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A good knowledge of material properties is a critical aspect for modeling high-temperature superconductor (HTS) devices. However, the knowledge of the electrical resistivity of coated conductors above the critical current is limited. The major challenge in characterizing this regime lies in the fact that for $I>I_{\rm {c}}$ , heating effects and thermal instabilities can quickly destroy the conductor if nothing is done to protect it. In previous work we extracted overcritical current data, obtained by combining fast pulsed current measurements with finite element analysis (Uniform Current (UC) model). In this work, we assessed the impact of the uncertainties of the input parameters on the quantities calculated with the UC model (temperature, current in each layer of the tape and resistivity of HTS). Firstly, sensitivity and uncertainty analyses were performed and it was found that the input parameters that mostly affect the accuracy of the UC model are the electrical resistivity and the thickness of the silver layer. Afterwards, an optimization method was developed to correctly estimate the geometry and the resistivity of the silver layer. This method combines experimental measurements of resistance $R(T)$ of the tape and pulsed current measurements. The development of this strategy allowed us to determine the parameters that significantly impact the results of the UC model and helped to minimize their uncertainties. This enables a more accurate estimation of the resistivity in the overcritical current regime.

As a high-temperature superconducting (HTS) conductor with a large current capacity applicable to a nuclear fusion experimental device, REBCO (REBa 2 CuO y ) tapes and high-purity aluminum sheets are alternately laminated, placed in a groove of an aluminum alloy jacket having a circular cross section, and the lid is friction-stir welded. To make the current distribution and mechanical characteristics uniform, the conductor is twisted at the end of the manufacturing process. In the early prototype conductor, when the I c was measured in liquid nitrogen under self-magnetic field conditions, I c degradations were observed from the beginning, and the characteristic difference between the two prototype samples under the same manufacturing conditions were large. Furthermore, I c degradation was progressed by repeating the thermal cycle from room temperature to liquid nitrogen temperature. This I c degradation did not occur uniformly in the longitudinal direction of the conductor but was caused by local I c degradation occurring at multiple locations. If the conductor was not manufactured uniformly in the longitudinal direction, the difference in thermal shrinkage between the REBCO tape and the aluminum alloy jacket caused local stress concentration in the REBCO tape and buckling occurred. Element experiments to explain this mechanism were conducted to clarify the conditions under which I c degradation due to buckling occurs. Then prototype conductors were tested with improved manufacturing methods, and as a result, I c degradation could be suppressed to 20% or less. We have achieved the prospect of producing a conductor with uniform characteristics in the longitudinal direction.

The primary aim of this study is to understand how the thermally induced strain varies with change in thickness of REBCO tapes. During the production stage of REBCO tapes, a residual thermal strain is induced in it due to cool down from 1020 K to 77 K. The 4 mm long and 0.091 mm thick REBCO tape cools down from 1020 K to a 0.17% residual compressive strain at room temperature and cools down to a 0.242% residual compressive strain at liquid nitrogen temperature. But these values are dependent upon the thickness of other elements in the REBCO tape. A mathematical model for the system is developed and validated using experimental data. The model is then subjected to different practical conditions and its thermal behaviour is evaluated. The performance of the superconductor is closely monitored and the degradation is determined for each case. The study has given more information on how the thermally induced strain differs with changes in thickness of REBCO tapes. The results obtained are expected to make modifications in the current production process for REBCO tapes and in its eventual usage.

Optimization of the helically-winding structure required for the helical fusion reactor utilizing the high temperaturesuperconductor (HTS) tape is studied by numerical calculation. When the HTS tapes are wound to bethe helical structure without the torsion, a significant edgewise strain appears. This might lead to the degradationof the performance of the HTS tape. Therefore, the edgewise strain must be minimized. It is found that the edgewise strain can be suppressed by the proper determination of the torsion by the optimization utilizing the Fourier decomposition of the distribution of the edgewise strain. The HTS tapes with edgewise strain are intrinsically deformed due to the torsion to reduce the edgewise strain so to be an “edgewise-strain-free” helical shape.

A structural model has been developed for a Cable-in-Conduit Conductor (CICC) made with 2G HTS superconductors inserted in a helically slotted aluminum core. The cable is particularly suited for high-field applications and consists of a twisted aluminum core and five helical slots. Each slot accommodates a stack of twenty REBCO tapes. The cable is equipped with a central cooling channel and an aluminum jacket. In this work, finite element analysis was used to predict the electrical performance of the cable as a function of the bending diameter. The model consists of three parts: compaction of the aluminum jacket on the slotted core (each slot filled with the HTS stack of tapes), cable's bending and thermal cooldown (from room temperature to 77 K). It was found that the compaction process affects the performance for the top and bottom tapes of the stack. To mitigate this effect, a new design option which includes one structural tape (either copper or stainless steel) on the top and bottom of the stack was evaluated, showing a significant reduction of the strain in those tapes. By adding the strain results generated by the three processes (compaction, bending and cooldown), the normalized critical current was calculated as a function of the bending diameter and compared with experimental data. With a friction coefficient of 0.02 between the tapes, the agreement between model and experimental data is good. The numerical modeling presented here will provide important information on the strain state of the tape-stacks and can be used to investigate the behavior of more recently developed cable designs and optimize future configurations.

We present a simple liquid-assisted processing (LAP) method, to be used in situ during pulsed laser deposition growth to give both rapid growth rates (50 Hz deposition leading to >250 nm min���1 with a single plume) and strong pinning (improved ��5���10 at 30 K and below, over plain standard YBCO films grown at similar rates). Achieving these two important features simultaneously has been a serious bottleneck to date and yet for applications, it is critical to overcome it. The new LAP method uses a non-stoichiometric target composition, giving rise to a small volume fraction of liquid phase during film growth. LAP enhances the kinetics of the film growth so that good crystalline perfection can be achieved at up to 60�� faster growth rates than normal, while also enabling artificial pinning centres to be self-assembled into fine nanocolumns. In addition, LAP allows for RE mixing (80% of Y with 20% of Yb, Sm, or Yb + Sm), creating effective point-like disorder pinning centres within the rare earth barium cuprate lattice. Overall, LAP is a simple method for use in pulsed laser deposition, and it can also be adopted by other in situ physical or vapour deposition methods (i.e. MOCVD, evaporation, etc) to significantly enhance both growth rate and performance., Grant from SuNAM Co. Ltd. Henry Royce Institute Equipment Grant: EP/P024947/1

Superconducting magnets are widely used in accelerator science applications. Muon production solenoids are applications that have recently attracted considerable public attention, after the approval of muon-related physics projects such as coherent muon to electron transition or muon-to-electron-conversion experiments. Based on its characteristics, muon production solenoids tend to be subjected to high radiation exposure, which results in a high heat load being applied to the solenoid magnet, thus limiting the superconducting magnet operation, especially for low-temperature superconductors such as niobium titanium alloy. However, the use of high-temperature superconductors may extend the operation capabilities owing to their functionality at higher temperatures. This study reviews the characteristics of high temperature superconductor magnets in high-radiation environments and their potential for application to muon production solenoids.

Author(s): Teyber, R; Brouwer, L; Godeke, A; Prestemon, S | Abstract: A compact gantry delivering 70-220 MeV protons with fixed field in the superconducting magnets could reduce the cost and improve the adoption of proton therapy. While a number of magnet and cryogenics designs have been proposed, the combined capital and operating costs of state-of-the-art superconducting materials have not been analyzed. In response, we develop a thermoeconomic model of a multi-stage, conduction cooled gantry lattice and analyze the cryocooler operating cost, cryocooler capital cost and conductor capital cost for Nb-Ti, Nb3Sn, REBCO and Bi-2223 over a continuous range of magnet temperatures, and a differential evolution algorithm is used to identify the optimal combination of thermal intercept temperatures. Although Nb3Sn yields the lowest Net Present Value (NPV) of $111.7k at a magnet temperature of 9.4 K, the optimized Bi-2223 design at 12.8 K approaches the realm of commercial feasibility by offering improved thermal stability and forgoing the need for costly conductor heat treatment and magnet quench training. Furthermore, it was found that Nb3Sn was more cost effective than Nb-Ti and that REBCO was not economically viable for the parameters of this investigation. The thermoeconomic model developed herein can optimize conductor choices, magnet temperatures and thermal staging which has value for any conduction-cooled superconducting magnet.

REBCO coated conductor has demonstrated high current capacity that can enable high-field magnets for high energy physics and fusion applications. However, quench protection is still one of the main challenges to be addressed for these applications. In addition, ${I}_{\text{c}}$ and $n$ value variations along the length of REBCO tapes exist in commercial production. The inter-tape contact resistance plays a key role to develop the self protection capability in cables and magnets by enabling current sharing and suppressing excessive eddy currents. Here we propose an electric-circuit model to describe the inter-tape contact resistance and its impact on the current sharing between REBCO tapes. We report the experiments on a 2-stacked tape REBCO cable with local ${I}_{\text{c}}$ drop to validate the model. With the developed model, we study the upper limit of the contact resistance which allows current sharing between tapes. We also study the impact of variation in ${I}_{\text{c}}$ and $n$ values in tapes on the cable performance. Our model is expected to provide useful insight into the current sharing and target values for inter-tape contact resistance in REBCO cables and magnets for various applications.

Within the framework for establishing standards of test methods for superconducting technical wires, various standards have been issued by the International Electrotechnical Commission (IEC) (standard documents IEC 61788-1 to -20). Following the successful round robin test (RRT) for tensile testing REBCO wires at room temperature (Osamura K et al 2014 Supercond. Sci. Technol. 27 085009), this effort is extended to tensile test HTS wires at cryogenic temperatures and is coordinated by the CryoMaK lab at Karlsruhe Institute of Technology. Five different commercially available REBCO wires from five different manufacturers and one BiSCCO wire from another supplier were provided by the Versailles Project on Advanced Materials and Standards (http://vamas.org) for testing. Samples were distributed between eight participating labs from five different countries for testing according to the specified guidelines. After the test results were delivered by all participants, the data were evaluated with statistical tools to investigate the main source of scatter and its magnitude in the test results. The final goal of the RRT is issuing an ISO/IEC standard for a cryogenic temperature tensile test for REBCO wires. In this report the results of the RRT for tensile testing REBCO wires at cryogenic temperatures are presented and discussed.

WOS:000476530300001; International audience; Thanks to their very high current carrying capabilities even under high magnetic field conditions and their outstanding mechanical properties, high temperature superconductors (HTSs) such as REBCO (rare-earth BaCuO) tapes are very attractive for high magnetic field applications. Depending on the magnet design goals and constraints, it can be advantageous in some cases to reduce the electrical margins of the conductor. Considering the uncertainty in locally evaluating the critical current, and the inhomogeneities of present-day REBCO tapes, there is a significant risk of the critical current being overstepped locally, thus triggering local damaging hotspots. Such an event does not have the sudden occurrence and/or fast spreading quality usually associated with the concept of 'quench' and should be simply seen as thermal runaway induced by dissipative zones (DZs). The development of numerical models to evaluate the occurrence and propagation of such zones inside windings is critical in the development of a HTS magnet fully using REBCO tape performance while guaranteeing safe operation conditions. In this work, we have developed a transient electro-thermal model adapted to pancake-based coils. It accounts for both the nonlinear electrical and thermal behavior of the material and considers the local inhomogeneities of the critical current I c along the tape. The electrical part is one-dimensional (1D) and computes the nonlinear dissipation in the conductor depending on the local operation conditions while the thermal part is two-dimensional (2D) to account for the heat propagation along the conductor length and from turn to turn. In order to improve computation efficiency, adaptive time-stepping methods have been introduced with the objective of ensuring good accuracy of simulation results.

In this work, stacked model conductors consisting of dozens of YBCO tapes were fabricated and experimentally investigated to determine whether the hysteresis and coupling losses affect each other. When an oblique magnetic field is applied to the tape, hysteresis and coupling losses are expected to be generated simultaneously. A uniform AC transverse magnetic field was applied to the sample in liquid nitrogen, and the AC losses were measured via the pickup coil method. The measurements were carried with the magnetic field applied at various angles with respect to the tape wide surface. The measured AC losses were separated into hysteresis and coupling losses. The measured hysteresis losses well agreed with the theoretical values for a square cross-section superconductor. This indicates that hysteresis losses can be considered in terms of the magnetic field component perpendicular to the flat face of the YBCO tape. The coupling time constant was found to be independent of the applied magnetic field angle. This shows that coupling losses can be considered in terms of the magnetic field component parallel to the tape's wide surface, and the permeability in the conductor can be assumed to be the vacuum permeability.

Both the pancake coiling and conductor on round core (CORC) cabling processes consist of winding second-generation high-temperature superconducting tape (REBCO) in a core. The contact behavior between the tape and the core during the winding process directly affects the critical current, cable flexibility, mechanical support and electrical contact resistance. Therefore, the winding process needs to be optimized to produce pancake coils and CORC cables with the desired electrical and mechanical properties. This paper comprehensively considers the role of the relaxation and Poisson effects. The theoretical model and the finite element model (FEM) for calculating the contact stress during the winding process are established, and a formula for the estimation of the contact force is proposed. The suitable winding pre-tension force, winding curvature radius of SCS2030 (2 mm width and 30 μ m substrate thickness) and SCS4050 (4 mm width and 50 μ m substrate thickness) of REBCO tapes are obtained and discussed by accounting for the relaxation effect and the critical current degradation limit. With consideration of the Poisson effect, the nonlinear contact behavior is investigated. In addition, the relaxation effect, Poisson effect and plasticity of the REBCO tape are taken into account in the FEM model. The results show that the axial strain in the REBCO layer during the winding process is related to the contact behavior. The greater the winding pre-tension force, the smaller the helical curvature radius, and the greater the contact force. The distribution of contact stress for several winding factors (winding pre-tension force, winding angle and the core radius) is divided into three contact types: the single-line contact, the double-line contact and the surface contact. The FEM model results are consistent with the theoretical evaluation and are more suitable for practical winding cases. This research is the basis for designing and optimizing pancake coils and CORC cables.

The European Organization for Nuclear Research (CERN) is developing high-current ReBCO-CORC strand-based cables for use in future large-scale detector magnets. A six-around-one, forced flow gas-cooled ReBCO-CORC cable-in-conduit conductor (CICC) is envisioned for application in magnets operating in the 20-40 K temperature range. A CICC, rated for 45 kA at 4.2 K and 10 T, is designed and in production. The CICC comprises a cable of six CORC strands helically wound around a tube. The cable has an expected current density of 105 A/mm2 at 10 T/4.2 K, which corresponds to an overall current density of 53 A/mm2. A cable current density of 110 A/mm2 can be reached when increasing the temperature to 20 K and operating in a magnetic field of 5 T.

CERN is developing high-current ReBCO conductor on round core (CORC) cables for application in future detector and accelerator magnets. A characterization test on a ReBCO-CORC cable sample and its joints is performed in the 10-T FRESCA cable test facility at CERN. The sample is taken from the first 12-m-long CORC production. Key is the characterization of the field- and temperature-dependent critical currents of the CORC cable at 1.9 K and 4.2 K. Secondary objectives include evaluating the response of the CORC cable to quenches and the performance of cylindrical low resistive cable terminals especially designed and manufactured for use on CORC cables. The 7.6-mm CORC cable features 8 kA at 4.2 K and 10 T, and the joint terminals show a $25\pm 5-{n}\Omega$ resistance for 20-cm length.

An R&D project on high-temperature superconducting (HTS) magnets using rare-earth Ba2Cu3O7 (REBCO) wires was started in 2013. The project objective is to investigate the feasibility of adapting REBCO magnets to ultrahigh field (UHF) magnetic resonance imaging (MRI) systems. REBCO wires are promising components for UHF-MRI magnets because of their superior superconducting and mechanical properties, which make them smaller and lighter than conventional ones. Moreover, REBCO magnets can be cooled by the conduction-cooling method, making liquid helium unnecessary. In the past two years, some test coils and model magnets have been fabricated and tested. This year is the final year of the project. The goals of the project are: (1) to generate a 9.4 T magnetic field with a small test coil, (2) to generate a homogeneous magnetic field in a 200 mm diameter spherical volume with a 1.5 T model magnet, and (3) to perform imaging with the 1.5 T model magnet. In this paper, the progress of this R&D is described. The knowledge gained through these R&D results will be reflected in the design of 9.4 T MRI magnets for brain and whole body imaging., Proceedings of the 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

A detailed knowledge of the resistivity of high-temperature superconductors in the overcritical current regime is important to achieve reliable numerical simulations of applications such as superconducting fault current limiters. We have previously shown that the combination of fast pulsed current measurements and finite element analysis allows accounting for heating effects occurring during the current pulses. We demonstrated that it is possible to retrieve the correct current and temperature dependence of the resistivity data points of the superconductor material. In this contribution, we apply this method to characterize the resistivity vs. current and temperature of commercial REBCO tapes in the overcritical current regime, between 77 and 90 K and in self-field conditions. The self-consistency of the overcritical resistivity model ρ O C ( I , T ) is verified by comparing DC fault measurements with the results of numerical simulations using this model as input. We then analyze by numerical simulation to what extent using the ρ O C ( I , T ) model instead of the widely used power-law model ρ P W L ( I , T ) affects the thermal and electrical performance of the tapes in the practical case of a superconducting fault current limiter. A remarkable difference is observed between the measured overcritical current resistivity model ρ O C ( I , T ) and the power-law resistivity model ρ P W L ( I , T ) . In particular, the simulations using the power-law model show that the device quenches faster than with the overcritical resistivity model. This information can be used to optimize the architecture of the stabilizer in superconducting fault current limiters.

Since the discovery of high-temperature superconductivity, significant progress in the fabrication of REBCO-based (Rare Earth Barium Copper mixed Oxides) thin-films superconductors has been achieved. In our review, we described the approaches and possibilities of the improvement of superconducting properties by the introduction of nanosized pinning centers. We focused on the synthesis and viability of the material for artificial pinning centers and methods used for the introduction of the pinning centers into superconducting REBCO-based thin-films. This article summarizes available materials and procedures regardless of the financial cost of the individual method. According to available literature, the most significant superconducting REBCO tapes can be obtained when a combination of 1D and 0D nanoparticles are used for nanoscale pinning.

Many high-temperature superconductor (HTS) applications require superconducting cables with high currents while operating in an alternating magnetic field. HTS cables should be composed of numerous superconducting tapes to achieve the required current capacity. Alternating current and magnetic fields cause AC losses in such cables and can provoke conductor instability. AC losses and contact resistances were measured of several cable designs based on commercially available REBCO tapes at the University of Twente. The AC loss was measured under identical conditions for eight REBCO conductors manufactured according to three types of cabling methods—CORC® (Conductor on Round Core), Roebel, and stacked tape, including a full-size REBCO CICC (cable in conduit conductor). The measurements were done at T = 4.2 K without transport current in a sinusoidal AC magnetic field of 0.4 T amplitude and frequencies from 5 to 55 mHz. The AC loss was measured simultaneously by calibrated gas flow calorimeter utilizing the helium boil-off method and by the magnetization method using pick-up coils. Also, the AC loss of two CORC® conductors and a Roebel cable was measured at 77 K. Each conductor was measured with and without background field of 1 T. The measured AC coupling loss in the CORC® and Roebel conductors is negligible at 4.2 K for the applied conditions while at 77 K coupling loss was observed for all conductors. The absence of coupling loss at 4.2 K can be explained by shielding of the conductor interior; this is confirmed with measurement and calculation of the penetration field of CORC® and Roebel cables. The inter-tape contact resistance was measured for CORC® and stacked tape samples at 4.2 and 77 K. It was demonstrated that a short heat treatment of CORC® conductor with solder-coated tapes activates tape-to-tape soldering and decreases the contact resistance. The reduction of contact resistance by two orders in magnitude to tens of nΩm is comparable with the interstrand contact resistance in ITER Nb3Sn type conductors.

A conductor on round core (CORC (R)) cable is composed of several layers of helically wound high-temperature superconducting (HTS) tapes on a round core with the winding direction reversed in each successive layer. The cable is flexible but the flexibility is limited by the critical strain value causing breakage of the HTS layer when this strain level is exceeded. The cables for magnets in fusion reactors experience large mechanical and electromagnetic loads. These loads arise from the cabling, coil manufacturing, cooling, and magnet operation. In order to optimize the manufacture and operating conditions, the mechanical behavior of CORC (R) cables must be understood for the different relevant loading conditions. The cable configuration with many contact interactions between tapes and the non-linear behavior of the materials during the production and operating conditions makes the modeling challenging. Detailed finite element (FE) modeling is required to account for these complexities. The FE modeling allows an accurate calculation of the stress-strain state (SSS) of the cable components under various loads and avoids time-consuming large-scale experimental optimization studies. This work presents a detailed FE modeling of the 3D SSS in a CORC (R) wire under bending load. The elastic-plastic properties of the individual tape composite materials and its temperature dependence are taken into account. The FE model is experimentally validated by a multilayer CORC (R) bending test performed by Advanced Conductor Technologies LLC. A critical intrinsic tensile strain value of 0.45% is taken as the threshold where the individual tape performance becomes irreversibly degraded. The proposed FE model describes the bending test of the CORC (R) wire adequately and thus can be used to study other types of loads. A parametric study is ongoing with dependent variables to pursue a further optimization of CORC (R) cables and wires for various applications.

VAMAS TWA 16 promoted an international round robin test (RRT) on critical current (I c ) measurement of RE-BaCu-O (REBCO; RE = rare earth) superconductors to establish the transport I c measurement method in liquid nitrogen bath without an external magnetic field. Ten laboratories from five countries participated in the RRT and tested commercially available REBCO superconductors. I c and measurement conditions were reported. In parallel, intrinsic I c nonuniformity of the superconductors were evaluated by reel-to-reel scanning Hall-probe microscopy for 10-m conductors. Statistical analyses of I c provided uncertainty of the measurement method. Combined standard uncertainty was analyzed considering the intrinsic nonuniformity of I c . The analysis indicated that the intrinsic nonuniformity has the largest contribution to the uncertainty of I c .

High temperature superconducting magnetic energy storage systems (HTS SMES) have attracted significant attention for fast response and ensure a reliable power supply. However, the current carrying capacity of single superconducting tape often meets limitation for the large scale HTS SMES applied in the power grid. Therefore, a high temperature superconducting composite cable with inner helical cooling tunnel and kA class current carrying ability is proposed for SMES magnet by using REBCO tape. The critical current characteristics of this internal cooling composite cable are analyzed considering the influence of anisotropy in magnetic field. A 100 m length, high temperature composite cable is manufactured in China Electric Power Research Institute (CEPRI). The critical current experimental system with a 3 kA DC current power source and a high-precision Digital Data Acquisition system have been set up to investigate the current carrying ability of a straight and a bending demo composite cable in LN2. The results show that critical current of HTS composite cable consisted of 4 REBCO tapes can achieve 780 A at 77 K self-field and the experimental I-V curve of each REBCO tape in the composite cable is not uniform because of the influence of anisotropy. When the HTS composite cable is bent, its critical current is about 90% of the straight HTS composite cable. Therefore, the design method and the proposed experimental system are proved to be effective as well.

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values Ic at 77 K (liquid N2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance (Rinter) has also been measured and the observed dependence of Rinter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA. © 2014 IEEE.

A project to develop an ultra-high-field magnetic resonance imaging (MRI) system based on HTS magnets using (RE)Ba2Cu3O7 (REBCO; RE=rear earth) coils is underway. The project is supported by the Japanese Ministry of Economy, Trade and Industry and aims to establish magnet technologies for a whole-body 9.4 T MRI system. REBCO wires have high critical current density in high magnetic fields and high strength against hoop stresses, and therefore, MRI magnets using REBCO coils are expected to have cryogenic systems that are smaller, lighter, and simpler than the conventional ones. A major problem in using REBCO coils for MRI magnets is the huge irregular magnetic field generated by the screening current in REBCO tapes. Thus, the main purpose of this project is to make the influence of this screening current predictable and controllable. Fundamental technologies, including treatment of the screening currents, were studied via experiments and numerical simulations using small coils. Two types of model magnets are planned to be manufactured, and the knowledge gained in the development of the model magnets will be reflected in the magnet design of a whole-body 9.4 T MRI system., Proceedings of the 27th International Symposium on Superconductivity (ISS 2014) November 25-27, 2014, Tokyo, Japan

The critical current angular distributions in the magnetic field range from 0 up to 8 T have been measured for Zr doped MOCVD (metalorganic chemical vapor deposition) tape produced by Super Power Inc. The vortex path model was used to fit these distributions to specify a tilted peak near c-axis that corresponds to the presence of BaZrO3 columnar defects in the tape. The orientation of such defects was found to be 10.3 o with respect to the c-axis direction. Peak in the ab direction was observed in the whole field range and it could be interpreted as pinning on spacer layers between CuO planes. The appearance of gauss distribution was found in high fields that confirms chosen model.

A cryogen-free 25 T superconducting magnet using a ReBCO insert coil that generates 11.5 T in a 14 T background field of outer low-temperature superconducting (LTS) coils is currently under development. The AC loss of the insert coil during field ramping is approximately 8.8 W, which is difficult to dissipate at the operating temperature of the LTS coils (4 K). However, since a ReBCO coil can operate at a temperature above 4 K, the ReBCO insert coil is cooled to about 10 K by two GM cryocoolers, and the LTS coils are independently cooled by two GM/JT cryocoolers. Two GM cryocoolers cool a circulating helium gas through heat exchangers, and the gas is transported over a long distance to the cold stage located on the ReBCO insert coil, in order to protect the cryocoolers from the leakage field of high magnetic fields. The temperature difference of the 2nd cold stage of the GM cryocoolers and the insert coil can be reduced by increasing the gas flow rate. However, at the same time, the heat loss of the heat exchangers increases, and the temperature of the second cold stage is raised. Therefore, the gas flow rate is optimized to minimize the operating temperature of the ReBCO insert coil by using a flow controller and a bypass circuit connected to a buffer tank.

CERN has designed and manufactured the first 1.7-m-long fully operational ReBCO CORC cable-in-conduit conductor. This CORC cable-in-conduit conductor is envisioned for use in large, high current and high energy magnets. The conductor has been successfully tested at 4.5 K/9 T and in 77 K/self-field. At 4.5 K, the CICC is fully superconducting up to the current limit of the test facility. The critical current of the conductor at 77 K is 12.3 to 13.0 kA with an n-value of 12 to 15, depending on ramp rate. The current distribution is observed to be homogeneous at both test temperatures. A resistance of 1.9 to 5.8 n \Omega is found for the joint terminals at 4.5 K in various magnetic fields and ramp rates, and 6.8 to 9.9 n\Omega at 77 K in self-field. The values found for the critical current and the resistance of the terminals at both 4.2 and 77 K are conform to the expectations based on a simulation model. The conductor underwent multiple thermal cycles no trace of degradation due to cabling, conduit assembly, joints and the high magnetic field tests was observed.

A 275 kV 3 kA high temperature superconducting cable (HTS cable), which could be used as a backbone power line in the future, was developed in the NEDO project called M-PACC. One of the most important developments of a high voltage HTS cable was the high voltage insulation technology. A design guideline and a test specification that was necessary to design, product and demonstrate of a 275 kV, 3 kA HTS cable have been studied by obtaining the various experimental data such as AC withstand voltage, impulse withstand voltage, partial discharge inception stress, and the V-t characteristics of the insulation, on the basis of the Japan Electrical Standards (JEC) and the International Electrotechnical Commission (IEC). Moreover, the 275 kV, 3 kA HTS cable with a length of 30 m was demonstrated under a long-term voltage and current loading test.

High temperature superconductors, especially rare-earth barium copper oxide (REBa₂Cu₃O₇₋ₓ or REBCO) tapes are an enabling technology for magnetic fields beyond 23.5 T, the limit of the existing technology based on the low temperature superconductors (LTS) niobium–titanium (NbTi) and niobium–tin (Nb₃Sn). Due to their higher prices, REBCO tapes are in general only employed as insert coils in an LTS outsert. Due to the tape geometry, the effect of the screening currents is high in REBCO insert coils. The resulting screening current-induced field (SCIF) can distort the shape and magnitude as well as affect the temporal stability of the field after charging. Their influence has to be well understood and it can be a limiting factor in field quality critical applications such as nuclear magnetic resonance magnets. In this work, we present the test results of a layer-wound 4 T REBCO insert coil operated in the background of a 21 T LTS outsert reaching a combined field of 25 T. Using an array of 13 calibrated Hall sensors, we investigate the spatial and temporal effects of the SCIFs at different background fields. Series expansion is employed to separate the contributions of the insert and of the outsert. We present in this work the characterization of the effect of the SCIF on the shape of the field and on its temporal stability in background fields from 0 T up to 19 T. We compare the experimental results with existing simulation models for the validation of the model at high magnetic fields, which had never been done for layer-wound REBCO coils. Based on the experimental data, we demonstrate the necessity of SCIF-aware simulations for REBCO insert coils, regardless of the insert's relative operating current. Before summarizing and concluding the work, we give a short overview of proposed methods for SCIF reduction in REBCO coils from the literature.

EuCARD-2 aims to research ReBCO superconducting magnets for future accelerator applications. The properties of ReBCO conductors are very different from low-temperature superconductors. To investigate dynamic field quality, stability, and normal zone propagation, an electrical network model for coated conductor cables was developed. To validate the model, two identical samples were prepared at CERN, after which measurements were taken at the University of Twente and Southampton University. The model predicts that for a Roebel cable, in a changing magnetic field applied in the perpendicular direction, hysteresis loss is much larger than coupling loss. In the case of a changing magnetic field applied parallel to the cable, coupling loss is dominant. In the first case, the experiment is in good agreement with the model. In the second case, the data can only be compared qualitatively because the calibration for the inductive measurement is not available.

The European Organization for Nuclear Research (CERN) is developing a six-around-one conductor on round core (CORC)-strand-based cable-in-conduit conductor (CICC) for use in detector and other large magnet systems. The CICC comprises six ReBCO-CORC strands helically wound around a central tube or rod and inserted in a square aluminum jacket. A major design challenge is finding a simple yet low-resistive method of injecting current homogeneously into the CORC strands of the CICC. In the production of joints for single-CORC cables, we are currently pursuing a method in which the different ReBCO layers at both ends of the CORC cable are trimmed into a staircase-like geometry. A similar trimming method is developed for joint terminals for the ReBCO-CORC-based CICC. A demonstration joint terminal is made to test the various steps of the trimming and manufacturing process before fabricating a joint terminal with real CORC strands. This paper presents an overview of CIC-joint terminal design, simulation results, and the different steps in the manufacturing process.

Roebel cable with second-generation YBCO strands is one of the promising high-temperature superconducting solutions of fully transposed high current conductors for high field accelerator magnets. Following the considerable research effort on the manufacturing of Roebel cables in recent years, sample conductors are now available in useful lengths with reproducible performances to allow for detailed characterizations beyond the standard critical current measurements. The ac loss and strands coupling are of significant interest for the field quality of the accelerator magnets. We report a set of systematic ac loss measurements on two different Roebel cable samples prepared for the EuCARD-2 collaboration. The measurements were performed over a wide range of temperature values between 5 and 90 K, and the results were analyzed in the context of strands architecture and coupling. The results show that the transposed bundles are partially decoupled, and the strands in transposition sections behave as an isolated single tape if the strands are insulated.

An extensive characterization of commercially available High-Temperature Superconducting (HTS) REBCO tapes has been recently performed at KIT. The main thermo-physical properties of the tapes have been measured, and heat slug and quench propagation have been investigated in vacuum at LN 2 temperature, using a resistive heater as driver and recording the voltage and temperature evolution after the pulse at several locations along the tapes. In this paper, we present a study of thermal disturbance propagation in a HTS tape with brass stabilizer. The experimental data are analyzed first, to identify the phenomena that influence heat propagation in the tape, and namely the heat loss to the sample holder and the non-ideal efficiency of the resistive heater. A numerical tool is then developed, which solves the 1D transient heat conduction equation in each layer of the tape and accounts for the thermal coupling between layers. The heat loss to the sample holder and the non-ideal efficiency of the resistive heater are taken into account in the model. A first validation of the thermal part of the model against an extended database of heat slug propagation tests is then performed: the comparison between simulation and experiment confirms the very good capability of the model to reproduce the measured temperature evolution. Finally, the results of the simulations of quench propagation are compared with experimental data, showing the capability of the model to reproduce the experiment, within the uncertainty in the input parameters.

RE1Ba2Cu3O7-≏(RE: Rare Earth, Y, Gd and so on, REBCO) superconducting tapes have high performance in critical current density, Jc , property even in high magnetic field and at liquid nitrogen temperature. We have proposed the ac loss reduction method of REBCO superconducting tape, which are the combination of a laser–scribing into a multifilamentary structure and a special winding process. Using the ac loss reduction technique, we could realize REBCO superconducting transformers with the advantage of light weight, compactness and high efficiency in comparison to conventional one. Further we aim to add a current limiting function to REBCO superconducting transformers. Recently we manufactured a 10kVA four-winding transformer with REBCO tapes. We carried out a sudden short circuit test and investigated the response against the fault excess current. We developed a numerical analysis program so as to quantitatively simulate the phenomena taking into account of a flux-flow resistance. As a result, it was shown that the whole of the REBCO winding did not shift to normal state even though the current exceeded the critical current all over the length. In this paper, we report the outlines.

Rare-earth-barium-copper-oxide (REBCO) tapes are now available from several industrial manufacturers and are very promising conductors in high field applications. Due to diverging materials and deposition processes, these manufacturers' tapes can be expected to differ in their electro-mechanical and mechanical properties. For magnets designers, these are together with the conductors' in-field critical current performance of the highest importance in choosing a suitable conductor. In this work, the strain and stress dependence of the current carrying capabilities as well as the stress and strain correlation are investigated for commercial coated conductors from Bruker HTS, Fujikura, SuNAM, SuperOx and SuperPower at 77 K, self-field and 4.2 K, 19 T.

Conductor on Round Core (CORC�) technology has achieved a long sought-after benchmark by enabling the production of round, multifilament, (RE)Ba2Ca3O7-x coated conductors with practical current densities for use in magnets and power applications. Recent progress, including the demonstration of engineering current density beyond 300 Amm-2 at 4.2 K and 20 T, indicates that CORC� cables are a viable conductor for next generation high field magnets. Tapes with 30 μm substrate thickness and tape widths down to 2 mm have improved the capabilities of CORC� technology by allowing the production of CORC� wires as thin as 3 mm in diameter with the potential to enhance the engineering current density further. An important benefit of the thin CORC� wires is their improved flexibility compared to thicker (7-8 mm diameter) CORC� cables. Critical current measurements were carried out on tapes extracted from CORC� wires made using 2 and 3 mm wide tape after bending the wires to various diameters from 10 to 3.5 cm. These thin wires are highly flexible and retain close to 90% of their original critical current even after bending to a diameter of 3.5 cm. A small 5-turn solenoid was constructed and measured as a function of applied magnetic field, exhibiting an engineering current density of 233 Amm-2 at 4.2 K and 10 T. CORC� wires thus form an attractive solution for applications between 4.2 and 77 K, including high-field magnets that require high current densities with small bending diameters, benefiting from a ready-to-use form (similar to NbTi and contrary to Nb3Sn wires) that does not require additional processing following coil construction.

Early quench detection and thermal stability of superconducting coils are of great relevance for practical applications. Magnets made with second generation high temperature superconducting (2G-HTS) tapes present low quench propagation velocities and therefore slow voltage development and high local temperature rises, which may cause irreversible damage. Since quench propagation depends on the anisotropy of the thermal conductivity, this may be used to achieve an improvement of the thermal stability and robustness of 2G-HTS coils. On pancake type coils, the thermal conductivity along the tapes (coil's azimuthal direction) is mostly fixed by the 2G-HTS tape characteristics, so that the reduction of anisotropy relies on the improvement of the radial thermal conductivity, which depends on the used materials between superconducting tapes, as well as on the winding and impregnation processes. In this contribution, we have explored two possibilities for such anisotropy reduction: by using anodised aluminium or stainless steel tapes co-wound with the 2G-HTS tapes. For all the analysed coils, critical current distribution, minimum quench energy values and both tangential and radial quench propagation velocities at different temperatures and currents are reported and compared with the results of similar coils co-wound with polyimide (Kapton®) tapes., This work was supported by the Spanish Ministerio de Economía y Competitividad and the European FEDER Program (Projects MAT2011-22719 and ENE-2014-52105-R), and by the Gobierno de Aragón (research group T12).

In this paper, we reported design, fabrication and test of a prototype 100-kA-class high-temperature superconducting (HTS) conductor, especially for joint section, to be used for segmented HTS helical coils in the FFHR-d1 heliotron-type fusion reactor. The conductor has a geometry of three rows and fourteen layers of Gadolinium Barium Copper Oxide HTS (GdBCO) tapes embedded in copper and stainless steel jackets and has a joint section with bridge-type mechanical lap joint. We introduced improved method to fabricate the joint based on pilot experiments and we were able to apply a current of ∼ 120 kA at 4.2 K, 0.45 T to the sample without quench at joint. The obtained joint resistance was ∼ 2 nΩ, which was lower than our previous data. Though joint resistance increased with a rise in current and magnetic field, predicted joint resistance in the environment of actual helical coil in the FFHR-d1 was small enough to properly run the cryoplant of the reactor.

A flywheel energy storage system (FESS) is a promising electrical storage system that moderates fluctuation of electrical power from renewable energy sources. The FESS can charge and discharge the surplus electrical power repetitively with the rotating energy. Particularly, the FESS that utilizes a high temperature superconducting magnetic bearing (HTS bearing) is lower loss than conventional FESS that has mechanical bearing, and has property of longer life operation than secondary batteries. The HTS bearing consists of a HTS bulk and double-pancake coils used 2nd generation REBCO wires. In the development, the HTS double-pancake coils were fabricated and were provided for a levitation test to verify the possibility of the HTS bearing. We successfully confirmed the magnetic field was achieved to design value, and levitation force in the configuration of one YBCO bulk and five double pan-cake coils was obtained to a satisfactory force of 39.2 kN (4 tons).

We studied the relationship between microstructure and flux pinning properties of REBCO films fabricated by a MOD technique using metal-naphthenate and 2-ethylhexanate (2-EH) solutions. 2-EH-(Sm0.3, Ho0.7) Ba2Cu3Oy film achieved high J c of 2.19 MA/cm2 at 77 K due to introduction of effective pinning sites. GdBa2Cu3Oy film prepared using complex solutions (CS) obtained high J c of 30 MA/cm2 at 4.2 K in spite of having low J c at 77 K. Anomalous temperature dependence of Jc was also observed for CS-GdBa2Cu3Oy film. It is speculated that lower- T c regions act as strong pinning centers in the CS-GdBa2Cu3Oy film by comparison analysis with flux pinning for NbTi with artificial pins.

EuBa 2 Cu 3 O 7- δ (Eu123) and EuBa 2 Cu 4 O 8 (Eu124) films oriented in c -axis were deposited on SrTiO 3 (100) substrates with eutectic NaOH-KOH flux at 450 °C. Synthesized phase has changed by using various types of barium source materials. Pure Eu124 films showed superconducting transition at ∼70 K, zero-resistance was not observed for Eu123/124 two-phase films. One of the possible reasons of this is Eu/Ba substitution of Eu123 phase. According to T c of the Eu124, the molten hydroxide method enables to deposit high-quality Eu124 films.

RE1Ba2Cu3O7-x (REBCO, RE=rare earth) coated conductors have attracted the attention of many scientists due to the high superconducting properties, etc. Due to the great progress of R&D on coated conductors, the activities have shifted to specific applications. The new Japanese national project, named as “Development of Fundamental Technologies for HTS Coils”, started last year aiming for the applications of MRI and medical accelerators. To meet the requirements of those applications, the R&D of REBCO coated conductors (CCs) are being performed with that of coiling technology in parallel. The theme has two sub-themes of “development of long CCs with high in-field performance” and “development of CCs with extremely low heat generation” in this national project. The ambitious goals of these themes are set as intermediate ones due 2016 Mar; the theme of in-field: 100m-500A/cm-w@65K, 3T -850A/cm-w@35K, 10T, the theme of low heat generation: 100m-500 μm of filament width w/ Icfilament inhomogeneity of distribution within 10% and joint resistance below 5nΩ. The final goals of these themes due 2018 Mar. were set as follows; the theme of in-field: 200m-600A/cm-w@65K, 3T -1000A/cm-w@35K, 10T, the theme of low heat generation: 200m-500 μm of filament width w/ Icfilament distribution within 5% and joint resistance below 3nΩ. The objectives, goals and progress of this project will be reviewed in this article. As for the progress, for example, the combination of EuBCO+BHO was found to be effective to improve the in-field performance of PLD derived coated conductors. A 94 m long wire with Icmin of 108A/cm-w@77K, 3T (corresponding to 472A/cm-w @65K, 3T) was successfully fabricated. It is as twice Icmin of the long tape as the highest one in the previous project.

Measurementsof the quasi-adiabatic normal zone propagation velocity and quench energies of a Superpower SCS4050 copper stabilised ReBCO superconducting tape are presented over a temperature range of 23 − 47K; in parallel applied magnetic fields of 6, 10 and 14 T; and over a current range from 50% to 100% of Ic. The data are compared to results of analytic predictions and to one-dimensional numerical simulations. The availability of long lengths of ReBCO coated conductor makes the material interesting for many HTS applications operating well below the boiling point of liquid nitrogen, such as magnets and motors. One of the main issues in the design of such devices is quench detection and protection. At higher temperatures, the quench velocities in these materials are known to be about two orders of magnitude lower compared to low temperature superconductors, resulting in significantly smaller normal zones and the risk of higher peak temperatures. To investigate whether the same also holds for lower temperatures more extended data sets are needed, both as input and as validation for numerical design tools.