the heat removal characteristics in HTS magnets, the cryogenic OHP is proposed to be imbedded in magnet windings. The feasibility of cryogenic OHP has been confirmed by fabricating proto-types and by observing stable operations using hydrogen, neon and nitrogen as the working fluid. A high thermal conductivity was achieved that surpasses those of high-purity metals. We also propose a modified-type OHP to mitigate the orientation dependence.

From the viewpoint of a cryogen-free high-field superconducting magnet, the thermal stability properties of YBa2Cu3O7 (Y123) coated conductor tapes before thermal runaway are examined under the conduction-cooling condition by a GM-cryocooler. Under such cryocooling condition, Ag-sheathed Bi2Sr2CaCu2O8 (Bi2212) wires with Jc ~ 104 A/cm2 reveal stable behaviors of 3 times larger current properties before thermal runaway than critical currents, although conventional Nb3Sn wires with Jc ~ 104 A/cm2 quench almost at their critical currents under the cryocooling condition. In order to determine the maximum current-carrying capacity of the Y123 tape in high fields, we use a zero-dimensional heat balance model. On the basis of the measured basic properties for the Y123 tape, the calculations were done at B=20 T, bath temperature T0=4.2 K, Jc ~ 107 A/cm2, and cryocooling heat transfer coefficient h=10-3 W/cm2K. We found that the Y123 tape with Jc ~ 107 A/cm2 does not have the over-critical current but rather the sub-critical current before thermal runaway.

Transverse flux enhanced type superconducting synchronous machines have been studied for large-capacity wind power generation. The generators have multiple circular superconducting field coils in the rotor and copper armature windings in the stator. A parametric design study and electromagnetic characteristic analysis of the superconducting wind turbine generators (WTGs) have been carried out. The target output is 10 MW for a rotational speed of 10 rpm, and the diameter of the whole coil system is 5 m. Superconducting technology is quite effective for compact and lightweight design of large power capacity, low-speed and high torque generators. Armature and field coil dimensions and the number of poles are key parameters in the design study. FEM analysis was carried out to obtain magnetic flux density, magnetic flux linkage, induced voltage, fundamental generator characteristics, etc. Required superconducting wire length was also estimated.

A new Japanese national project, called M-PACC, to develop high temperature superconducting electric power devices started in June last year (FY2008–FY2012). This project aims to develop three different types of electric power devices that are expected to provide stable power supplies with large capacity and small size by using YBCO coated conductors. The first program is the development of a 2 GJ class superconducting magnetic energy storage system to control stable electric power systems. It is planned to develop several sets of element coils for a 20 MJ class system as a technological feasibility study for a 2 GJ class coil. The second program is the development of two different types of power cables with higher performance than existing power cables; one is a three-core 66 kV–5 kA class large current cable and the other is a single-phase 275 kV–3 kA class high voltage cable. These cable were required several technological developments, namely, large current and low AC loss, high voltage insulation and low dielectric loss, and power and heat balance for both cables. The third program is the development of a 20 MVA class power transformer with 66 kV/6.9 kV as a distribution transformer. In this project, power transformer systematization technology including 2 kA class large current coil technology, anti short-circuit wire winding technology, AC loss reduction technology, and winding technology will be developed.

The possibility of the occurrence of quench caused by mechanical disturbance in high-temperature superconductors (HTS’s) is much lower than that in low-temperature superconductors. However, to enhance the reliability and safety of HTS coils, it is necessary to establish a stability criterion and a protection scheme on the basis of the assumption that the quench is caused by the failure in power supply, cooling systems, etc. In this study, we assume that the stored magnetic energy in the HTS coil is dissipated by an external resistance that acts as a quench protection. Under this assumption, we derive a function that examines the heat balance of quenching YBCO coil under an adiabatic condition and propose a criterion using the derived function to determine an appropriate stabilizer thickness of YBCO conductors.

AC losses in a superconductor strip are numerically evaluated by means of a finite element method formulated with a current vector potential. The expressions of AC losses in an infinite slab that corresponds to a simple model of infinitely stacked strips are also derived theoretically. It is assumed that the voltage-current characteristics of the superconductors are represented by Bean’s critical state model. The typical operation pattern of a Superconducting Magnetic Energy Storage (SMES) coil with direct and alternating transport currents in an external AC magnetic field is taken into account as the electromagnetic environment for both the single strip and the infinite slab. By using the obtained results of AC losses, the influences of the transport currents on the total losses are discussed quantitatively.

Ion Beam Assist Deposition/Chemical Vapor Deposition(IBAD/CVD)-processed YBCO-coated conductors with high critical current density Jc at high magnetic fields are expected to be applied to superconducting equipments such as superconducting magnetic energy storage (SMES). For application to superconducting magnet in SMES one of the most important properties for superconductors is the relaxation property of superconducting current. In this paper, the relaxation property is investigated for IBAD/CVD-processed YBCO-coated conductors of the superconducting layer in the range of 0.18–0.90 μm. This property can be quantitatively characterized by the apparent pinning potential, U 0 ∗ . It is found that U 0 ∗ takes a smaller value due to the two-dimensional pinning mechanism at high magnetic fields for conductor with thinner superconducting layer. Although U 0 ∗ decreases with increasing thickness at low magnetic fields at 20 K, it increases at high magnetic fields. The results are theoretically explained by the model of the flux creep and flow based on the dimensionality of flux pinning. Scaling analysis is examined for the dependence of U 0 ∗ on the magnetic field, temperature and the layer thickness.

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of a low-temperature superconducting (LTS) coil. A HTS conductor has a greater volumetric heat capacity at the operating temperature envisaged for practical applications. Therefore, a HTS coil is much less likely to be quenched than a LTS coil by mechanical disturbances such as the heat generated by the cracking of the impregnation material or by the friction resulting from wire movements. However, the HTS conductor is cyclically subjected to tensile strain because electrical charging and discharging are repeated in real applications involving the Superconducting Magnetic Energy Storage (SMES) system. The superconducting characteristics may locally deteriorate due to this cyclic strain. Therefore, to enhance the reliability and safety of the HTS coil, a quench protection scheme is needed. Because the normal-zone propagation velocity is quite low, detecting a non-recovering normal zone is difficult in HTS coils, and quenching produces excessive overheating that may cause the conductor to melt. In this study, we focus on a coil wound with a YBCO bundle conductor used in SMES applications and investigate the redistribution characteristics of the transport current in and the thermal behavior of the coil during a quench; we use a newly developed computer code based on the finite element method (FEM) and an equivalent circuit. We also discuss a protection scheme to dump the magnetic energy stored in the coils on an external resistance connected in parallel.

A highly effective cooling technique for a superconducting magnet is proposed by incorporating the cryogenic oscillating heat pipes (OHP) as cooling panels in the coil windings. The OHP is a high performance two-phase heat transfer device, which can transport several orders of magnitude larger heat loads than heat conduction of solids. The cryogenic OHP using , Ne, and as working fluids have been developed and tested at the operating temperature ranges of 17-25 K (H2), 26-32 K (Ne), and 67-80 K (N2). The measured effective thermal conductivities were reached to 500-3,000 W/m · K (H2), 1,000-8,000 W/m · (Ne) and 10,000-18,000 W/m · K (N2). The high thermal transport properties of the cryogenic OHP and its application as the cooling components of superconducting magnets are also discussed.

To achieve large current capacity and high mechanical flexibility, YBa 2 Cu 3 O 7 (YBCO) superconducting cables consist of a number of YBCO coated conductors that are assembled and wound spirally on a Cu former. In practical applications, superconducting cables are vulnerable to short-circuit fault currents that are 10–30 times greater than the operating current. Therefore, in order to ensure the stability of YBCO superconducting cables in such a situation and to protect them from the fault currents, it is important to investigate the redistribution of the transport current and electromagnetic coupling between the conductor layer, shield layer, and Cu former. In this study, we carried out experiments on a 10-m-long YBCO model cable, which was manufactured by Furukawa Electric. An over-current with a peak of 31.8 kA rms and a duration of 2.02 s was applied to the model cable. We performed numerical simulations using a novel computer program developed using the 3D finite element method to elucidate the electromagnetic and thermal behavior of the YBCO model cable in the presence of an over-current.

High-temperature superconducting (HTS) power cables transmit bulk power with lower loss than conventional cables. Moreover, HTS cables are expected to be constructed as a new underground cable in urban areas at lower cost compared to a high voltage XLPE cable. To put promising HTS cables to practical use, we need (RE)BCO tapes with long length, high critical current, and low cost. Recently many organizations have improved the performance of the (RE)BCO tapes, such as YBCO tapes, or other coated conductor tapes that are made with a variety of different processes. We have fabricated the conductors for the HTS power cable that was constructed of different kinds of (RE)BCO tapes and measured the I c and AC losses. We achieved significantly low AC loss of 0.1 W/m at 1 kA in the HTS conductor using narrow slit tapes that were cut by laser. Moreover, a 20 m long HTS power cable model and a cable intermediate joint were developed. Short circuit current tests were conducted on the cable system that consisted of two 10 m cables, a cable joint, and two terminations. The cables and the joint withstood the short circuit current of 31.5 kA for 2 s without damage.

Five years of Japanese national project (FY2003–FY2007) was ended last spring with remarkable success. The national project was originally aimed for development of coated conductors which have high superconductivity performance and long length enough to fabricate high temperature superconducting (HTS) electric power devices. Preliminary research and development of HTS electric power devices were carried out as well. A series of R&D results will be summarized and reviewed in this paper. The new 5 years Japanese national project has started last June (FY2008–FY2012) to develop HTS electric power applications including SMES, cables and transformers and to develop/produce coated conductors satisfying the requirements from the respective power devices. Collaborative R&D efforts by national laboratories, universities and private companies have been started with the supports of NEDO and METI. The accomplishment of the previous project will be summarized and the plans and goals of the new project will be presented in this paper.

We fabricated 1 m high temperature superconducting conductor (HTS conductor) using YBa 2 Cu 3 O 7−x coated conductors (YBCO tapes) on textured metal substrates, which are expected to be lower in cost than YBCO tapes using ion-beam assisted deposition. Those substrate and intermediate layers were manufactured by Furukawa Electric, and YBCO and a protective layer were applied to the intermediate layer by Chubu Electric Power. Before fabricating the conductor, a 0.1 mm thick copper tape was soldered to the YBCO tape, and 10 mm wide YBCO tape was divided into three strips by a YAG laser. To have sufficient current capacity for 1 kA, a two-layer conductor was fabricated, and its critical current ( I c ) was 1976 A, but the magnetic properties of the textured metal substrates affected the increase in AC loss. In a low current region, the AC loss in this conductor was much higher than the Norris strip model, but approached the Norris strip model in the high current region because the magnetization was almost saturated. Low AC loss of 0.144 W/m at 1 kA rms was achieved even though the conductor had a small outer diameter of 20 mm and was composed of YBCO tapes with magnetic substrates.

The thickness dependence of the critical current characteristics was investigated for YBa 2 Cu 3 O 7− δ (YBCO) coated conductors fabricated by Ion Beam Assist Deposition (IBAD)/Chemical Vapor Deposition (CVD) method in the range of 0.18–0.90 μm to find out the optimum thickness for various applications. The transport and magnetization critical current densities were estimated using the electric field criterion of E c = 1.0 × 10 −4 V/m and E c = 1.0 × 10 −9 V/m, respectively. The critical current density decreased with increase thickness in the low magnetic field region in the both electric field region because of the structural degradation of superconducting layer. This decreasing rate was lower than in Pulsed Laser Deposition (PLD)-processed YBCO coated conductors. It is found that the thickness dependence of irreversibility field B i differed between the low and normal electric field region. In the low electric field region, B i increased with increasing thickness. On the other hand, B i is almost independent of the thickness in the normal electric field region. The thickness dependence of B i was also superior to that of PLD-processed YBCO coated conductors.

YBCO tapes are expected to be used in future high temperature superconducting (HTS) applications because of their good J c characteristics at high temperatures and in high applied magnetic fields. In applications to electric power devices such as transmission cables, transformers, and fault current limiters, the HTS conductors will be subjected to short-circuit fault currents that are 10 to 30 times the normal operating current. These overcurrents are greater than the critical current, and degrade or burn-out the HTS conductors. Therefore, it is important to clarify the mechanism of the degradation caused by such overcurrent pulses. We carried out preliminary experiments on damage caused by overcurrent pulse drive, focusing on the temperature limitation without suffering degradation for overcurrent pulse operation. A 10-mm-wide YBCO tape was cut into 2-mm-wide sample tapes by a laser beam, and the sample tapes were soldered on silver-deposited 100-mum-thick copper plates. Overcurrent tests were carried out on these sample tapes and Ic degradation was investigated. In addition the contact interface between YBCO and the Ag layer or buffer layer before and after the overcurrent drives has been investigated in order to clarify the correlation between the degradation and delamination of sample tapes.

The quality of high-temperature superconductors (HTSs) is rapidly increasing, and they can be used in power systems such as SMES systems and in industries as Si single-crystal growth pullers. In the future, HTS conductors are expected to be used for the magnets in nuclear fusion reactors or accelerators. In such applications, HTS conductors will be exposed to radiation. As a result, HTS conductors become radioactive making the operation and maintenance of the reactors or accelerators difficult; this also leads to an increase in the amount of nuclear waste generated. Therefore, we carry out experiments on the radiation damage in HTS conductors to study the characteristics of HTS conductors in radiation environments and to create an industrial database of the radiation effects on HTSs. We also carry out radioactive analyses on HTS conductors in order to clarify the relationship between the irradiation dose and the activity of HTS conductors. In this study, we prepared YBCO and Bi-2223 tapes and carried out the following preliminary experiments: (a) irradiation of the sample HTS tapes with 2-MeV neutrons at room temperature without transport current and (b) study of neutron-induced radioactivity by gamma-ray spectrometry using a Ge detector after the irradiation.

To achieve large current capacity and mechanical flexibility, high-temperature superconductor (HTS) power transmission cables consist of a number of YBCO coated conductors, which are assembled and wound spirally on a Cu former. In practical applications, superconducting cables might be subjected to short-circuit fault currents that are 10 to 30 times the operating current. Therefore, in order to ensure the stability and feasibility of HTS power cables and protect them from fault currents, it is important to estimate the redistribution of the transport current and electromagnetic coupling among the conductor layer, shield layer, and Cu former. In this study, we carried out experiments on a 20-m-long YBCO model cable, which was composed of two jointed 10-m-long YBCO model cables. Over-current with a peak of 31.8 kArms and a duration of 2.02 s was applied to the model cable. We performed numerical simulations using a newly developed computer program based on the 3D finite element method (FEM) in order to clarify the electromagnetic and thermal behaviors of the YBCO model cable in the presence of an over-current.

YBCO superconducting wire has a relatively low decrease in power distribution at high temperatures and under a high magnetic field. A high-intensity substrate is used for the wire, so the wire has high machine characteristics. Therefore, it is expected that this wire can be used for large-scale high magnetic field coils. Here, coordination between the SMES system for 100 MVA/2 GJ class load fluctuation compensating was conducted using IBAD/CVD-YBCO wire. The SMES system includes a toroidal type YBCO coil consisting of 180 compact, high magnetic field multi-unit coils, a large coil cooling system that uses the conduction cooling method, which does not use a refrigerant medium, and a multi-cell power converter that achieves multi-unit coil connection with relatively low current and low voltage. Studies were conducted for each individual device and for the whole system. Based on the study plan in this paper, it has become possible to develop and coordinate each device of the 100 MVA/2 GJ class power system load fluctuation compensation SMES system using YBCO wire, which up until now had seem impossible as an actual system.

A textured Cu {100} lang001rang substrate for practical use in low cost coated conductor applications has been successfully fabricated. A textured Cu substrate is a promising substrate for a coated conductor template. This is the result of a low material cost base, no magnetization loss and good electro conductivity. One of the issues for a Cu substrate is surface oxidation. During REBCO deposition, the Cu substrate is exposed to an oxygen atmosphere, and de-lamination of the buffer layer can occur. To prevent this issue a thin Ni layer was formed on the textured Cu substrate by electroplating. After a further annealing process the Ni electroplated Cu substrates exhibit a good sharp texture and a smoother surface. The Deltaphi and Deltaomega of the substrate were about 4.6 and 4.5 degrees, respectively. The surface roughness was estimated at 6.8 nm Ra by AFM. Using this substrate and clad type Cu/SUS316 substrates, we deposited a CeO2/YSZ/CeO2 layer by pulsed laser deposition (PLD) and an YBa2Cu3O7-delta layer by PLD and metal-organic chemical vapor deposition (MOCVD). The results show no occurrence of de-lamination. Typical critical temperature of the coated conductors was 90.5 K and critical current density was a maximum of 4.5 MA/cm2 (at 77 K in a self-field).

YBa2Cu3O7−δ, (YBCO) coated conductors with an additional Cu layer are expected to be applied as coils used in superconducting magnetic energy storage, SMES. In the operation of SMES, large cyclic hoop stress is applied to the coated conductor in the longitudinal direction. In the present study, we carried out fatigue tests in liquid nitrogen, LN2, and measured the critical current, Ic, after a specific number of fatigue cycles to clarify its fatigue fracture mechanism. All fatigue tests were carried out under a stress ratio (the ratio of the minimum to maximum load) of 0.5. The frequency of stress cycling was 30 Hz. Our results showed that the addition of a Cu layer increased the fatigue strength after 106 cycles, F N = 10 6 , by about 19%. Decrease of Ic was more than 30% of the initial critical current, Ic0, without loading at the point of final overall fracture when the maximum load in the stress cycles was close to that corresponding to irreversible strain. Furthermore, microscopic observation and the change in Ic showed that fatigue fracture mainly initiated from the Hastelloy C-276 substrate. Fatigue fracture also initiated from the Cu layer, but only when the maximum load was approximately F N = 10 6 .

La0.6Sr0.4Ru0.9Mg0.1O3-δ (LSRM) powder was prepared by co-precipitation method using ammonium carbonate solution and nitrate solutions. The freeze-dried suspension was decomposed at 1000°C in air. The obtained LSRM powder was sintered in air at 1200°C for 4 h to measure the electrical conductivity. The LSRM solid solution, which was stable in the temperature range from 1000 to 1500°C in air, was referred to an orthorhombic structure. The measured true density of LSRM powder was close to the density calculated using X-ray diffraction data. No chemical reaction occurred between LSRM and Gd-doped ceria electrolyte at 1000°C in air. The electronic conductivity of LSRM measured by a four-terminal DC method was 423 and 361 S·cm-1 at 300 and 800°C, respectively.

Transport losses in a coated conductor with a textured-metal substrate with reduced magnetism were studied experimentally. The substrate is with a clad structure, and HoBCO superconductor layer is deposited on the substrate with buffer layers. The measured transport loss of a sample whose critical current is 126.0 A falls between Norris’s strip value and Norris’s ellipse value. The increase in the measured transport loss from Norris’s strip value can be attributed to its non-uniform lateral J c distribution. The same buffered clad tape was placed under an IBAD–MOCVD coated conductor with a non-magnetic substrate, and its transport loss was measured. The comparison between the measured transport loss of this sample and that of the identical IBAD–MOCVD coated conductor without the buffered clad tape indicates that the increase in the transport loss due to this buffered clad tape is small. The transport losses of hexagonal assemblies of IBAD–MOCVD coated conductors, whose structure simulates that of superconducting power transmission cables, were also measured where the buffered clad tapes were under-lied or over-lied on the coated conductors. The increase in the transport loss of hexagonal assemblies of coated conductors due to the buffered clad tapes is at an allowable level.

High temperature superconducting (HTS) cables using YBCO tapes are expected to be more economical because AC losses will be much smaller than conventional cables. To reduce AC loss, 10 mm wide YBCO tapes were divided into five strips using a YAG laser. Using narrower strips and optimizing the space between the strips were effective in reducing AC loss. A 1 m conductor was fabricated, and AC loss was 0.048 W/m at 1 kA and 50 Hz. Based on the successful AC loss reduction in the 1 m conductor, we will fabricate a 10 m HTS cable with a three-layer HTS conductor, electrical insulation, and a one-layer HTS shield and cupper protection layer for overcurrent. In addition, we have developed a prototype of the HTS cable joint that can withstand an overcurrent condition of 31.5 kA for 2 s.

Prior to the development of fabrication technique for the chemical vapor deposited (CVD) YBa 2 Cu 3 O 7-δ coated conductor on a IBAD-Gd 2 Zr 2 O 7 , investigations on the improvement of spatial homogeneity have been done. By using spatially resolved measurements and combined multiple microanalysis techniques with length scale of several μm, physical and transport properties of the CVD samples have been investigated before and after fabrication modification. Structural inhomogeneity was visualized using thermoelectric voltage imaging (TVI) technique using a laser scanning microscope. Laser scanning microscopy at superconducting temperature is used to visualize flux flow dissipation; furthermore, mappings of 2D local current flow density distribution have been done using a scanning SQUID microscopy. It has been shown that the superconducting layer consisted of YBCO matrix with localized defects originating from the buffer layer. This led to current non-uniformity and caused high flux flow dissipation within the vicinity of the defects. Process conditions have been modified effectively based on those insights. After fabrication modification, our measurement analyses shows that the texturing of the YBCO layer improved significantly and the appearance of spatially distributed obstacles that are responsible for non-uniform current distribution and localized dissipation are reduced. Our complementing, quick yet non-invasive technique not only can quantify the improvement of YBCO homogeneity but also shed light on the basic understanding of the current limiting mechanism in the IBAD based coated conductors.