Table of contents

Twisted stacked-tape cable (TSTC) is one of the methods to produce flexible current carrying elements (CCE) with high current density made of 2G coated conductors. The critical current of CCE in self-field is less than the sum of critical currents of all tapes used to make CCE. This critical current reduction is attributed to self-field influence of critical currents of tapes. To describe the current reduction accurately in TSTC it is necessary to take into account current and field distribution in a stack cross-section using dependencies of critical current on magnetic field for each tape. In other words one has to make self-consistent calculations of current and magnetic field distribution in a stack. In this paper we present the self-consistent analysis of magnetic field and current distribution in TSTC with different number of tapes using the generalized Kim model. We also measured current degradation in TSTC made of from two to ten 2G tapes and compared it with calculations. We found good coincidence of calculated and measured current degradation that confirm the validity of our approach.

A series of Ag-doped GdBaCuO superconducting films were prepared by low fluorine MOD process on buffered Hastelloy substrates. The effects of Ag doping on the microstructures and superconducting properties of GdBCO thin films were investigated. It was found that an optimum addition of about 5 mol % Ag lead to better c-axis orientation and surface microstructure of the resultant films. Meanwhile, the pyrolysis process shortened and the superconducting performance is improved with a wide available window.

HTSC 2G YBCO based short samples were formed by PLD (pulsed laser deposition) on the cube textured Ni-W tapes. The influence of buffer layer architecture on the critical properties of HTS films was investigated with scanning and transmission electron microscopy, X-ray diffraction and four point probe electrical measurement. The best critical properties were found in the HTS grown on tapes with triple buffer layers: Y₂O₃, YSZ and CeO₂. After CeO₂ layer deposition the blister-like defects appeared on the buffers surface. These defects being inherited by YBCO film could lead to degradation of critical properties.

E-J characteristics of SuperPower YBCO 2G tapes have been measured in the temperature range 15K-80K. It was found that the E-J characteristics deviate significantly from the standard power-law behaviour with apparent power exponent decreasing continuously at high voltage with increasing current. The deviation of E-J characteristic from the standard power-law was found to be consistent with a Weibull distribution of critical current. The identical scaling of E-J characteristics above 40K suggests a common critical current distribution. At lower temperature the critical current distribution becomes narrower to give higher apparent power exponent at lower temperature. With the critical current distribution model it is shown that the dissipation can be correlated directly to the flux flow resistance of Bardeen. The distribution of the critical current can be associated with a distribution of pinning potential of collective pinning and the headline pinning range is obtained as function of temperature.

AMSC produces Second Generation (2G) HTS wire for utility power applications as well as coil, motor and generator solutions. In this paper, various types of AMSC's Amperium^® wire suitable to power cables, fault current limiters and coils are reviewed. In addition, recently developed performance-improvements in amperage, reduced ac power loss and mechanical properties are summarized. The introduction of thicker HTS layers coupled with optimized heat treatments to enhance critical current density dramatically improve both cable and coil wire current-carrying capability. A non-magnetic RABiTS^TM substrate has now been developed to the point where it is compatible with the manufacturing process and capable of sustaining large critical currents. Finally, the ability of Amperium^® wires to withstand cable-winding stresses, and to exhibit the high transverse c-axis strength critical to the reliability of the wire in coils, are discussed.

Bruker HTS manufactures YBCO based superconducting wires of the second generation on low- cost Stainless Steel substrate (100 μm thick). With 250 – 500 A/cm@77 K, SF, 650 MPa tensile strength and 6 mm bending radius excellent electrical and mechanical properties are achieved. As complementation of the 2G fabrication technology an automated 2 km copper pulse plating facility has been installed in 2012. We report here the operation requirements and the experiences of the copper plating technique.

REBa₂Cu₃O_y, (RE : rare earth elements, RE123) coated conductors are expected to show high performance in superconducting applications, due to their high mechanical strength and high current density in magnetic fields. Fujikura has developed ion-beam-assisted-deposition (IBAD) and pulsed-laser-deposition (PLD) technique, and today we routinely manufacture coated conductors with length over 500 m and I_c over 500 A/cm-width at 77 K, self field. We have also been able to fabricate long conductors with higher Ic by thickening RE123 layer. Although I_c performances at 77 K, self field are important, but performances in magnetic fields at lower temperatures below 77 K are more important for coil applications. In this work, we evaluated coated conductors with thick RE123 layer in magnetic fields at low temperatures. All samples are fabricated with long length. We measured samples by four-probe transport method at wide temperature range from 77 K to 10 K by using cryo-cooled cryostat. A conductor with 5.5 μm thick RE123 exhibits high I_c values of 937 A/cm-width at 77 K, self field, 637 A/cm-width at 50 K, 5T and 976 A/cm-width at 40 K, 5T.

This paper focuses on a flat-type inductive superconducting FCL (FIS-FCL) consisting of a pancake coil and a YBCO thin layer disc. AC current injection experiments and magnetic field analysis were carried out for two kinds of FIS-FCL, single-disc model and double-discs model. In the former, the pancake coil was putted on the YBCO disc. In the latter, the pancake coil was sandwiched with two YBCO discs. The double-discs model cancels out the magnetic flux density more effectively than the single-disc model. In the double-discs model, the superconducting state period is longer than in the single-disc model. Thus, it may be concluded that the double-discs model is considered to be suitable for FIS-FCL.

In previous studies, the secondary (impurity and non-reactive) phase and voids were observed in MgB₂ matrix after the heat treatment, and then these are the lowering factors of critical current density (J_c) property. In order to improve J_c property by microstructure control of MgB₂ matrix, the fine elemental boron powder as the raw material was carried out using the high-speed vibrated milling with tungsten carbide (WC) jar. The average particle size of metal boron powder was decreased from 1.14 μm to 0.20 μm by the high-speed vibrated milling. The various fine particle boron powders as the function of milling time were also prepared, and in-situ Cu addition MgB₂ multifilamentary wires using these fine boron powders were fabricated. Critical transition temperature (T_c) value of Cu addition MgB₂ wire using fine boron powder obtained to about 37 K. No change of the T_c property by the different particle sized boron powders was confirmed. In this paper, the comparisons of microstructure and superconducting properties between the different boron particle sizes were investigated.

Considerable progress has been made with the development of REBCO coated conductors in recent years, and high performance conductors are available commercially. For many applications, however, the cost remains prohibitive, and AC losses discourage their selection for higher frequency applications. Chemical solution deposition (CSD) methods are attractive for low-cost, scalable preparation of buffer and superconductor layers, and in many respects inkjet printing is the method of choice, permitting non-contact deposition with minimal materials wastage and excellent control of coating thickness. Highly textured coatings of YBCO and Gd-doped CeO₂ have previously been reported on buffered metal substrates. Inkjet printing also introduces the possibility of patterning - directly depositing two and three dimensional structures without subtractive processing - offering a low-cost route to coated conductors with reduced AC losses. In this contribution, the inkjet deposition of superconducting YBCO tracks is reported on industrially relevant buffered metal substrates both by direct printing and an inverse patterning approach. In the latter approach, ceria tracks were printed reported, which are a candidate both for resistive filament spacers and buffer layers. TFA-based precursor solutions have been printed on SS/ABAD-YSZ/CeO₂ and Ni-W/LZO/CeO₂ RABiTS substrates, and the resulting multifilamentary samples characterised by microscopy and scanning Hall probe measurements. The prospects for future inkjet-printed low AC loss coated conductors are discussed, including control of interfilamentary resistivity and bridging, transposed filamentary structures and stabilisation material.

The average temperature of the heated metal conductor are usually measured by the hot wire method. This method is not applicable to measure the temperature of superconductor. As an alternative we proposed the Double Thickness Method. In DTM two identical elements of superconductor are glued together and miniature thermometer is placed between them. This method allows to measure temperature on superconducting tapes and bulks under the condition of intense cooling (for example, in liquid nitrogen). DTM can be used in AC, DC and pulse heating regimes.

The pinning mechanism of MOCVD-grown YBCO coated conductors with Y₂O₃ precipitates was investigated by angle-resolved transport measurement of J_c in a wide range of temperature and magnetic fields. Aside from the Y₂O₃ nanoprecipitates, a-axis grains and threading dislocation along the c-axis were found in the YBCO layer. The Y₂O₃ precipitates are less effective pinning centers at lower temperature. The tapes with precipitates show a higher anisotropy with larger J_c at H || ab than H || c. This behavior was attributed to the preferred alignment of the nanoprecipitates along the ab-plane.

In order to generate fields of more than 20 T using pure superconducting magnets, the implementation of HTS coated conductors (CCs) is an option. Amongst the characteristics of the REBCO is its intrinsic anisotropy which means its critical current, I_c, varies as a function of its orientation with respect to the external magnetic field, ϕ, as well as being dependent on external field, B, and temperature, T. For the design of the high field insert coils for the experimental magnet facility HOMER II, the behaviour of Ic at fields of about 24 T and at ϕ approximately 80 ° is required. Within our JUMBO facility it is possible to measure the U(I) behaviour of commercial CCs at liquid helium and liquid nitrogen temperatures. The free bore available is 100 mm at 4.2 K; the applied magnetic field can be varied up to 10 T, and ϕ can be set between 0 and 180 °. Measurements were carried out on commercially available CCs with emphasis on ϕ values near to 90 °. The critical current was determined by means of a power law function fitted to the resulting U(I) curves using specially written Matlab® programs. A number of different three dimensional functions were fitted to the measurement data and compared with each other.

ITER Poloidal Field (PF) systems consist of 6 independent coils with different dimensions and require NbTi superconductor and copper strands. Western Superconducting Technologies Co.,Ltd.(WST) will supply PF2-5 NbTi strand for ITER, and over 14,000 km of NbTi strands have been produced in the past two years. Main performance of NbTi strands, including critical current, n value, wire diameter, Cu/non-Cu ratio, hysteresis loss and RRR are reported and analysed in this paper.

YBa₂Cu₃O_7−x (YBCO) films were fabricated at various laser repetition rates on CeO₂/YSZ/CeO₂ buffered Ni-W tapes by pulsed laser deposition (PLD). The dependence of structural and superconducting properties on the laser repetition rates was investigated. The microstructure and surface morphology of YBCO films were characterized by X-ray diffraction and atomic force microscopy. And the critical current (I_c) of YBCO films was measured by the conventional four-probe method. The results show that with increasing laser repetition rate the island density decreased and the island size increased. It was found that the texture and I_c of YBCO films were largely dependent on the laser repetition rate. And furthermore it was observed that the film thickness was not in directly proportional to laser repetition rate. Under optimum experimental condition, high quality YBCO films with critical current I_c above 500 A/cm at 77 K and 0T were obtained.

Twisting of multi-filamentary superconductors is an important step in the development of wires with AC losses at an acceptable level for AC applications. The necessary twist pitch depends on wire architecture, critical current density, matrix material, and external factors such as temperature, frequency and applied magnetic field. The development of an AC optimized MgB₂ superconductor would be facilitated by a fast method to set the requirements for the twist pitch. A problem often encountered when comparing wires with different twist pitches is the degradation in critical current occurring at small twist pitches due to mechanical deformation. In this work we propose to use a non-twisted conductor to estimate the influence of twisting on the AC losses. A long superconductor is cut into smaller lengths, each simulating one third of the twist pitch, and the AC losses due to applied magnetic fields are compared between samples of different lengths. With this method, the effect of reducing the size of the loop of the coupling currents is studied without changing the superconducting parameters. AC loss measurement results are presented for a round titanium matrix MgB₂ wire with simulated twist pitches between 9 mm and 87 mm.

The self-consistent interaction of Abrikosov vortex with both point and extended ferromagnetic defects was taken into account in the 2D model of layered HTSC. Magnetization curves for HTSC with magnetic nanoparticles as bulk pinning centers were obtained by Monte-Carlo method. The nonlinearity of the interaction of the superconductor with ferromagnetic defects and irreversibility of initially reversible magnetization processes due to nonlinearity were demonstrated.

Varying the thickness of the Pd layer, the role of the Pd films deposited on the cube-textured Ni-based substrates, was assessed in order to investigate the issues related to substrate oxidation phenomena and their impact on the quality of the buffer layer structures. Using a low temperature annealing, 600 °C, a threshold for the Pd film thickness, beyond which the Pd layer hinders the oxides formation on the substrate, can be defined. When the Pd-Ni interdiffusion is complete, oxide phases of Ni and of Ni-W coexist in the substrate. Besides, at temperatures as high as 800°C, the NiWO₄ formation is favoured as the thickness of the Pd layer increases. Despite of these phenomena, development of a buffer layer template suitable for YBCO growth are reported with a thickness for the Pd layer as low as 20 nm.

We studied effects of 3-MeV gold-ion irradiation on critical current density (J_c) in YBa₂Cu₃O₇ films prepared by three different deposition methods: fluorine-free metal-organic deposition, thermal co-evaporation, and metal-organic chemical-vapor deposition. The three films showed strikingly distinct responses to the extrinsic pinning-center introduction in their J_c values in magnetic fields, namely, about 300%, 70%, and no enhancement in the above order. Transmission electron microscopy results suggest that this characteristic process dependence is closely related to the microstructure of the as-deposited films.

Oxford Superconducting Technology (OST) has been continuously improving Bi-2212 round wire performance because of its potential for application in high-field magnets (> 25 T). We focused on Bi-2212 wire configuration design, filament densification and reducing carbon and hydrogen contamination to improve the engineering critical current density (J_E). Several wire configurations have been developed to meet different wire diameter and operating current requirements. The swaging, cold isostatic pressing (CIP) and over-pressure heat treatment processes have been demonstrated to effectively increase Bi-2212 filament mass density in the final wire and result in high performance over long length. The J_E values exceeding 550 A/mm² at 4.2 K, 15 T have been achieved on the CIPed 1 m long sample using a 10 bar over-pressure (OP) heat treatment. The twisted Bi-2212 wire significantly reduced ac loss without the critical current degradation.

We studied growth substrate temperature dependent morphologies of BaHfO₃ (BHO) nano-rods for SmBa₂Cu₃O_y, (SmBCO) films with BHO fabricated by pulsed laser deposition (PLD) method. The morphology of BHO nano-rods within the SmBCO matrix could be controlled by changing substrate temperatures adoping a Low Temperature Growth (LTG) technique. Using the LTG technique, we could fabricate SmBCO films showed c-axis orientation even at low substrate temperature during deposition without a degradation of superconducting properties, which is one of the most advantages of the LTG technique. In this study, we found an optimal morphology of the BHO nano-rods and the morphology was fine, straight and various growth directions. The BHO nano-rods were more effective to improve the J_c in a magnetic field than other morphologies of the BHO nano-rods.

In the YBa₂Cu₃O_7−x coated conductor, the buffer layer is crucial for the texture transfer from the Ni-5%W substrate to the superconducting layer. We developed original La_2−xGd_xZr₂O₇ buffer layers (x ≤ 1) by chemical solution deposition to decrease the structure mismatch between the substrate and the buffer layer. They show a strong cube texture up to the surface, a crack free surface and a very low roughness. In conclusion, the La_2-xGd_xZr₂O₇ layers with 0.4 < x < 0.9 are promising and should be employed in the future as buffer layer in coated conductor to replace La₂Zr₂O₇ buffer layer.

Due to their high current carrying capability, coated conductors are regarded as the most promising high-temperature superconductor tapes for power applications. However, their high aspect ratio causes too high magnetization losses. To reduce the ac loss, one way is to striate the wide tapes into filaments. We used a picosecond laser for the structuring of (RE)BCO coated conductors. The laser allows to burn 18 μm to 21 μm wide grooves (Ag-cap) with a depth between 0.5 μm to more than 100 μm into the coated conductors, with negligible heat effects at the edges of the structures. Different numbers of filament were structured in Cu-and Ag-cap coated conductors. Patterns with up to 120 parallel filaments in 12 mm wide conductor were made. The critical current and the total ac-magnetization loss were measured as a function of the filament count. With an increasing number of filaments I_c degradation occurs. This current reduction has two contributions, the removed HTS material and current inhomogeneities within the superconductor for instance defects along the tape causing secondary phases. For 120 filaments Ag-cap tapes the hysteresis loss reduction is about two orders of magnitude, as expected. The observation of some remaining filament coupling was investigated.

Coated conductors, although designed to carry transport current, are ideally suited to carrying persistent current and can therefore be cut and stacked to form a type of composite bulk which has superior thermal properties compared to existing bulks despite having less than 2 % superconductor by volume. The magneto-thermal modelling reported follows on from previous experimental work on pulsed magnetization of a 12 mm square tape stack. The magnitude of the applied field has a strong effect on the trapped field and flux. The optimum applied field depends on sample height and the maximum trapped field and flux saturates as the height reaches the diameter of the stack. The nature of a composite bulk made from a stack of tapes gives complete control over the height of the stack which needs to be optimised for pulsed magnetization.

We present experimental data and analysis of application of coated conductors (CC) for levitation and trapped field magnets. The magnetization loops and magnetization relaxation curves were obtained in fields up to 14 T and temperatures from 4.2 K to 77.4 K. The levitation force and its relaxation were measured for the stack of superconducting tapes at T=77 K. The measurements of levitation force for different stack sizes demonstrate the linear growth of the force amplitude and the improvement of the relaxation characteristics as a number of tapes in a stack increases.

We theoretically explore a competition between magnetism and superconductivity for the three-layered ferromagnet/superconductor (F/S) structures. The critical properties of F₁/F₂/S trilayers in an external parallel magnetic field H are investigated. The different mutual orientations of the F layer magnetizations are examined. Under this condition the triplet component of the superconducting condensate is arisen. Assuming that all layers are dirty we solve boundary problem for the Usadel function. To find the critical parameters for trilayer as function of the F layers thicknesses we use Gor'kov's self-consistency equation. The results of numerical calculations for critical temperature T_c and critical magnetic field H_c at various parameters of F/S structure show that the external magnetic field and triplet correlations can essentially influence on the critical properties of considered systems. Unusual dependencies are discussed.

RABiTS production process was developed for tapes from Ni-W alloys with W content in the range of 4-10.5 at.%. Tapes' composition, structure and magnetic properties were investigated. High purity Ni-W ingots were smelted and tapes with thickness of 0.06-0.09 mm and various total accumulated deformations in the range of 90-99.2% were fabricated. Texture quality dependence on accumulated deformation in the tapes with W various content was studied. X-ray analysis data including pole figures show that high quality cube texture can be formed in the tapes with W content up to 9 at.% inclusive, but not formed in the tapes with 10.5 at.%. According Curie temperature values and saturation magnetization measurements of alloys with W content more than 8 at.% are paramagnetic at liquid nitrogen temperature. Summarizing it can be stated that RABiTS tapes with 8-9 at.% W content have optimal combination of structure and magnetic properties for using in coated conductors.

The basic physical reasons rooted in the current instability phenomenon of AC regimes are discussed. To explain their stable formation, the conduction-cooled high-temperature superconductors (HTS) are investigated studying mutually dependent thermal and electrodynamics states of a tape. It is proved that there exist characteristic times defining time windows of stable development of overloaded AC regimes. They lead to the states for which the limiting peak values of charged current and induced electric field before instability onset much more than the corresponding critical values of a superconductor. Moreover, the limiting stable peak values of the temperature, electric field and current are higher than the corresponding quench ones calculated at DC modes. It is shown that the stability boundary of overloaded AC modes slowly depends on the current frequency. As a result, the high-T_c superconducting magnets may stably operate under very high AC losses even at the conduction-cooled conditions. The obtained results should be taken into consideration when the range of stable losses is defined in HTS during AC regimes.

The influences of bending strain on the critical current density J_c in coated conductor (CC) tapes were investigated at 77 K by contactless method based on scanning Hall probe magnetometry (SHPM). In order to clarify the reasons of variations in critical current for different bending radius we carried out local studies of critical currents in CC tapes before and after bending strain. By using SHPM we defined two-dimensional maps of critical current distribution at various bending radiuses. It was demonstrated that the superconducting layer in CC tape cracked at critical bending radius into periodical regions with unchanged and decreased transport properties. Appearance of local cracks with reduced value of the critical current leads to degradation of the critical current of the whole tape. It is found that the both critical bending radius and kind of tape cracking depend on the type of metallic substrates. The results of contactless studies are in good agreement with direct transport measurements.

The pulsed laser deposition (PLD) targets used for HTSC 2G fabrication should be uniform and chemically suitable to make long tapes with reproducible critical properties. We investigated inner density distribution of flat YBCO targets using non-destructive neutron introscopy and tomography techniques. The methods are based on absorption of monochromatic neutron beam which is going through the specimen. In first set of targets we measured variance of material density (up to 10-25%) with maxima density located in central layer of the target body. After updating of target fabrication technology no significant density inhomogeneity were detected. The dependence of critical current of superconducting films with thickness of 300 nm deposited on sapphire substrate with appropriate buffer layers using PLD YBCO targets with various oxygen content was also investigated. Oxygen index of YBCO targets vary from 6.12 to 6.85. No significant influence of target oxygen index on critical current density of fabricated films was detected.

Liquid hydrogen (LH₂), which has large latent heat, low viscosity coefficient, is expected to be a candidate for a cryogen for superconducting wires, not only MgB₂ but also other HTC superconductors. LH₂ cooled superconducting wires are expected to have excellent electro-magnetic characteristics, which is necessary to be clear for cooling stability design of LH₂ cooled superconducting device, however, due to handling difficulties of LH₂, there are only few papers on the properties of LH₂ cooled superconductors, especially under external magnetic field. We designed and made an experimental setup which can energize superconducting wires immersed in LH₂ with the current of up to 500A under the condition of external magnetic field up to 7 T and pressure up to 1.5 MPa. In order to confirm experimental method and safety operation of the setup, over current tests were carried out using MgB₂ superconducting wires under various external magnetic field conditions. Critical current of the test wire at the temperature 21, 24, 27, 29 K under external magnetic fields up to 1.2 T was successfully measured. The resistance of the wire also was measured, while the transport current exceeded the critical current of the wire.

YBa₂Cu₃O_7-δ (YBCO) films with a thickness of up to 3 μm containing nano-sized BaHfO₃ (BHO) have been grown on Y₂O₃/Y-stabilized ZrO₂/CeO₂ buffered Ni-9at% W tapes by pulsed laser deposition (PLD). Structural characterization by means of X-ray diffraction confirmed that the YBCO layer grew epitaxial. A superconducting transition temperature T_c of about 89 K with a transition width of 1 K was determined, decreasing with increasing BHO content. Critical current density in self-field and at 0.3 T increased with increasing dopant level.

Flexible rods of single crystals of 9% Y₂O₃-stabilized ZrO₂ (YSZ) were used as substrates for deposition of high-critical temperature superconducting (HTS) thin films. YSZ fibers were prepared by mini-pedestal method with laser heating and had average diameter of 300 micrometers and 30 mm length. X-ray diffraction analysis demonstrated high crystalline quality of obtained fibers and also indicated the presence of 15° deviation of the fiber axis from the [001] YSZ direction. Thin YBa₂Cu₃O_7−x films were grown by pulsed laser deposition on YSZ rods using CeO₂ buffer layer. Films have shown high critical temperature of 90 K with sharp superconducting transition. Critical current density was estimated to about 3×10⁴ A/cm² at 80 K. Temperature dependence of critical current density suggests granular structure of films with grain size about several microns. Our results demonstrate feasibility of flexible YSZ fibers coated by HTS thin films for practical use.

In this work, an experimental and theoretical study of the transport and magnetization AC losses of a single layer ReBCO cable is presented. Examined cable consisted of five coated conductor tapes wound in parallel helically on the round core with 40 mm pitch and six full transposition resulting in the 24 cm total cable length. Transport currents up to 600 A amplitude with frequencies 36 Hz and 72 Hz were used in the transport loss measurement. AC fields up to 50 mT were applied during the magnetization loss measurement at 36 Hz and 72 Hz, respectively. Very good agreement between the experimental results and the simulations for magnetization loss was achieved. However, the experimental results for transport loss are markedly greater than the prediction reached from simulation. Nevertheless, they are lower than the transport AC loss theoretically assumed for a straight superconducting tape with the identical critical current.

Industrial Nb₃Sn wires with Ti and Ta additives (RRP process) and with Ta additives (PIT process) with a diameter of 1 mm have been irradiated at room temperature with protons of 65 MeV and of 24 GeV at various fluences up to 1×10²¹ p/m². A steady increase of J_c vs. fluence was observed for all the wires up to the highest fluence. The observed increase of J_c at 4.2K in all wires was quite similar in spite of the very different proton energies. With increasing fluence. the radiation induced pinning force was found to increase. the enhancement J_c/J_co after 5.04×10²⁰ p/m² reaching 1.4 for Ta and 1.8 for Ti alloyed wires at 10T. The present results were quantitatively analysed by assuming a radiation induced point pinning mechanism in addition to grain boundary pinning. The results are compared with those of an ongoing neutron irradiation study undertaken on the same Nb₃Sn wires in collaboration with the Atominstitut Vienna. Proton irradiation was found to produce considerably higher damage than neutron irradiation.

The paper concerns the analysis of transient state (quench transition) in HTS tapes loaded with the current having DC component together with a ripple component. Two shapes of the ripple were taken into account: sinusoidal and triangular. Very often HTS tape connected to a power electronic current supply (i.e. superconducting coil for SMES) that delivers DC current with ripples and it needs to be examined under such conditions. Additionally, measurements of electrical (and thermal) parameters under such ripple excitation is useful to tape characterization in broad range of load currents. The results presented in the paper were obtained using test bench which contains programmable DC supply and National Instruments data acquisition system. Voltage drops and load currents were measured vs. time. Analysis of measured parameters as a function of the current was used to tape description with quench dynamics taken into account. Results of measurements were also used to comparison with the results of numerical modelling based on FEM. Presented provisional results show possibility to use results of measurements in transient state to prepare inverse models of superconductors and their detailed numerical modelling.

We have developed a cryogen-free critical-current (I_c) measuring system comprising a conduction-cooled 8 T HTS magnet and convection-cooled sample, both cooled by commercial cryocoolers. The sample can be rotated and transport currents of up to 800 A delivered with less than 0.5 K temperature rise during the Ic measurement. The system is automated with respect to variations in temperature (30–90 K), field (0–8 T), and field angle (0–360°). We have used this system to measure HTS wire samples, concentrating on metal-organic deposited YBCO on RABiTS substrates. Particular emphasis is given to the evolution of I_c anisotropy with temperature, and the dangers of extrapolating from 77 K to 30 K.

In the 200 m high temperature superconducting (HTS) cable test facility at Chubu University, a coaxial power cable is used and composed of two BSCCO (Bi-2223) superconducting layers. The tapes are wound closely to reduce effects on the critical current of BSCCO at self-field. Accordingly, each superconducting layer has a different number of BSCCO tapes. Previously, we have investigated dependence of the critical current (Ic) on the gap in order to optimize the HTS DC cable design. We have been studying the effect on the performance of HTS tapes for the superconducting DC power cables by critical current measurements. In the present experiments several HTS tapes are used and set as a similar structure in the cable with a two-layer structure. The critical current of HTS tapes are measured against the gap between the tapes in the same layer. The experiments show the improvement of the critical current by optimizing the tape arrangements due to magnetic field interaction between the tapes. We will present the experimental results and discuss the design of the HTS DC cable.

It is considered as a challenge for RABiTS route to get cube textured Ni-W alloy substrates with high mechanical and magnetic properties for coated conductors. The works of our group in recent years are summarized about different Ni-W substrates with high W content and composite tapes made by RABiTS technique. The fabrication process and the mechanism of cube texture formation in above different tapes are studied systematically. Compared with commercial Ni-5 at.%W substrate, these alloy substrates show a comparable texture quality and improved mechanical properties as well as reduced or zero magnetization especially in the novel composite substrates.

Performances of Twisted Stacked-Tape Cables (TSTC) made of YBCO tapes were discussed for self-field and high-field operations. Power transmission cables of a single stack cable and a 3-channel conductor were evaluated by self-field distributions. A new concept of a double coaxial cable was discussed to obtain a high performance, high current density power cable. High field test results of 50-tape and 40-tape TSTC conductors were summarized, and their degradations were evaluated with single-tape data measured.

Three kinds of superconducting wires based on BaFe₂As₂ with different substitutions are fabricated using powder-in-tube method and characterized including magneto-optical imaging. In the case of (Ba,K)Fe₂As₂ wires processed by hot isostatic press, critical current density (J_c) of 32 kA/cm² has been achieved at 4.2 K under self-field. Wires fabricated in a similar way with Ba(Fe,Co)₂As₂ resulted in much lower J_c of 7.8 kA/cm² at 4.2 K under self-field. In the case of BaFe₂(As,P)₂, J_c of ambient-pressure processed wire has J_c of only 1.0 kA/cm² at 4.2 K under self-field. Origins of these differences are discussed.

An experimental system has been developed to investigate electro-magnetic properties of high-Tc superconductors cooled by liquid hydrogen under the external magnetic field of up to 7 T. A LH₂ cryostat is concentrically mounted on the inside of a LHe cryostat to cool a NbTi superconducting magnet. The experimental system is installed in an explosion-proof room. Explosion proof electrical devices are used and current leads are covered with an enclosure filled with nitrogen gas. A remote control system has been developed. Furthermore, the effects of stray magnetic field on the existing and the new devices are investigated and electro-magnetic shielding panels and enclosure made of iron were designed. It is confirmed through the cryogenic test that the experimental system meets the design requirements.

For the improvement of superconducting properties of BaMO₃ (M=Zr, Sn, HQ-doped REBa₂Cu₃O_y films, we focused on the T_c reduction caused by lattice misfit between BaMO₃ and REBa₂Cu₃O_y. We compared the influence on the Tc reduction of BaZrO₃, BaSnO₃ and BaHfO₃-doped SmBa₂Cu₃Or (SmBCO) films with various BMO contents. From the correlation between configuration of nanorods and c-axis expansion, it was suggested that the lattice strain working on the SmBCO matrix became stronger in the order of BaHfO₃, BaZrO₃ and BaSnO₃. Therefore, the combination of SmBCO and BaHfO₃ is better regarding the suppression of the Tc reduction compared with other combinations of SmBCO and BaMO₃.

We report on successful fabrication of superconducting FeSe wire using hot gas extrusion (HGE) ex-situ PIT (powder in tube) method. Length of the obtained wire was about 60cm with cross-sectional superconducting core area nearly 2.5 × 10⁻³ cm². For the wire sample we observed superconducting transition temperature, T^onset_c ≈ 11 K, about 1.2K lower compared to preliminary prepared FeSe powder. Heat treatment in argon atmosphere at 350^oC resulted in transition width decrease from ΔT(10% − 90%) ≈ 1.75 K in sample without heat treatment down to ΔT ≈ 0.9 K in annealed samples. Estimated derivative of the upper critical field as a function of temperature of the sample annealed during 72h in argon atmosphere at 350^oC is dH_c2/dT ∼ −2.9 T/K. Applying WHH theory to our data allows to define H_c2(0K) = 0.69T_c × (dH_c2/dT) ≈ 19.8T. The untreated wire shows critical current density, J_c ∼ 75 A/cm² at T=4.0 K in zero field. Increasing annealing time up to 72 hours at 350^oC in argon atmosphere gives rise to Jc increase of about 60% approaching 120 A/cm² at T=4.0K and H=0T . Also J_c measurements were made in magnetic fields up to 9T. Our results show applicability of the HGE PIT method for fabrication of superconducting wires based on FeSe compound. Long-range heat treatment is necessary to improve superconducting properties of the samples.

The structure of superconducting DC cable made with high temperature superconducting tapes is designed to improve the current-carrying capacity by reducing the Lorentz force acting on flux lines. For this purpose an axial magnetic field is produced by the outer shield conductor. The analysis was performed for the optimum structure for various superconducting tapes with different J_c(B,θ) characteristics. It is found that the current-carrying capacity increases appreciably when the difference in the critical current density of the tape is large enough between normal and parallel magnetic fields.

YSZ buffer layers and YBCO superconducting layers were grown on biaxially textured NiW tapes by a reel-to-reel pulsed laser deposition (PLD) technique. It was found that orientation and surface morphology of YSZ buffer layers were very sensitive to deposition parameters, especially to oxygen pressure and laser repetition rate. Relationships between microstructure, surface morphology of YSZ buffer layer and PLD deposition parameters were systematically studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effects of surface morphology of YSZ buffer layer on the superconducting performance of YBCO layer were investigated. As increasing oxygen pressure from 0.5 to 10 mTorr, the surface morphology of YSZ buffer layer became better first and then worse, however, it deteriorated with the increase of laser repetition rate. It was found that the surface morphology of YSZ buffer layer had great influence on the superconducting transport properties of YBCO layer. Under optimized experimental condition, high quality YBCO layer with critical current density J_c above 4 × 10⁶ A/cm² was obtained at 77 K and self field.

This project is partly supported by NSFC Grant 51207146, RAEng Research Exchange scheme of UK and EPSRC EP/K01496X/1.

Superconducting cable is an emerging technology for electricity power transmission. Since the high power capacity HTS transmission cables are manufactured using a multi-layer conductor structure, the current distribution among the multilayer structure would be nonuniform without proper optimization and hence lead to large transmission losses. Therefore a novel optimization method has been developed to achieve evenly distributed current among different layers considering the HTS cable structure parameters: radius, pitch angle and winding direction which determine the self and mutual inductance. A prototype HTS cable has been built using BSCCO tape and tested to validate the design the optimal design method. A superconductor characterization system has been developed using the Labview and NI data acquisition system. It can be used to measure the AC loss and current distribution of short HTS cables.

It is well known that the recrystallization texture of heavily cold-rolled pure copper is almost completely cubic. However, one of the main drawbacks concerning the use of pure copper cube-textured substrates for YBCO coated conductor is the reduced secondary recrystallization temperature. The onset of secondary recrystallization (i.e., the occurrence of abnormal grains with unpredictable orientation) in pure copper substrate was observed within the typical temperature range required for buffer layer and YBCO processing (600–850 °C). To avoid the formation of abnormal grains the effect of both grain size adjustment (GSA) and recrystallization annealing was analyzed. The combined use of a small initial grain size and a recrystallization two-step annealing (TSA) drastically reduced the presence of abnormal grains in pure copper tapes.

Another way to overcome the limitation imposed by the formation of abnormal grains is to deposit a buffer layer at temperatures where secondary recrystallization does not occur. For example, La2Zr2O7 (LZO) film with a high degree of epitaxy was grown by metal-organic decomposition (MOD) at 1000 °C on pure copper substrate. In several samples the substrate underwent secondary recrystallization. Our experiments indicate that the motion of grain boundaries occurring during secondary recrystallization process does not affect the quality of LZO film.