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The Japanese national project started on standardization of the test method of the DC critical current of superconducting cables. A round robin test of critical current measurement is carried out to survey underlying problems for the standardization. The sample cables are made of Bi-2223 tapes and the inner core conductor has two layers and the outer shield conductor is composed of a single layer. The diameter of the shield layer is about 40 mm and the length of the cable is 2.00 m. The critical current was determined by the electric field criterion of 1 μV/cm. The n value is also measured for the inner core and the outer shield conductor. Three institutes participated in the test for two sample cables. The obtained experimental relative standard deviation for the critical current at 77.30 K is in the range of 0.2 to 1.3%, and is sufficiently small. Hence, it can be said that the present method that is widely used is suitable as a standard test method. On the other hand, the experimental relative standard deviation of the value is in the range of 2.8 to 18.6%, which is relatively large. This is caused by the measurements done at different temperatures.

The critical current of a coaxial cable made of Bi-2223 tapes was investigated to survey underlying problems, especially for the resistive voltage caused by current transfer, for the standardization of test method of the critical current of superconducting AC power cables. Five voltage taps were attached on the copper sheath of 80 mm long, which is used as a part of the current lead, on both ends for the inner conductor layer and outer shield layer. The current transfer was theoretically examined especially for the outer shield layer of a simple structure. The resistive component in the measured voltage was calculated as a function of the position of the voltage tap, and it can be shown that the critical current is correctly obtained using the electric field criterion after subtracting the resistive component, as is usually done in experiments. These results are compared with the observed results.

In this study, we designed a power cable based on the longitudinal magnetic field (LMF) effect using an iterative approximation method. An evaluation of the cable shows that it is more efficient than conventional-design power cables. A cable with four inner layers and four shield layers and a maximum winding angle of 30° had a current capacity of more than 10 kA. Based on calculation results, we fabricated a 2-m-long LMF power cable and a conventional-design power cable. Experimental results show that the proposed power cable can carry more current (8.8% more current per tape and 18% more current overall) than a conventional-design power cable due to the LMF effect. These results well agree with the theoretical calculation results.

The trapped magnetic field properties of superconducting MgB2 bulk magnets with various shapes such as a triangular, a quadrangular, a hexangular bulk were calculated by the Finite Elements Method (FEM). The effect for the combination of several numbers of bulks was also investigated for several kinds of shapes to obtain large area of bulk surface in spite of one large bulk. In this calculation, the simple magnetization process replaced by the field-cool magnetization was used to obtain the equivalent distribution of the magnetic field, and the thermal equation in FEM was omitted. The trapped magnetic field for the triangular bulk by FEM was compared with the experimental result. It was found that the calculated results agreed well with the experimental result. The maximum trapped magnetic field was obtained in the cylindrical shape among several kinds of shapes. The trapped magnetic field was increased by the combination of multi-bulks. It was confirmed that the trapped magnetic field of the multi-bulks was larger than that of the single bulk. The trapped magnetic field increases with increasing the number of the bulks., 28th International Symposium on Superconductivity(ISS 2015), November 16-18, 2015, Tokyo, Japan

The critical current density (Jc) properties of the Artificial Pinning Center (APC) introduced YBa2Cu3O7 (YBCO) films in the longitudinal magnetic field were measured. Y2O3 or Y2BaCuO5 (Y211) was introduced as APCs to YBCO, and YBCO films with APC were fabricated on SrTiO3 single crystal substrate. The sizes of Y2O3 and Y211 were 5—10 nm and 10—20 nm, respectively. As a result, Jc enhancement in the longitudinal magnetic field was observed in Y2O3 introduced YBCO films. However, it was not observed in Y211 introduced YBCO films. Therefore, it was considered that Jc properties in the longitudinal magnetic field were affected by introducing of small size APC, and it was necessary that APC does not disturb the current pathway in the superconductor., 28th International Symposium on Superconductivity, ISS 2015, November 16-18, 2015, Tokyo, Japan

Innovative dc superconducting cables with higher current-carrying capacity than conventional cables were proposed in previous works. The superior property is brought about by the longitudinal magnetic field that enhances the critical current density. In this work, the current-carrying capacity of a three-layer cable composed of Bi-2223 tapes was theoretically calculated based on the measured critical current density in the longitudinal magnetic field. The obtained current-carrying capacity of the proposed cable is 13.2% higher in comparison with the conventional cable of the same size.

We proposed the innovative force-free superconducting cable with higher current-carrying capacity than conventional superconducting DC cables utilizing the longitudinal magnetic field effect that enhances the critical current density. That is, the critical current density can be enhanced by the longitudinal magnetic field that the conductor itself and the outer shield conductor produce. It means that, if the longitudinal magnetic field can be reduced, the critical current density is appreciably reduced, resulting in an accelerated resistive transition. Such situation can be realized by arranging the copper conductor layers in a suitable structure that transport currents when the superconductors are in resistive state. We propose to arrange the copper conductor layers in the inner and outer conductors to reduce the longitudinal magnetic field by twisting in opposite directions to respective superconducting layers in each conductor region. Various conditions of fault-current limiting can be achieved by arranging the winding angle of the copper layers. Discussion will be given on the fault-current limiting properties of the proposed cable.

The trapped magnetic field properties in superconducting MgB2 bulk magnets with various kinds of shape such as a disk, a ring and a pair of disks were calculated by the finite element method (FEM). For simplicity, field cool magnetization was replaced by a simple magnetization process at constant temperature to obtain equivalent distribution of magnetic field, and the thermal equation in FEM was omitted. It was confirmed that the result of FEM agreed well with the result by analytical method in infinite long cylinder. We compared the trapped magnetic field property between FEM result and experimental result in reference in order to research the simple evaluation method of the trapped magnetic field of MgB2 bulk magnet. It was found that the result of FEM agreed with the experimental result and it can explain the distribution of trapped magnetic field of superconducting MgB2 bulk magnet. From these results, it was found that it was possible to be calculated in various kinds of shape with using simple evaluation by FEM. Therefore, the optimization of the maximum trapped magnetic field in superconducting MgB2 bulk magnet can be discussed., Proceedings of the 27th International Symposium on Superconductivity (ISS 2014) November 25-27, 2014, Tokyo, Japan

As a method of largely reducing the transmission loss in the electric power grid, superconducting direct current (DC) power transmission cable has been investigated. Using superconducting DC power transmission cables, large amounts of current and energy can be transferred compared to conventional copper cables. In this case, an alternating current (AC) is converted to DC and superposed AC which is known as ripple current, and the energy loss by the ripple current is generated. Therefore it is desired to estimate the energy loss density for the case of DC current and superposed AC current for a design of DC transmission cable system. In this study, the hysteresis loss for DC current of 2 kA rectified from 60 Hz alternating current is calculated using the Bean model, and coupling loss was also estimated. The diameter of the cable was 40 mm. The ripple currents generated by multi-pulse rectifiers, 6-pulse, 12-pulse, and 24-pulse were considered. It is found that the total AC loss including the hysteresis loss and the coupling loss is considerably smaller than the supposed heat loss of 0.5 W/m which is obtained with a newly developed cable., 26th International Symposium on Superconductivity (ISS 2013), November 18-20, 2013, Tokyo, Japan

The depairing current density in superconductors is theoretically investigated for magnetization and transport currents. It is found that the depairing current densities in both cases are higher than those predicted by Tinkham. One of the reasons for the higher current densities is that those are obtained at the transition point to the normal state, while Tinkham obtained in the superconducting state far from the transition boundary. Another reason is that the order parameter is larger than the equilibrium value, which contributes not only directly to a higher current density but also to a lower kinetic energy due to the current.

The high upper critical field and low anisotropy of the 122-type iron-based superconductor BaFe2As2 makes it promising for use in superconducting high field magnets. However, its critical current density (J c) in high magnetic fields needs to be further improved. Here we show that for the film prepared by pulsed laser deposition method by controlling the deposition parameters (higher substrate temperature and lower growth rate), the crystallinity of BaFe2(As0.66P0.33)2 (Ba122:P) matrix is improved while maintaining a high density of incoherent BaZrO3 (BZO) nanoparticles (NPs) which together lead to significantly increased self field J c. Our Ba122:P nanocomposite films also exhibit increased in-field J c, reduced angular anisotropy of J c and reduced detrimental effects of thermal fluctuations (creep rate) over a wide range of temperatures and magnetic field strength. The BZO NP doped Ba122:P films show high in-field J c over 2.1 MA cm−2 even at 4 K and 9 T (μ 0 H∣∣c), which is significantly higher than that of standard Ba122:P films and conventional alloy superconducting wires. To understand the contribution of the various pinning centers, we applied a simple model, which was developed for cuprates, to Ba122:P film with all the parameters used derived by fitting to a limited set of experimental data (no free parameters) such that temperature, angle and field properties at other experimental conditions are then calculated. This simple model fits very well to the experimental results in these two very different material systems. We discuss the effectiveness of natural defect and BZO NPs on the ratio of J c to the depairing current density. The superconducting properties for 122-type iron-based superconductors obtained through this work are considered promising for high-field applications.

It is known from experimental data that the critical current of superconducting wires in a longitudinal magnetic field is higher than that in a transverse magnetic field. This property was proposed to apply to DC superconducting cables in our previous paper. Here, we propose a new calculation method of the current-carrying capacity for a new DC superconducting cable. An iteration method is used to calculate the magnetic field and critical current density in each layer and the current-carrying capacity of the cable. This enables us to calculate these quantities even for the cable with a different structure from the force-free one. When the critical current density increases with a longitudinal magnetic field, the optimal structure is found to be close to that of the force free cable. The optimal structure is obtained also when the critical current density decreases slightly with the longitudinal magnetic field. The reason for the enhanced current-carrying capacity even for such a case is a sufficiently larger critical current density in the longitudinal magnetic field than that in the transverse magnetic field., Proceedings of the 25th International Symposium on Superconductivity (ISS 2012) Advances in Superconductivity XXV , December 3-5, 2012, TOWER HALL FUNABORI, Tokyo, Japan

The loss energy density (AC loss) with ripple current is numerically calculated by finite element method for cylindrical and strip superconductors based on Irie–Yamafuji model in which the magnetic field dependence of the critical current density is taken into account for design of DC transmission cable system. It is confirmed that calculated result of the AC loss in the cylindrical superconductor with the ripple current agrees well with theoretical estimation which was reported in the previous work. On the contrary, the AC loss in the strip superconductor with the ripple current is obtained only by numerical calculation. It is found that the AC loss in the strip superconductor of the ripple current becomes larger than that without DC current at small ripple current amplitude, since the penetration depth of magnetic field becomes large. Therefore, it is recommended that strip superconductor is better to use as cylindrical hollow superconductor for DC transmission cable system to reduce the AC loss., Proceedings of the 25th International Symposium on Superconductivity (ISS 2012) Advances in Superconductivity XXV , December 3-5, 2012, TOWER HALL FUNABORI, Tokyo, Japan

The superconducting Ba1-xKxFe2As2(x = 0.4) single crystals were prepared by the so-called FeAs self-flux method. The critical temperature by ac susceptibility measurement was estimated to be about 36 K. The magnetic field and temperature dependences of critical current densities were investigated by an ac inductive measurement (Campbell’s method). Unlike the phenomenon of co-existence of the global and local critical current densities observed in many polycrystalline Fe-based superconducting pnictides, it was found that only a uniform critical current density (Jc) flows through the whole sample. The value of Jcat 20 K and 1 T was about 5×108A/m2, which is much smaller than the local critical current density observed in polycrystalline samples. This result implies that a dissimilarity of flux pinning mechanism exists between these two kinds of materials. The force-displacement characteristic of fluxoids in sample was investigated. The Labusch parameter was found to increase monotonously with increasing magnetic field, while the interaction distance was proportional to the fluxoid spacing. These results are consistent with the prediction based on a simple flux pinning mechanism.

AC susceptibilities (real χ ′ and imaginary χ ″) of Sr 0.6 K 0.4 Fe 2 As 2 (122 type) polycrystalline with Ag addition are analysed by the grained Bean model. A variety of characteristics, double peak in χ ″ and shoulder transition in χ ′, appear in the model simulation. Comparing the measured χ ′ and χ ″ with the model allows more clear insight on the polycrystalline structure. Estimated critical current densities J cg and J cl of the grain and the link in the iron-based pnictides show that the addition of 20 wt.% Ag increases J cl 9 times larger. Improvement of intergrain characteristics with Ag addition is clearly indicated.

```  Abstract-- In this review paper, we report the characteristics of the critical current density in FeAs based superconductors which is newly discovered by Hosono group of Tokyo Institute of Technology on 2008. Since the many specimens in present stage are not single crystals, there are two kinds of critical current density observed in the specimens which are so-called local and global critical current densities. Therefore, it is necessary to evaluate both kinds of critical current densities. The history effect in which the global critical current density shows different values in increasing and decreasing magnetic field is also observed when the specimens have the local and the global critical current densities. The wire which critical current is 180 A is successfully developed with using the knowledge of abovementioned characteristics of two kinds of critical current densities and the history effect. kinds of critical current density in the specimens. They are so-called local and global critical currents. That is, the connection between the grains in specimen is too week to carry large current, and the global critical current density is far lower than the local critical current density which flows only inside the grain. They are also called intra- and inter-grain shielding current density. It is essential to measure both local and global critical current densities, because the local critical current density will be achieved when the specimen will be prepared as single crystalline in near future which is already done in RE-123 (RE: Rare Earth) superconducting coated wire. Moreover, the global critical current is corresponding to transport critical current density which is usually measured by four probe method. In the present study, the local and the global critical current

It is important to investigate condensation energy density of superconducting material, since it determines its pinning property. According to the heavy-ion irradiation, the columnar defects are introduced to the superconductor which act as the strong pinning centers, and the critical current density is enhanced by the new pinning centers. Since the number density and the size of the columnar defects can be observed, the condensation energy density is estimated by using the pinning summation theory. In the present study, we prepared Ba(Fe0.93Co0.07)2As2 (Ba-122) pnictide superconductors by self-flux method. 200 MeV Au ions were irradiated into the specimens along c-axis and the matching field was 2 T. After the irradiation, the critical current density was 6 times larger than that before the irradiation. The estimated condensation energy density is in the order of 104 J/m3 and is slightly smaller than those of cuprate superconductors. The temperature dependence is similar to that of YBa2Cu3Ox oxide superconductors, since the anisotropy parameter is small in Ba-122. Therefore, it is expected to use Ba-122 at high temperatures near the critical temperature., Superconductivity Centennial Conference 2011 - EUCAS–ISEC–ICMC, Sep 18 - 23, 2011, Hague, Netherlands

The pinning parameters for numerical calculation based on the flux creep-flow model are determined by using genetic algorithm (GA), which has been applied to many practical determination for parameters. Several estimation functions which describe the distance between the experimental and calculated results by GA were proposed, and the difference between the results were calculated. It is found that the pinning parameters of the flux creep-flow model are successfully deduced by GA. The difference between the calculated and experimental results and the calculation time are found to be largely depended on the estimation functions., Proceedings of the 24th International Symposium on Superconductivity (ISS 2011), October 24-26, 2011, Tokyo, Japan

The loss energy density (AC loss) in cylindrical superconductor with ripple current based on Irie-Yamafuji model in which the magnetic fiel dependence of critical current density is taken into account is theoretically calculated for design of DC transmission cable system. It is confirme that the AC loss does not changed for the cases with and without DC current when the critical current does not depend on magnetic fiel which is corresponding to Bean-London model. On the contrary, it is found that there is current region where the AC loss becomes smaller than that for the case without DC current. The AC loss of ripple current is seems to be enough small in layered structure of DC transmission cable made by thin tape superconductor., Proceedings of the 24th International Symposium on Superconductivity (ISS 2011), October 24-26, 2011, Tokyo, Japan

The effect of distribution of powder size on the estimation of intra-grain critical current density was investigated for polycrystalline samples of Sr0.6K0.4Fe2As2 with and without addition of Ag 20 wt.%. It turned out that the value of intra-grain critical current density calculated using the average of powder size Jcl is more overestimated than that calculated taking account of the distribution of powder size JclP as the distribution of powder size is widened, since the magnetic moment drastically increases due to the larger contribution from larger particles. From the distribution of powder size obtained by the microscopic observation using SEM, the factor of overestimation of Jcl/JclP is found to be larger than two in both samples. Moreover, the distribution of powder size in the Ag-added sample is found to be smaller than that of pure sample. Therefore, the factor of overestimation of Jcl is considered to be smaller in the Ag-added sample. It is recommended to take account of the distribution of powder size for precise estimation of intra-grain critical current density.

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.

The improvement of critical current density properties of REBCO coated conductors for applications in superconducting power devices is desired. It is known that the critical current density Jc of REBCO thin films made by the TFA-MOD method in a magnetic field is greatly improved by the introduction of BZO-nanoparticles of a non-superconducting phase as artificial pinning centers. In addition, the field angle anisotropy can also be reduced by isotropic flux pinning interactions of nanoparticles. However, a detailed report on the mechanism of flux pinning with a quantitative investigation in such coated conductors has not been given. In this study, the field angle dependence of the critical current density is measured for TFA-MOD-processed YGdBCO coated conductor with a superconducting layer of 0.65 μm thick, and the results are theoretically analysed to clarify the flux pinning mechanism.

The upper limit of critical current density by the flux pinning mechanism is theoretically investigated by taking into account the influence of kinetic energy. The upper limit is estimated to be 67% of the depairing current density in the London limit, which is about 23% higher than the estimation by Tinkham. The reason for the higher value is attributed to the enhancement of the order parameter that reduces the kinetic energy caused by the current. The possibility of such a strong flux pinning is discussed for nano-rods introduced to REBCO coated conductors.

The flux pinning properties in a magnetic field of various directions are investigated for trifluoroacetates-metal organic deposition (TFA-MOD) processed Y 1− x Gd x Ba 2 Cu 3 O y (YGdBCO)-coated conductors with the superconducting layer of 0.5 μm thick with artificial BaZrO 3 (BZO)-nanoparticles. It is found that the critical current density is enhanced in a wide range of field angle except around the direction parallel to the a − b plane, resulting in low field-angle anistropy. The enhancement of J c in the wide range of field angle is attributed to the isotropic pinning of nanoparticles. The decrease in the critical current density in the field direction parallel to the a − b plane is considered to be caused by limitation of growth of stacking faults by distributed BZO nanoparticles. The observed results were theoretically explained by the flux creep-flow model and the above speculation of the pinning mechanism was confirmed.

The critical current property of Bi 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) silver-sheathed tape has been remarkably improved by application of ConTrolled Over Pressure (CT-OP) process with adjusting temperature and oxygen partial pressure. For further improvement of the property of Bi-2223 tapes, the effect of number of filaments on the critical current density Jc was investigated. It was found that Jc was higher and its decrease with increasing magnetic field was weaker in the tape with larger number of filaments. In addition, the irreversibility field Bi increased with increasing filament number. The field-angle anisotropy Jc(0°)/Jc(90°) of the tape with the largest filament number was smallest, where the field-angle was measured from the a–b plane. However, the out-of-plane misorientation angle is fairly small in this tape. Hence, the low field-angle anisotropy in this tape seems to be attributed to improved superconductivity in the block layers in the superconducting region near the interface with silver matrix.

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

Critical current properties at high magnetic fields are analyzed using the percolation theory for polycrystalline MgB 2 superconductors. In the theory the effect of anisotropic irreversibility field of each grain due to random orientation is considered. Relatively low irreversibility field of this superconductor in spite of fairly strong flux pinning strength is attributed to the percolating nature for superconductor with low electrical connectivity. This theoretical approach explains the experimental result showing higher irreversibility field for samples with higher packing factor. It is concluded that the high packing factor is inevitable for high critical current density at high magnetic fields.

We have proposed MgB2 multifilamentary tapes with high critical current densities and low ac losses under external magnetic fields which is parallel to the tape face. The purpose of this proposal is to develop the MgB2 new conductors composed of plural tapes as windings of superconducting coils with both large current capacities and low losses for fusion reactors, superconducting magnetic energy storage systems, and so on. Our MgB2 multifilamentary tapes are fabricated by rolling process from round wires. The tapes are fabricated by using in-situ method. The purpose of this paper is to clarify the effects of deformation to tape shape on the electromagnetic properties of MgB2 filaments in multifilamentary wires. For the purpose, measurements of both critical currents and magnetizations have been carried out on MgB2 fine single filament wires which have simulated a filament in multifilamentary wires. Measurements on wires with large diameter have been also carried out, in order to clarify the effect of diameters of original round wires. We prepared pure and SiC-added MgB2/Nb/Cu composite wires as the original round wires before rolling process. The diameters of round wires have been varied and then the aspect ratios of tapes have been varied. The observed critical current densities of the tapes are about 7 times larger than that of the original round wire at a maximum. The ratios of Jc of wires to tapes don't have dependence of the diameter of the original round wire. In addition, the results of measurements of magnetizations have shown that magnetization losses in MgB2 tapes decrease to 1/3 compared with original round wires.

The critical current densities of polycrystalline bulk SmFeAsO$_{1-x}$F$_x$ prepared by the powder-in-tube (PIT) method and by a conventional solid-state reaction were investigated using the remnant magnetic moment method and Campbell's method. Two types of shielding current, corresponding to global and local critical current densities $J_{\rm c}$ were observed using both measurement methods. The global and local $J_{\rm c}$ were on the order of $10^7$ A/m$^2$ and $10^{10}$ A/m$^2$ at 5 K, respectively. The local $J_{\rm c}$ decreased slightly with increasing magnetic field. The global $J_{\rm c}$ was independent of the preparation method, while the local $J_{\rm c}$ was larger for samples prepared by PIT than for those prepared by solid-state reaction., 20 pages, 5 figures. to be published in Physica C

It was previously shown that the apparent pinning potential U 0 ∗ associated with the relaxation of superconducting current at low magnetic fields depended on the thickness of superconducting layer for IBAD/PLD-processed YBCO-coated conductors. It is necessary to clarify the thickness dependence of the relaxation property at high magnetic fields for application of YBCO-coated conductors in a persistent current mode at such magnetic fields. In this study, the thickness dependence was investigated in the field range of 1–6 T at 20 K for IBAD/PLD-processed YBCO-coated conductors with the superconducting layer thicknesses of 0.25 – 1.0 μ m . It was found that U 0 ∗ decreases monotonically with decreasing thickness at 6 T. This seems to be attributed to the two-dimensional pinning mechanism because of the longitudinal flux bundle size limited by the film thickness.

Critical current density was measured for oxygen-controlled (Bi, Pb)-2223 single crystals before and after the irradiation with gold ions in a magnetic field parallel to the irradiation-induced defects along the c-axis. Eleven specimens prepared in different annealing conditions were measured. The condensation energy density of each specimen was evaluated from the observed critical current density by using the summation theory of pinning forces of columnar defects and the flux creep theory. It was found that the specimen heat-treated at 1 atm in oxygen atmosphere has the highest condensation energy density among all specimens. Hence, it is speculated that the optimum oxygen pressure for the anneal is around 1 atm.

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.

Flux pining properties of columnar defects nucleated by gold ion irradiation was investigated for DyBCO coated conductor. The energy of irradiated gold ions was 320 MeV and 200 MeV and the matching field was 1.0 and 2.0 T, respectively. It was found that the flux pinning strength was significantly enhanced at high magnetic fields. These experimental results were analysed using the summation theory of the elementary pinning forces with the theoretical model of flux creep and flow. It was found that the experimental results can be explained by the theories. The obtained result suggests that columnar defects with larger radius are more effective for pinning because of the larger probability of catching flux lines.

A practical Bi-2223 superconducting magnet, working in liquid nitrogen (L.N 2 ), was designed and fabricated. Bi-2223 tape with a critical current of 147 A was prepared by a controlled overpressure (CT-OP) process at 77.3 K in self-field. Ten double-pancake coils were resistively connected by copper terminals. The bore diameter was 54 mm ϕ , the magnet outer diameter was 122 mm ϕ , the height of the magnet was 124 mm, and the weight of the magnet was about 3 kg. The maximum magnetic field at the center of the bore was 0.48 T with an operating current of 50 A. The experimental results agree well with design predictions calculated by finite element method. AC operation was also performed, and no distortion of the voltage waveform was observed. Therefore, this Bi-2223 superconducting magnet is a suitable replacement for copper magnets designed for applications in science and technology.

The relaxation properties were investigated for PLD processed YBCO coated conductors deposited on IBAD substrates in the region of 0.25–1.50 μm for the thickness of superconducting layer to clarify the thickness dependence of the relaxation of magnetization. The apparent pinning potential U 0 ∗ was estimated from the relaxation rate. It was found that U 0 ∗ increases with increasing temperature in the low temperature region. The weak thickness dependence of U 0 ∗ in this temperature region comes from the three-dimensional flux pinning mechanism. Then, U 0 ∗ has a peak followed by a decrease with increasing temperature. The peak value of U 0 ∗ increases with increasing thickness. Hence, the thicker superconductor is more advantageous for applications in the middle temperature range and higher. The flux pinning mechanism in the medium and higher temperature region is two-dimensional and the larger U 0 ∗ value in the thicker specimen originates mainly from the thickness. For the improvement of U 0 ∗ in the low temperature region, it is required to enhance the flux pinning strength and to increase the thickness without deterioration in the critical current density.

Many recent reports on the critical current density ( J c ) in superconducting MgB 2 bulks indicated that improving the grain connectivity is important, since the obtained J c values were generally much lower than those in other metallic superconductors and it was ascribed to the poor connectivity between grains in polycrystalline MgB 2 . In this study, we focused on the estimation of the global critical current density, super-current path, grain connectivity and their relationships with the faults volume fraction in the MgB 2 bulks prepared by a modified PIT (powder in tube) method. Campbell’s method was applied for the purpose of obtaining the penetrating AC flux profile and the characteristic of AC magnetic field vs. penetration depth from the sample’s surface. A computer simulation on the penetrating AC flux profile in MgB 2 bulks with randomly distributed voids, oxidized grains and other faults was also carried out. J c obtained by Campbell’s method turned out to be smaller than that obtained from the SQUID measurement, implying that the global super-current was reduced by the existence of various faults and the lack of the electrical connectivity. It was verified that the relationship between the global critical current characteristics and the faults contained in MgB 2 samples can be quantitatively clarified by comparing the simulated critical current densities and other factors with the experimental results.

The critical current property of Bi-2223 silver sheathed tape has been significantly improved by application of ConTrolled Over Pressure (CT-OP) process with adjusting temperature and oxygen partial pressure at the final heat treatment. In this paper, the field-angle dependence of the critical current density was measured at 77.3 K and the crystalline misorientation angle was estimated for four specimens with different critical current densities. It is found that in a parallel magnetic field, the critical current density of the latest Bi-2223 tape of 190 A class is remarkably higher than the other specimens over a wide region of magnetic field. On the other hand, although the critical current density of the latest tape in a normal magnetic field is higher than the other specimens, the difference is relatively small especially at high magnetic fields. Thus, the field-angle anisotropy of the latest tape is higher than the other specimen. The observed results can be explained by the improved condensation energy density and the better c-axis alignment in the latest tape.

The critical current properties were investigated in the superconducting layer thickness range of 0.5–1.5 μm for PLD processed YBCO coated tapes deposited on IBAD substrates. The measurements were done in low and high electric field regions around the order of 10 −8 V/m and 10 −4 V/m, respectively. As a result, it was found that the critical current density J c decreased with increasing thickness in both measurements at low magnetic fields. However, the thickness dependence of the irreversibility field B i were different between the two measurements: B i increased with decreasing thickness in the high electric field region, while B i decreased with decreasing thickness in the low electric field region. The n value increased monotonically with increasing thickness. The observed results were approximately explained by the irreversible thermodynamic principle that the transverse flux bundle size is determined so that the critical current density is maximized.

Significant overestimation of the critical current density J c is found for the third harmonic voltage measurement method for thin YBCO superconductors. This is caused by a reversible flux motion in pinning potentials, which is known to be significant for superconductors of thickness comparable to or smaller than Campbell’s AC penetration depth. In this paper the effect of reversible flux motion on the estimation of J c by this method with different thicknesses is also theoretically investigated by using the Campbell model for the force–displacement characteristics of flux lines. Although the factor of overestimation can be approximately explained by the Campbell model, agreement is not satisfactory. It is considered that the variation in the flux motion from reversible to irreversible states seems to take place much faster than assumed in the Campbell model.

The hot topics of the critical current properties in YBCO coated conductors and Bi-2223 silver-sheathed tapes are discussed. The dependence of the critical current density in YBCO coated conductors on the superconducting layer thickness is partly associated with the defects nucleated in thicker films and is also caused by the thickness dependent influence of the flux creep. Experimental results on PLD processed conductors with different thicknesses are compared with the theoretical analysis based on the flux creep-flow model. A discussion will also be given for the optimum layer thickness for applications. In Bi-2223 tapes introduction of CT-OP (ConTrolled Over Pressure) process has significantly improved the critical current properties. This progress can be attributed to the improvement of the mean value of the flux pinning strength and the decrease in its distribution width. The possibility of further improvement of the critical current properties is discussed using the flux creep-flow model.

Investigation is done on the effect of ConTrolled Over Pressure (CT-OP) processes on the critical current properties in multifilamentary Bi-2223 tapes. It is found that the critical current density at low magnetic fields is remarkably improved by the new CT-OP process. This result with those on irreversibility field and n-value are compared with the numerical calculation based on the flux creep and flow model. The analysis reveals that the most probable value is increased, although the statistical distribution width is increased for the flux pinning strength. This result indicates that the critical current density at high magnetic fields is deteriorated by the widened distribution. If such an inhomogeneity can be reduced to the level of previous tapes by improving the CT-OP process, the high field performance is shown to be significantly improved.

The thickness dependence of pinning properties is investigated in detail under various conditions of temperature, magnetic field and electric field for IBAD/PLD YBCO-coated conductors. The thickness-dependent critical current density in the three-dimensional pinning regime at low magnetic fields is attributed to the change in the morphology of the superconducting layer. It is found that while the irreversibility field increases with the thickness at low electric fields according to DC magnetization measurements, it decreases at usual electric fields corresponding to resistive measurements. This can be explained by the theoretical model of flux creep and flow with the irreversible thermodynamic principle that the transverse flux bundle size is determined so that the critical current density under the flux creep is maximized. The former model assumes the limitation of the longitudinal flux bundle size by the superconducting layer thickness, while the latter principle assumes a change in the transverse one from the value expected from the elasticity of flux lines. The observed n-value, the parameter describing the relationship between the electric field (E) and the current density (J) as , increases monotonically with the thickness in both electric field regions. This can be simply attributed to the increase in the pinning potential with the thickness.

Flux pinning properties were investigated by focusing on the origin of the high peak critical current density and the high irreversibility field for Nd0.33Eu0.38Gd0.28Ba2Cu3Oy (NEG-123) bulk superconductors with the addition of 211 secondary phase particles of volume fractions up to 10 mol%. It was found that a negative correlation exists between the peak critical current density Jcp and the irreversibility field Bi for various fractions of added 211 phase. This suggests that the mechanisms that determine Jcp and Bi are different. The 211 particles do not contribute to the peak effect neither directly through the pinning mechanisms of the condensation energy interaction nor indirectly with the aid of the order–disorder transition of flux lines. Other possible defects are nano-lamella structures. Although these defects do not directly contribute to the peak effect through the pinning mechanism of the condensation energy interaction, they may contribute to it with the aid of the order–disorder transition of flux lines. On the other hand, as Bi deteriorates with the addition of 211 particles, it is not determined by the flux pinning of 211 particles. The decrease in Bi is considered to be caused by the proximity effect of nano-lamella structures.

The dependence of superconducting layer thickness on the critical current characteristics was investigated in the range of 0.5–1.5 μm for YBCO-coated conductors made by the PLD process. Since the dimension of the pinning is considered to be three as indicated by the pinning correlation length, it is concluded that the observed thickness dependence of the critical current density at low fields come simply from a degradation in the superconducting layer structure with increasing thickness. The irreversibility field and the n-value increase with the thickness. These dependencies are well described by the theoretical model of the flux creep and flow.

The effects of low temperature sintering and C-doping on the previous termcritical currentnext term density of MgB2 superconductor was investigated and the obtained results were compared with the theoretical results of the flux creep-flow model. The previous termcritical currentnext term density increased by these treatments over a wide range of magnetic fields. It is proposed that the improvement of the previous termcritical currentnext term density at low magnetic fields comes from the reduction of the grain size in the specimen sintered at low temperatures and from the increase in the elementary pinning force of grain boundaries in the C-doped specimens. On the other hand, the improvement of the previous termcritical currentnext term density at high fields is mainly caused by the enhancement of the upper previous termcriticalnext term field due to the electron scattering by grain boundaries and C atoms.

The critical current properties of a superconducting Bi-2223 tape produced by the over pressure processing at the final heat treatment were investigated. The critical current density increased more than the factor of densification of the superconducting region. In addition, the irreversibility field also increased. This means that the quality of flux pinning is improved by the new process. From the numerical calculation based on the flux creep and flow model, it is speculated that the fraction of weakly pinning regions decreases due to the improved connectivity of grains.

Pinning properties such as the critical current density and the irreversibility field of superconducting (Bi,Pb)-2223 single crystals with different O2 annealing conditions were investigated before and after a heavy ion irradiation which nucleated columnar defects parallel to the c-axis. The critical current density was enhanced by a factor of 3 to 4 after the irradiation due to the strong pinning effect by the columnar defects. In addition, the irreversibility field was also improved, and the improvement becomes larger at higher temperatures. The enhancement of the pinning properties was larger in O2 annealed specimen, suggesting an improvement of the dimensionality of the superconductivity. The experimental results were theoretically analyzed using the flux creep-flow model. It was found that the theoretical prediction can explain the experimental results of the critical current density and the irreversibility field.

The critical current density in Bi-2212 superconductors with various anisotropies irradiated by heavy ions was investigated in the medium temperature region to understand the effects of defect size and the anisotropy of the superconductor. It was found that the critical current density and the irreversibility field were larger for the specimen with larger defect and/or with smaller anisotropy. Introduction of stronger pinning centers and the optimization of the doping condition to improve the dimensionality are desired for further improvement of the critical current properties.