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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.

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.

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.

A new Japanese national project has started in 2008 to develop high temperature superconducting electric power devices. In this project, we have developed a superconducting magnetic energy storage (SMES) system, which are highly expected as a stable power supply. Main issues are a large current capacity conductor made of YBCO-coated tapes and a compact coil wound the conductors. Aiming at the manufacturing of compact coil with higher energy density through the creation of higher magnetic field, we produced YBCO coil, and by carrying out hoop stress tests. Also, aiming at the development of 20 MJ class SMES component coils necessary for picturing the technological outlook of 2 GJ class SMES coil for power system control, we manufactured component coils (outer diameter 650 mm), adopting the bundled conductor that would enable the realization of high strength and low loss, proving that its current capacity was large enough to exceed 2.6 kA, through basic verification test for energization. In addition, we have challenged the development of high efficiency coil conduction cooling technology, stable manufacturing technology of coated conductors for SMES and the highly reliable/highly durable SMES coil component technology.

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.

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.

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.

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.

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 .

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.

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.

For the application to power transmission cables, a number of YBCO tapes would be assembled and wound in spiral on a Cu-former. YBCO tapes and the Cu-former are connected in parallel and they might be subjected to short-circuit fault currents 10–30 times the operating current. In this study, we constructed a 1 m long YBCO model cable. Over-current with a peak of 31.5 kArms and a duration of 2.0 s, which was established by JEC (Japanese Electrotechnical Committee), was applied to this cable in a liquid nitrogen bath. The redistribution of the transport current between YBCO tapes and the Cu-former were examined by using Hall sensors. The numerical simulations were carried out using a newly developed computer program based on the 3D finite element method (FEM) in order to clarify the over-current characteristics in the cable. From the comparison of the experimental and simulation results, the validity of the developed computer program was confirmed. Therefore, we performed the simulations of a 10 m long model cable when carrying a fault current of 31.5 kArms applied for a durations of 2.0 s, and also estimated the influence of the cross-section area of the Cu-former and the Cu shield layer on the thermal behavior of the model cable by using the developed computer program.

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.

Transport characteristics of IBAD/ CVD-YBa2Cu3O7 (YBCO) coated conductor were measured at 4.2 K under hoop stress. The conductor was fabricated by a multi-stage metal-organic chemical vapor deposition (CVD) method. The YBCO layer was deposited on Hastelloy substrate with PLD-Ce02 and IBAD-Gd2Zr207 buffer layers. A 20-mum silver layer was sputtered as a protective and stabilizing layer. The hoop stress test coils were fabricated by winding the conductor on a 250-mm diameter stainless-steel bobbin by five turns. Two coils, denoted as coils A and B, were fabricated. The Hastelloy substrate located outside for coil A and inside for coil B. Both coils were tested in magnetic field at 4.2 K under hoop stresses. Coil A and B experienced 1028 and 777 MPa at 11 T, 4.2 K. The measured stress-strain curves provided that the Young's modulus of the conductor was 190 GPa. The tolerable stress of ~1000 MPa and the Young's modulus of 190 GPa are consistent with the values obtained by a tensile test. The hoop stress test results indicates that the YBCO coated conductor is promising for application under huge hoop stress.

Toward the industrialization of YBa2Cu3O7-x (YBCO) coated conductors for practical applications, metal-organic chemical vapor deposition (MOCVD) has been investigated. We applied multi-stage MOCVD to achieve high-speed production of coated conductor because the increase of the number of depositing stages placed in a line could enlarge deposition area. Using 12-stage CVD, we succeeded in deposition of highly biaxially oriented YBCO layer at tape transfer speed of 50 m/h on the CeO2 capped ion-beam-assisted-deposition (IBAD) - Gd2Zr2O7 (GZO) buffered Hastelloy tape, and achieved high critical current density ( Jc) exceeding 2 MA/cm2 in a short sample. And it was found that lowering substrate temperature according to YBCO layer growth was effective to increase Ic. Using this method, high critical current (Ic) of 174 A was achieved by 1 mum thick YBCO layer which passed through 12-stage CVD, and 227 A by 1 mum thick YBCO layer deposited by 6-stage one. Based on these results, three 200 m-class YBCO layers were deposited by multiple depositions at tape speed of 50 m/h each, using the 12-stage CVD system. A 203 m-long YBCO layer was successfully deposited with end-to-end Ic of 92.8 A, so that the multiplication of Ic and length reached 18.8 kA ldr m. These results indicate that a multi-stage CVD technique is useful for a rapid fabrication of long YBCO coated conductor.

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.

High Tc superconducting (HTS) toroidal coil using YBCO coated conductor is designed for 70 MJ class superconducting magnetic energy storage (SMES) for power system control. Its configuration and shape are optimized by means of genetic algorithm (GA) to minimize the required length of the conductor. The optimization is performed for two kinds of constrains, i.e., maximum electric field or flux flow loss of the coil, which are calculated by means of finite element method (FEM). The FEM analysis considers quantitative current density (J)-electric field (E) expressions based on percolation transition model. It is shown that the great transport performance against magnetic field of YBCO coated conductor can realize a very compact SMES coil compared with an existing Nb-Ti's one.

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.

YBCO coated conductors have been expected for the application to a coil for superconducting magnetic energy storage (SMES). In the application to a superconducting coil, the coated conductors experience bending, uniaxial tensile strain and their combined strain. Therefore, the influence of simultaneous bending and tensile strain on critical current should be revealed. In this work, we developed the test method of critical current under such combined strain state. As a result, it was confirmed that compressive pre-bending can improve the stress tolerance of the YBCO coated conductors. On the other hand, compressive bending strain suppresses the initial critical current by the intrinsic strain effect. These results indicate that optimal bending radius should be selected in order to realize superior stress tolerance and high current capacity simultaneously.

We have developed YBCO coated conductors by Multiple-Stage Chemical Vapor Deposition (MS-CVD) technique, which can make YBCO layer with high deposition rate and easy to scale up by increasing the number of reactors. We confirmed that the high speed production by the twelve-stage CVD equipment, which was designed to scale up CVD process, did not affect the property of YBCO coated conductors and that the YBCO coated conductors with I c of 100 A-class could be fabricated by the high speed production of 10 m/h. In order to demonstrate the advantage of the MS-CVD technique for longer YBCO coated conductors, we have also fabricated the 92-m-long YBCO conductor on an IBAD substrate. The YBCO conductor was fabricated with a tape moving at the speed of 25 m/h and was deposited several times to control the thickness of the YBCO films. The end-to-end I c of the 92-m-long YBCO tape was measured by criterion of 1 μV/cm and was 96 A ( J c of 1.9 MA/cm 2 ). We have successfully realized the fabrication of the YBCO coated conductor with the practical properties of 100 A-class in I c and 100 m-class in length. Recently, we have also obtained high I c of 227 A ( J c of 2.3 MA/cm 2 ) in a short sample of a YBCO conductor on IBAD substrates by optimization of the fabrication process.

The lateral J c distributions of YBCO coated conductors fabricated by IBAD/MOCVD process were measured by the magnetic knife method. In the samples, YBCO layer was deposited by MOCVD process on the IBAD template of GZO together with CeO 2 layer. Ag-protective layer was put on YBCO layer. A series of voltage taps were attached with 10 mm separation, and the lateral J c distribution was measured in each section between two voltage taps. A sample YBCO coated conductor in which YBCO layer was deposited five times using an up-to-date set-up shows a rather uniform J c distribution. The magnitude of J c reaches 2 × 10 10 A/m 2 (2 MA/cm 2 ) in wide area of the conductor.

This report describes a basic design of an HTS-SMES coil wound with large current capability Bi-2212 cables. The objective of this development is to evaluate fundamental properties of the coil in a high magnetic field. The coil is comprised of an HTS inner coil wound with Bi-2212 Rutherford cables and an LTS outer backup-field coil wound with NbTi conductors. The Bi-2212 coil is designed to generate a maximum field of 8 T at 4.2 K within the backup field at an operating current of 1584 A. In this study, general specifications of the coil were discussed with some important issues; winding dimensions for a peak field, current density limitation, elasticity of the winding, and protection from the thermal runaway. The coil is fabricated in FY 2006, and testing will be performed in FY 2007. In the experiments, superconducting characteristics of the coil in a high magnetic field will be evaluated

A high speed deposition technique for YBCO coated conductors has been required from the viewpoint of cost reduction for second generation wire. The multiple-stage chemical vapor deposition technique was employed for making YBCO coated conductors with a high deposition rate. In our earlier research, over 2.0 MA/cm 2 of J c was confirmed on IBAD (PLD-CeO 2 /IBAD-Gd 2 Zr 2 O 7 /Hastelloy) substrates with a maximum deposition rate of 25 m/h using the six-stage CVD system. To evaluate the availability of the multiple-stage CVD technique for making a longer YBCO tape, we have fabricated a 200 m-long YBCO tape on a non-textured silver substrate. Deposition rate was 10 m/h, and number of passing was two times. Total deposition time was 40 h (20 h × 2). During the deposition of a YBCO layer, the CVD system was quite stable. J c was measured at a voltage tap interval of 10 m with criterion of 1 μV/cm. J c of the YBCO tape remained in the order of 10 4 A/cm 2 due to the non-textured microstructure. However, J c distribution of the tape showed a good uniformity. A 210 m-long YBCO/(Ag + IBAD) tape was also prepared. A 210 m-long substrate combined with IBAD and Ag was used. I c of 60–78 A was obtained on the IBAD substrates part. This result suggested that the multiple-stage CVD technique was very useful to making a long YBCO tape.

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.

The multiple-stage CVD method is the promising technology, which can be realized for rapid and long formation of YBCO tape. In earlier researches, we have fabricated the six-stage CVD system, and successfully developed 100 m long YBCO tape at the deposition rate of 10 m/h. However, over 500 m long wires are required for the practical applications. In order to achieve this, we have studied the multi-coating of YBCO layer at further high-speed deposition. Five layers of YBCO tape were fabricated on roll milled non-textured Ag substrate directly. Each layer was deposited by the six-stage CVD system at deposition rate of 25 m/h. After each deposition, temperature of YBCO tape went down to room temperature. The distribution of J c was within 25% and its value is the same as single coating one. No significant degradation of J c was observed. From these results, it was indicated that the multiple-stage CVD system, which equips more reactors serially, is effective way for making longer YBCO tapes.

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.

MJ-class HTS SMES has been developed for bridging instantaneous voltage dips using newly-developed Bi-2212 cable. The developed Bi-2212 wire for SMES coils achieves high-performance conductive characteristics that do not deteriorate in high magnetic fields beyond 10 T, which enable compactly arranged SMES coils to be operated in a high magnetic field, and SMES coils of the Bi-2212 wire can be adequately insulated due to a high temperature margin. Therefore it is possible for the SMES coils to enhance dielectric strength and output power. The insulating, cooling and conductive characteristics of 4 unit coils (Outer Diameter: 700 mm, Height: 127 mm, Stored Energy: 90 kJ) were checked under a variety of conditions. Moreover, fundamental performance tests were done on bridging instantaneous voltage dips, using a 125 kW resistance and 50 kW motor as imitation loads. Testing showed that the HTS SMES operated reliably. Up to the present, 11 unit coils (Outer Diameter: 700 mm, Height: 390 mm, Stored Energy: 560 kJ) have been stacked and tested.

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.

The superconducting magnetic energy storage system (SMES) of output 5 MVA has been developed to bridge instantaneous voltage dips. The field examination is executed by setting up this system at the new and large liquid crystal factory in Japan. The developed SMES system can store up to 7.34 MJ of magnetic energy in superconducting coils using the NbTi Rutherford conductor. The maximum output of the developed SMES is 5 MVA, and the system can discharge the output for one s. The operating current of the coil is 2.7 kA, and the rated voltage is 2.5 kV. The field examination was started in July 2003. During the field test, we will confirm the performance of bridging instantaneous voltage dips by SMES, the long-term drive reliability and the standby loss characteristic.

We developed rectangular-shaped Bi-2212 conductors. At 4.2 K and 30 T, the 37- and 349-filament conductors exhibited J C values of 151 and 125 kA/cm 2 , respectively. Moreover, we measured the distribution of I C for a 25 m-class 349-filament conductor. The end-to-end I C value was 66 A at 63 K, self-field, and the variation in I C was about ±10%. We also fabricated a test coil with 50 m-class, 349-filament conductor. The test coil carried 510 A (using a criterion of 10 −13 Ω m) and generated B 0 =0.95 T at 4.2 K without backup field, resulting in a current density in the test coil of 150 A/mm 2 . The coil performance was almost 80% that of a short sample with a magnetic field applied perpendicular to the tape surface.

Cost reduction study of 100 MW/500 kWh SMES for load compensation and frequency regulation is performed under Japanese national project. A newly designed CIC (Cable In Conduit) Conductor is developed for the SMES (Superconducting Magnetic Energy Storage). The number of superconducting strand in the cable is reduced with segregated Cu strands. The cross-sectional structure of a strand with CuNi sheath is simplified from the previous design. To reduce AC loss time constant, strands are twisted up to second degree, which is much lower than that of the previous cable design. A short sample of the conductor is manufactured and tested. The test results show that the conductor has good performance for the SMES.

Textured Ag tape is one of the most ideal substrates for the next generation high-Tc wires operable in liquid nitrogen. {1 1 0}〈1 1 0〉 textured Ag tapes were obtained by cold rolling and subsequent heat treatments. The texture of the tape after cold rolling (before annealing) was typical “brass type”. For obtaining the {1 1 0}〈1 1 0〉 texture, oxygen should exist in the Ag tape during the heat treatment. We find a two-step annealing is very effective to achieve the better {1 1 0}〈1 1 0〉 textured Ag tape, compared with the samples annealed in one-step; the better grain alignment and the smoother surface could be obtained. Moreover, 300 °C annealing in O2 atmosphere allow us to obtain the {1 1 0}〈1 1 0〉 texture by only 3 min final heat treatment at around 800 °C. This is beneficial to achieve the higher production speed of YBa2Cu3O7 coated conductors.

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.

A cost-reduction design of the SMES system is proposed, using Nb3Sn conductors that in recent years have shown progress in cost and performance. The main feature of this design should be a combination of Nb3Sn single-strand conductors and a small bore solenoid (=CELL) coil. The cost reduction was intended to use mass production methods and to operate small or medium-sized factories. However, this simple and compact design brings up new issues. Quench protection is especially a key issue. It needs high-speed quench detection and forced-quench that will reduce the unbalance magnetic force at quench and prevent the CELL coil from burning up. This cost-reduction design of the SMES system using Nb3Sn conductors is sufficiently competitive with the other designs that use NbTi conductors.

AC losses in superconductor strips assembled face-to-face are numerically evaluated by means of a finite element method. The external magnetic field is applied perpendicular to their flat face. It is also assumed that the superconductor strips have the voltage–current characteristics represented by the critical state model with constant critical current density. The influences of the number of strips and the gap length between strips on the losses are quantitatively discussed as compared with the conventional theoretical expressions for some special cases in order to understand only the geometrical effects on the perpendicular-field losses in actual assembled conductors with the finite numbers of Y-based superconducting tapes.

We developed Bi-2223 narrow tape for a transposed segment conductor. The tape was 0.23 mm thick and 1.90 mm wide, almost half the conventional tape width. It used a Ag–Mg–Sb alloy for the sheath material and consisted of Ag/SC ratio of 2.2 and 37 filaments. We adjusted the Ag/SC ratio to improve the mechanical strength of the newly developed narrow tape and obtained a tensile strength of 108 MPa. The narrow tape kept the initial Ic value until a flat-wise bending strain was up to 0.6% or an edge-wise bending strain was up to 0.3%. Moreover, it was flexible enough to be machine-fabricated into a transposed segment conductor.

Intensive research work has been carried out in order to develop industrially available HTS REBCO-coated conductors under the NEDO project in Japan. Recently, several groups in the project succeeded in the development of high performance coated conductors. Their characteristic features have been evaluated in terms of mechanical properties and their influence on critical currents. The mechanical properties at RT and 77 K were analyzed on the basis of the rule of mixtures. The force-free strain (Aff) was analytically deduced, which indicates the strain at which the residual stress exerted on the superconducting layer becomes zero. Tensile strain dependence on critical currents could be divided into elastic and brittle regions. The reversible strain limit (Arev) was defined as a strain at which the critical current recovers elastically to the level of 99% Ico. Within the elastic region, the critical current showed a convex strain dependence, which is explained as Ekin's intrinsic strain effect. The degradation beyond the reversible strain limit was attributed to a fracture of the superconducting layer. As a whole, the present study made clear quantitatively the tensile strain behavior of critical currents and proposed a reasonable definition for the reversible strain limit.

Bending tolerance is one of the most fundamental mechanical properties of YBCO coated conductors for application to a superconducting coil. In order to reveal the origin of the difference in critical current (Ic) with bending radius among various conductors, it is essential to estimate the bending strain at a superconducting layer. In this study, the bending strain was evaluated both experimentally and analytically for YBCO coated conductors with and without a Cu stabilizing layer. The internal tensile and bending strain of YBCO film in a composite conductor was directly evaluated from the shift of the Bragg peaks of a YBCO film using synchrotron radiation. We also present an analytical method to calculate the neutral axis position by considering its shift due to the progress of plastic deformation of the components such as Cu, Ag and Hastelloy. The validity of our strain analysis was confirmed by agreement of the measured and calculated internal bending strains. The calculation result shows that yielding of the Cu stabilizing layer has a strong influence on the position of the neutral axis and results in a weaker dependence of Ic. A comparison with tensile strain measurements shows that the effect of bending can be predicted from the tensile strain dependence.

The intrinsic pinning mechanism of YBa2Cu3Oy coated conductors was investigated in terms of the anisotropy of Jc. The dissipation in the intrinsic pinning is mainly due to the double-kink excitation of the pinned vortices in the intermediate temperature region between 70 and 40 K. In this temperature region, the n-value is as small as about 8–10. At low temperature below 20 K, however, n-values become large and Jc is independent of the magnetic field because of the reduction of the double-kink excitation of the vortices.