Search

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.

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.

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.

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.

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.

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.

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.

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.

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.

This paper describes a fundamental study on the generation characteristics of a high Tc superconducting induction/synchronous machine (HTS-ISM), with a structure the same as squirrel-cage induction machines. Bi-2223/Ag multifilamentary tapes are utilized for the fabrication of the secondary windings, and a commercialized 3-phase, 4-pole stator is introduced. The fabricated HTS-ISM is immersed in liquid nitrogen, and then operated at a frequency of 60 Hz. It is shown for the first time that our HTS machine can generate active power at synchronous speed when the primary windings are excited. This indicates that synchronous generation can be realized with the simple structure of HTS-ISM.

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.

We are experimentally studying basic AC loss properties of IBAD/CVD-YBCO coated conductors with a copper layer for stabilizing, especially the temperature dependence of perpendicular field loss in alternating electromagnetic environments. We prepared two types of short specimens with and without a copper layer and measured AC losses by a saddle-shaped pickup coil in an alternating magnetic field perpendicular to the wide surfaces at liquid helium temperature. In the ranges of the amplitude up to 4 T and the frequency up to 0.2 Hz, the AC losses both of the two specimens are hardly dependent upon the frequency. The results show that hysteresis loss is a major component of the AC loss in the specimens and the effects of the copper layer can be negligible. We also measured AC losses for the specimens with the copper layer at liquid nitrogen temperature to estimate the dependence on measurement temperature. The results suggested that the AC loss vs. the amplitude of applied field can be scaled by a critical current at a zero magnetic field.

YBCO tapes are expected to be used in future high temperature superconductor (HTS) applications as they have better Jc characteristics at high temperatures and in high magnetic fields. For power applications such as transmission cables, YBCO tapes and a copper former are connected in parallel and they might be subjected to a short-circuit fault current that is 10 to 30 times the normal operating current. Therefore, the over-current behavior, including the hot-spot and the distribution of transport current between the copper-laminated YBCO tapes and the copper former, is very important to examine the stability and feasibility of the cable. This paper describes the experimental results for over-current pulses of a 1-m single-layer cable fabricated with five copper-laminated YBCO tapes and a copper former. Further, we performed numerical simulations by using a newly developed computer program based on the 3D finite element method.

Overcurrent characteristics and reduction of AC loss are essential for high temperature superconducting (HTS) cable in a real grid. AC loss in an HTS conductor using YBCO could be potentially small but protection for overcurrent was needed. A 0.1 mm thick copper tape soldered to the YBCO tape was effective as protection from overcurrent and did not affect the increase in AC loss. The 2 m HTS conductor with Cu strands of 250 mm 2 and YBCO tapes with copper was fabricated. This conductor could withstand overcurrent of 31.5 kA for 2 s. To reduce AC loss, 10 mm wide YBCO tapes were divided into five strips using YAG laser. Using narrower strips and decreasing the space between the strips were effective in reducing AC loss. In consideration of this configuration, a three-layer conductor was fabricated, and AC loss of 0.054 W/m at 1 kA rms was achieved even though it had a small outer diameter of 19.6 mm.

Development of the SMES using high temperature superconducting (HTS) coil has progressed as a Japanese national project since FY 2004. The objective of the project is to obtain around twice the normal energy density via an HTS-LTS "hybrid" coil system compared to an LTS coil system. The HTS coil is composed of sixteen double-pancake coils and the double-pancake coil is wound with a Bi2212 Rutherford-type cable. The HTS coil is operated at 4.2 K in liquid helium bath with the LTS coil, and operated at maximum field of 8.9 T. Before we made a hybrid coil system, we verified the design issues using HTS experimental coils. In this paper mechanical compression tests, DC operation tests and thermal runaway tests using the experimental coils are discussed.

Magnetic field dependencies of the Ic of the IBAD/CVD-YBCO short tape sample and its small coil sample were measured in high fields, up to 18 T at 77 K. Compared with the Ic of the tape sample, the Ic of the coil sample at 0.1 muV/cm showed the same tendency in high fields. If YBCO tape is applied to a high-field coil application, the application should be operated at a temperature which is lower than 77 K. Using long CVD-YBCO tape, six stacked pancake coils were fabricated. Various current tests were conducted using one of these stacked coils. In AC current tests, thermal stability of the YBCO coil was estimated. When the peak values of AC current were 1.2 times higher than the maximum DC current in a thermal stable state, Idcmax, and the average electric field of the coil at the first triangular wave was about 10 times higher than 1 muV/cm criterion, the peak values of the built-up voltage did not tend to increase even after the 99th triangular wave current, and thermal run-away in the coil was not observed. In DC current with overlapped pulse current tests, the maximum peak current of the coil in a thermal stable state was obtained as a function of DC current and sweep time. It was 1.3 times higher than Ic and 1.4 times higher than Idcmax in a test condition. These results indicate that the YBCO coil has high potential in short-time, over-current operations at high temperatures. In cases where built-up voltages did not disappear, they began to increase just after the coil currents reverted to the initial DC currents. It was found that DC current influenced the increasing speed of built-up voltages once the pulse current had decreased to zero.

For HTS (high-temperature superconductor) applications to electric power devices, superconductors are subjected to short-circuit fault currents 10 to 30 times the normal operating current. These over-currents will drive the HTS conductors normal and degrade or burn-out the HTS conductors. Therefore it is important to establish a novel thermal stability criterion for over-current pulse drive. In this study, we carried out preliminary experiments on the damage caused by over-current pulse drive focusing on the limitation of the temperature rise without degradation. We prepared five YBCO sample tapes fabricated with IBAD/PLD and IBAD/ MOCVD methods. Measurements were performed at 60 K and 80 K (conduction cooling with Gifford-McMahon cryocooler was adopted) in self-field. The experimental results indicated that the degradation of the sample tapes for the over-current pulse with a duration of 1 second depended on not the initial critical current Ic, but the peak temperature Tpeak. The permissible peak temperature without degradation existed at about 400-600 K. The sample tapes were also observed before and after the experiments by magneto-optic (MO) imaging to find the distribution of degradation area.

A superconducting magnetic energy storage system (SMES) was developed and has been tested in the field (at a large LCD TV plant in Japan) in order to confirm its performance at bridging instantaneous voltage dips, its long-term reliability and its standby loss. The developed SMES system can store up to 7.34 MJ of magnetic energy in superconducting coils using a NbTi Rutherford conductor with a maximum electric output of 5 MVA. The field tests began in July 2003. During operation, the performing compensatory action of the SMES system on instantaneous voltage dips within the actual system was confirmed. Through the field tests of the SMES system, technical proof of the specifications required to provide instant, effective compensation for instantaneous voltage dips was verified

Since FY 2004, a four-year Japanese national project has been ongoing to develop an HTS-SMES as a sub-program of the Development of Superconducting Power Network Control Technology. The purpose of this project is to confirm the performance of a high energy density SMES coil with Bi-2212 cables. This coil system is composed of an NbTi outer coil and a Bi-2212 inner coil, consisting of 16 double pancakes. A Bi2212 cable of 140 m length is used for the double-pancake windings, and reinforcing materials are co-wound with the cable. A winding inner diameter of 360 mm was set according to the bending tolerance of the cable. The magnet is to be cooled at 4.2 K by liquid helium in order to obtain high overall current density. In the first year of this project, a conceptual design and various experiments for HTS magnet were carried out. This paper describes the test results of the HTS experimental coil that was developed to evaluate the electrical and thermal characteristics of an HTS. It is important to minimize deterioration and damage to the cable, because such defects cause fatal impact to the coil. Consequentially, high accuracy measurement of V-I characteristics up to10−8 V/cm range was performed in this experiment.

Superconducting Fault Current Limiting Transformer (SFCLT) is expected to perform functions both of transformer in the normal operating condition and of fault current limiter in the system fault condition. As the Phase-3 of the SFCLT project, we have been developing SFCLT based on Bi2212/CuNi bulk coils at LN2 temperature and verified its technical feasibility. In this paper, we developed a numerical model for evaluation of the electrical and thermal behavior of HTc-SFCLT such as current limitation and recovery characteristics. This model took into account E-J characteristics of Bi2212/CuNi bulk coil and its electrical and thermal transient phenomena during the operation of HTc-SFCLT. The simulated current agreed well with the experimental data with the error of less than 5％. The excellent current limitation and self recovery characteristics obtained by the experiments could also be reproduced. With the numerical model, current and thermal behavior of HTc-SFCLT was simulated for different parameters of conductor configuration, which would be useful for the future design and optimization of HTc-SFCLT.

The multiple-stage Chemical Vapor Deposition technique was developed for making a long coated conductor with high deposition speed. In this paper, a multi-coating of YBCO layer using 6-stage CVD system was described. Substrates were PLD-CeO/sub 2//IBAD-Gd/sub 2/Zr/sub 2/O/sub 7//Hastelloy. Thickness of each layer was about 1.0 /spl mu/m, 1.0 /spl mu/m and 0.1 mm respectively. /spl Delta//spl phi/ of CeO/sub 2/ layer was about 9-10/spl deg/. Deposition rate of YBCO layer was 25 m/h and 5 m/h. Number of passing was 10 times and 2 times respectively. I/sub c/ of the tape which was prepared at a speed of 25 m/h /spl times/ 10 passes was 100 A/cm. Thickness of total YBCO layer was about 1 /spl mu/m and its J/sub c/ was 1 MA/cm/sup 2/. I/sub c/ of the tape which was prepared at a speed of 5 m/h /spl times/ 2 passes was 101 A/cm. Thickness of total YBCO layer was also 1 /spl mu/m and its J/sub c/ was 1 MA/cm/sup 2/.

Superconducting Fault Current Limiting Transformer (SFCLT) is one of the promising superconducting power apparatus, since it has both functions of superconducting transformer in normal condition and superconducting fault current limiter in fault condition. In this paper, current limiting and the recovery tests of high temperature SFCLT (HTc-SFCLT) with MCP-BSCCO2212 bulk coils were performed at liquid nitrogen temperature. Experimental results revealed that the HTc-SFCLT could successfully recover into the superconducting state by itself immediately after the fault clearance. Moreover, the recovery criterion of HTc-SFCLT as a function of load current was clarified.

A SMES system of MVA class for bridging instantaneous voltage dips has been developed using Bi-2212 wire. The Bi-2212 wire has high-performance conductive characteristics that do not deteriorate at a low temperature in high magnetic fields beyond 10 T. These characteristics enable a compact design of a SMES system of the Bi-2212 wire. In addition, coils of the Bi-2212 wire can be adequately insulated due to a high temperature margin. Therefore, the SMES system designed by using the coils has advantages to enhance dielectric strength and output power of the system. In our previous study, a SMES system consisting of 4 unit coils was constructed and the various properties were examined. Up to the present, the total 18 unit coils were stacked to make a coil system (outer diameter: 700 mm, height: 554 mm, stored energy: 984 kJ) and installed into a SMES system of 1 MVA for bridging instantaneous voltage dips. Also, the cooling system of the HTS SMES has been improved. The characteristics of the conduction cooled HTS coils of 1 MJ class were investigated in the operations of 1 MVA SMES system for bridging instantaneous voltage dips. Thermal reliability was verified during each operations of exciting, standby, bridging and current damping. Moreover, the repetitive bridging operations even worked out every 5 minutes. Advantages of the conduction cooled HTS coils for SMES were verified.

We investigated phase formations of multifilamentary Bi-2212/Ag wires during the heat-treatment. We carried out systematic quenching experiments. In the partial-melting process in pure O/sub 2/, when heating rate was 10/spl deg/C/h, melting of filaments was started from central filaments and spread to the edge-located filaments. However, when heating rate was 5/spl deg/C/h, melting of filaments was caused uniformly. In the partial-melting process in 8% O/sub 2/ also, melting of filaments occurred caused uniformly. Phase identification indicates that the existing alkaline earth cuprate was 2:1 alkaline earth cuprate in 8% O/sub 2/ while alkaline earth cuprate was 14:24 alkaline earth cuprate in pure O/sub 2/. These results showed effect of O/sub 2/ partial pressure in filaments. In the slow solidification process, the Bi-2212 phase grew up greatly and broke through the silver matrix in pure O/sub 2/, but the grain growth was suppressed in 8% O/sub 2/, resulting in the suppressing of bridging of Bi-2212 filaments.

In this paper, fundamental characteristics of conduction-cooled Bi2212/Ag wires and coils for HTS SMES were investigated, in terms of critical current at different temperatures at 4.2 K.55 K. Experimental results revealed that the critical current did not always increase with the temperature decrease, but was saturated or even decreased, depending on the test sample condition. Thermal analysis considering temperature dependence of specific heat and thermal conductivity of the materials could explain the critical current-temperature characteristics, and suggest the possibility to optimize the operating temperature of SMES.

NdBa 2 Cu 3 O 7 (Nd-123) and YBa 2 Cu 3 O 7 (Y-123) films were fabricated on {1 0 0}〈0 0 1〉 or {1 1 0}〈1 1 0〉 textured Ag tapes by a pulsed laser deposition technique and a metal organic decomposition method using metal naphthenates solution. Good biaxial crystal orientations were achieved in the Nd-123 films prepared on the {1 0 0}〈0 0 1〉 textured Ag tapes by the laser deposition technique. Biaxially oriented Y-123 films were also obtained on the {1 1 0}〈1 1 0〉 textured Ag tapes by the pulsed laser deposition technique and a metal organic decomposition method. We obtained the prospect that the high deposition rate and the thickening the film will solve both the groove problem and the Cu diffusion problem when the textured Ag tapes were used as substrates for coated conductors.

We have been developing a high temperature superconducting fault current limiting transformer (HT/sub c/-SFCLT) with functions of both superconducting transformer in normal operating condition and superconducting fault current limiter in fault condition. The ratings of the developed HT/sub c/ SFCLT are 3-phase, 6.25 kVA, 275/105 V. The secondary coil consists of melt cast processed (MCP)-BSCCO2212/CuNi composite bulk monofilar coils, whereas the primary coil is composed of a copper coil, both of which are immersed in liquid nitrogen bath together with the iron core. Experimental results revealed the operational feasibility of HT/sub c/-SFCLT through no-load and short-circuit tests, current limiting test and recovery test.

One of the advantages of Bi-2212 is that a high J/sub c/ value can be obtained in wires with various shapes and filament configurations. Realization of a high J/sub c/ round wire made it possible to fabricate many kinds of cable developed for applications of low temperature superconductors (LTC). We successfully fabricated a 30-strand Rutherford cable using 0.8 mm/sup d/ round wire. Optimization of cabling factor enables to reduce the I/sub c/ degradation at the cable edge. I/sub c/ values were about 650-750 A at 65 K in a noninductive winding condition, which corresponds to 11-13 kA at 4.2 K in self-field.

Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (hereinafter referred as Bi-2223) transposed segment conductors were developed for high-T/sub c/ superconducting power cables. As the conductors have good mechanical, electrical and thermal properties, the conductor has applicability for other application. So, we have been developing a solenoid coil with Bi-2223 transposed segment conductor. The fabricated coil with the conductor was demonstrated successfully to transport AC current. The performance test results indicate that the conductor is suitable for the coils of pulse or AC application, especially SMES coils.

Superconducting flywheel usually adopts carbon fiber resin mold plastic (CFRP) material that has the basic characteristics such as higher strength and lighter weight. Stabilized rotation of flywheel requires lower unbalance individually about flywheel and supporting disc. And supporting disc is needed to steadily follow the expansion of the inside diameter on the flywheel by centrifugal force and keep the concentricity of the flywheel and its own rotating axis. We developed the flywheel rotor, which was composed of CFRP flywheel, and partially spherical supporting disc made of high strength aluminum. They were combined with expanse fitting each other and most precisely machined to realize the extremely precise axial symmetric shape and enable to be supported and driven at the point near the center of gravity. We manufactured and confirmed that the flywheel rotor rotated up to peripheral speed of 628 m/s quietly without permanent set.

Flywheel with superconducting magnetic bearings requires the characteristics for the motor/generator such as lower loss, higher efficiency, lower bearing load and more displacement tolerance of the radial directions. We developed an extremely flat shape axial gap type motor/generator which consists of a rotor with permanent magnets and slotless windings to satisfy these characteristics. We introduced the system for adjusting intensity of the excitation to decrease the eddy current loss during the storage and to get the controllability of electromotive force for variable speed operation during charging and discharging. We manufactured the motor/generator of output power 17 kW at 10,000 rpm. It was tested to perform the fundamental functions of motor and generator at partial speeds up to 4000 rpm.

Superconducting magnetic bearing for flywheel requires the characteristics such as higher stiffness, lower loss and higher stability. There are two types of superconducting magnetic bearings, one is axial gap type and another is radial gap type and the characteristics of these types are quite different. We think that the supporting system of superconducting flywheel should support the rotor at one position near the center of gravity to minimize the cooling energy loss. We propose that the bearing composed of axial gap type and radial gap type is necessary from the result of this investigation, because the characteristics about both types of bearings should be compensated each other.

Super-ACE project started to research and develop the superconducting AC power equipment as a national project in 2000 fiscal year. This project is R&D of the basic technologies of high temperature superconducting (HTS) cable, HTS fault current limiter (FCL), HTS magnet for reactor and transformer. Main subjects of the cable are to develop an HTS conductor rated 3 kA, cooling technology of a 500 m HTS cable of 700 A rated current, and the analysis of integrated HTS power system. The paper describes the background of this project, its target, major results of studies, and testing schedule of 500 m cable.

We have been investigating the feasibility of a superconducting fault current limiting transformer (SFCLT) with the functions of fault current suppression and system stability improvement. This paper discusses these functions with consideration of overvoltage and thermal characteristics of SFCLT, and their dependence on recovery time into superconducting state after the quench of SFCLT. Simulation results using EMTP suggested that SFCLT would be coordinated with the background power system under the appropriate superconducting recovery time. The operating parameters of SFCLT were also optimized in a simplified power transmission model system. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(2): 40–47, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10070

We have been developing a superconducting fault current limiting transformer (SFCLT) with 3-phase, 500/275 kV, 625 MVA and optimized the main parameters by EMTP simulation. In this paper, we designed and fabricated an experimental scale-down model of SFCLT with 3-phase, 275/105 V, 6.25 kVA, using NbTi superconducting wire. We introduced the experimental model SFCLT into a transient network analyzer consisted of synchronous generators, transformers, transmission lines, circuit breakers and an infinite bus. It was revealed that experimental model had effective function-parameters as was simulated and experimental results clarified the effectiveness of SFCLT having both functions of the fault current suppression and the system stability improvement in a future superconducting power system.

We have successfully improved the J/sub c/ value of Bi-2212 round wire to 500 kA/cm/sup 2/ at 4.2 K in self-field. The J/sub c/ value strongly depended on the filament size, aspect ratio, and geometry. Adjusting the filament size made it possible to obtain wires with J/sub c/ values higher than 450 kA/cm/sup 2/ in a range of wire diameters from 0.8 to 1.3 mm. The J/sub c/ value remained constant up to an applied tensile strength of 150 MPa. A 1 + 6 stranded cable fabricated using 1.02 mm diameter wires carried an I/sub c/ value of 4.5 kA at 4.2 K in self-field. A 16-strand Rutherford cable could also be fabricated using this wire.

An axial air gap type thrust bearing of the outer diameter of 620 mm for 25 kW h flywheel which combined with the permanent magnet of Nd–Fe–B and 10 pieces of high temperature YBaCuO superconductors of the diameter of 150 mm directly cooled by the refrigerator was fabricated, and its performance tests were conducted. Load capacity which could support a planned rotor weight of 8.2 kN was confirmed, and also the stiffness of the radial direction and the axial direction were measured in case of several initial cooling conditions. Spring and damping coefficients in zero field cooling were also measured in the tapping test, and it was found that axial spring coefficients were about 10 times as large as radial spring coefficients, though damping coefficients were almost the same.

We have developed HTS cables using Bi-2212 round wires, which have high mechanical strength and an average J/sub c/ of 200 kA/cm/sup 2/ at 4.2 K in self-field. Four designs of cables were fabricated in this work and those tables could carry I/sub c/ values from 1 kA to 10 kA at 4.2 K and self-field depending on the number of strands. A 70m-long 20-strand Rutherford cable was successfully manufactured. Optimization of heat treatment conditions was effective to reduce I/sub c/ degradation observed in the case of heat treatment with a long length. I/sub c/ values of a Rutherford cable were measured under loading at 4.2 K in 4 T. The critical values of the stress on face loading and edge loading were 60 MPa and 100 MPa, respectively. We successfully obtained high J/sub c/ value of 500 kA/cm/sup 2/, which was almost double that of the conventional wires capitalize "Rutherford".

In this paper, the authors propose a "superconducting fault current limiting transformer (SFCLT)", a superconducting transformer equipped with the function of a superconducting fault current limiter. They discussed the operating characteristics of SFCLT introduced into a simplified power transmission model system. Fault current, transient stability, overvoltage and thermal characteristics of SFCLT were analyzed in the model system. It was finally revealed that SFCLT could satisfactorily bring about the functions of fault current suppression and power system stability improvement.

Flywheel energy storage systems with high temperature superconducting magnetic bearings are expected for load leveling use. A 1 kWh flywheel of 600 mm diameter was developed and the maximum energy storage of 1.4 kWh at 20,000 RPM was attained. For the development of a large capacity flywheel system, it is necessary to sophisticate the cooling system and improve the performance of the HTS magnetic bearing. So, an advanced cooling system of the bearing with a cryogenic refrigerator has been developed, and also the bearing characteristics in several cooling conditions have been investigated. It has been confirmed that the repulsive force of the bearing at 50 K was 1.5 times of its value at 80 K, and the rotating loss of the bearing at 50 K was one third of its value at 80 K, but the lateral magnetic stiffness at 50 K became small in comparison to that of 80 K. On the basis of the above results, an HTS magnetic bearing for 12.5 kWh flywheel system is under development.

A high temperature superconducting magnet using newly developed Bi-2212/Ag Rutherford cable was trial manufactured as a partial model for a 10 T magnet. The developed magnet consists of 4 double pancake coils constructed from a Rutherford cable with a critical current of 3.5 kA at 4.2 K, 0 T. The magnet has inside and outside diameters of 100 mm and 180 mm, respectively and is 75 mm long axially. The coil successfully operated with DC 1.25 kA and 1.5 kJ was stored stably. A maximum magnetic field of 2 T with A current of 1.47 kA was confirmed in liquid helium. Also, the authors performed excitation tests under the high-speed rate of change of magnetic field (0.54 T/S) and 300 cycles simulating alternating operation, and estimated the value of AC losses at 5.3 W using the change of liquid helium level.

A new CVD system equipped with a triple-reactor was developed to improve the forming rate of the superconducting tape. Three-layered Y123 films with good crystallinity covering the total thickness were obtained directly on roll milled non-textured Ag tapes by using this triple-stage CVD system. 11 m long Y123 tape with a uniform J/sub c/ of 3.0/spl times/10/sup 4/ A/cm/sup 2/ at 77 K, 0 T was successfully prepared on roll milled non-textured Ag tape at a forming rate of 3.0 m/h, Y123 films formed directly on the cube-textured Ag[100] tapes showed two types of orientation, cube-on-cube and diagonal-on-cube, between Y123 and Ag. On the other hand, Sm123 and Nd123 films showed the simple orientation, cube-on-cube, so these films were expected to improve the in-plane alignment of Y123 film by inserting them as the intermediate layer.