REBCO Wires Research Articles

Reduction of Screening Current-Induced Field in a REBCO Coil Wound by Solder-Square-Stacked Wire

REBa 2 Ca 3 O 7-x (REBCO, RE = rare earth) coated conductors are promising conductors for high magnetic field magnet applications. For REBCO magnets, the tape structures and their superconductive characteristic cause a persistent screening current flowing in the tapes. Due to the existence of the screening current induced field (SCIF), the overall magnetic field becomes unstable and shows a temporal drift. This problem limits the application of REBCO in high-precision magnets, such as MRI and NMR. In this paper, the soldered-stacked-square (3S) REBCO wire is proposed to reduce SCIF. To confirm the reduction effect, a solenoid coil and a benchmark coil were wound by the 3S wire and the commercial 5-mm-wide REBCO tape, respectively. The screening current distributions of 3S wire and 5-mm tape were analyzed. Then, the temporal drift of the overall magnetic field and SCIF hysteresis loop were both experimentally measured for comparison. The test results showed that the 3S wire can significantly reduce the negative effects of SCIF.

A Tear-Drop Bifilar Sample Holder for Full Excitation and Stability Studies of HTS Cables at 4.2 K Using a Superconducting Transformer

HTS cables proposed for next-generation main, hybrid, and insert magnets will have very high I c (>10 kA) at 14 T and above. Determining the quench, current-sharing, and other properties of these cables will require measurement systems, which will likely incorporate a superconducting transformer. The utilization of high-field persistent mode solenoids will make size and cost less prohibitive, allowing a higher frequency of measurements within a larger number of research facilities. Additionally, a system designed for use with solenoids will allow for experiments at higher maximum fields than those achievable using dipoles and split coils. Proposed in this document is a bifilar sample probe, which fits within a 77-mm bore solenoid capable of measuring up to a 6-mm outer diameter conductor-on-round-core REBCO cables or wires up to 20 kA in transverse fields at 12 T and 4.2 K. Splices and mechanical considerations will be discussed.

Optimum Copper Stabilizer Thickness for Symmetric Tape Round (STAR) REBCO Wires With Superior Mechanical Properties for Accelerator Magnet Applications

Symmetric Tape Round (STAR) wires are only 1.6-1.9 mm in diameter and are made by winding 2.5-mm-wide REBCO tapes on an AWG 18 (1.02-mm diameter) copper wire former. Such ultra-small diameter wires are possible by the use of symmetric tapes where the REBCO film is positioned at the geometric center, close to the neutral axis. Such symmetric tape architecture is achieved by a strategic selection of the thickness of the copper stabilizer to match the thickness of the Hastelloy substrate. The substrate itself is made thin, to enable good tolerance to bend strain. The optimized STAR wire architecture is found to be mechanically robust and retain nearly 98% of its critical current of over 500 A at 77 K, self-field even when bent to a small radius of 15 mm. The superior bend strain tolerance of STAR REBCO wires make them suitable for use in canted cosine theta coils for future compact accelerator magnet applications.

Modeling-Driven Optimization of Mechanically Robust REBCO Tapes and Wires

The bending properties of RE-Ba-Cu-O (REBCO, RE = rare earth) coated conductors are a stringent constraint for coil and cable applications. While the use of thinner substrates reduces the stresses/strains in the REBCO layer, it is also possible to enhance the mechanical properties of these tapes by optimizing the stabilizer thickness in order to reduce the distance between the neutral axis and the REBCO layer. In this paper, we present both experimental and analytical results showing a significant improvement in the critical current (I c ) retention at small bend diameters (less than 2 mm) for tapes with various substrate thicknesses by using an optimized copper thickness on the REBCO side only. The presented analytical method accounts for the neutral axis shift caused by the progressive plastic deformation in the various layers (Hastelloy, silver, and copper). The Ic dependence of different substrate tapes on the thickness of single-sided electroplated copper stabilizer was measured at bending diameters ranging from 0.81 to 12.5 mm by means of a single-turn helical winding on a cylindrical former. The strains were then calculated and an optimized copper thickness is determined for the various configurations. Based on these results, an optimal design for ultra-small diameter REBCO wires is also presented.

International round robin test for tensile testing HTS wires at cryogenic temperatures

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.

Superior critical current of Symmetric Tape Round (STAR) REBCO wires in ultra-high background fields up to 31.2 T

Symmetric Tape Round (STAR) wire fabricated with an ultra-thin RE–Ba–Cu–O (REBCO, RE = rare earth) coated conductor has a high engineering current density (Je) and remarkable flexibility. It is a competitive candidate for future canted cosine theta magnets with smaller winding radii that operate at fields above 20 T at 4.2 K. STAR wires consisting of six or eight REBCO coated conductor layers have been fabricated with an outer diameter smaller than 2 mm. We performed in-field tests of the critical current of STAR wires at 4.2 K in background fields up to 31.2 T. At a 15 mm bending radius, Je values of 438 A mm−2 at 20 T and 299 A mm−2 at 31.2 T were obtained. These are the highest Je values measured in any reported round-geometry REBCO wires at such ultra-high background magnetic fields. This exhibits the potential of STAR wires in future compact accelerator magnets.

Superconducting joint of REBCO wires for MRI magnet

A high temperature superconducting wire (HTS wire) is promising for various superconducting magnet applications because it operates at a higher temperature than a low temperature superconducting wire (LTS wire) is. Particularly, a MRI magnet using the HTS wires (HTS-MRI magnet) is expected to obtain light-weight, compact and low operating cost compared to a LTS-MRI magnet. The MRI magnets are generally operated by a persistent current mode. A magnet of the persistent current mode consists of multiple superconducting magnets and persistent current switch (PCS) that connects with superconducting joints. However, the superconducting joint of the HTS wires has not been realized stably at this time. We have developed a superconducting joint by using a commercial REBCO wire. The HTS-MRI magnet requires that resistivity of less than 10−12 ohm and current capacity of more than 100 A is achieved by a direct-joint between superconducting layers of the two REBCO wires. Moreover, measurement equipment for low joint resistivity was prepared as measuring the decay of the magnetic field in the one-turn loop. In our R&D, the joint resistance and the critical current were achieved with 155 A and 5.3×10−13Ω.

Overall Thermal Conductance and Thermal Contact Resistance in No-Insulation REBCO Magnet

In no-insulation (NI) REBCO coils, thermal contact resistance (TCR) between winding turns has important influence on thermal stability of REBCO magnets under varying magnetic field or even in case of quench. Reduction of TCR between winding turns is one of the crucial factors to develop thermally stable magnets cooled by conduction. To avoid a delamination problem, epoxy is not usually used between winding turns and the significant TCR exists in NI magnets. Then, thin epoxy layers between coil surfaces and metallic conduction plates are used to enhance thermal conduction in conduction cooled magnets. To design thermally stable NI REBCO magnets cooled by conduction, TCR, induced by dry contacts of winding turns, should be properly evaluated. This paper describes measurements of overall thermal conductance using a model coil without conduction plates and epoxy layers from 10 to 40 K in a conduction test apparatus. TCR is evaluated using the measured overall thermal conductance and previously reported transverse thermal conductivity of a REBCO wire. In another model coil with conduction plates and epoxy layers, enhanced overall thermal conductance is measured and compared with finite element analysis results that include the measured TCR.

Symmetric tape round REBCO wire with Je (4.2 K, 15 T) beyond 450 A mm−2 at 15 mm bend radius: a viable candidate for future compact accelerator magnet applications

Round REBCO (RE = rare earth) wires of 1.6–1.85 mm diameter have been fabricated using ultrathin REBCO tapes where the superconductor film is positioned near the geometric center. Such symmetric tape round (STAR) wires exhibit excellent tolerance to bend strain with a critical current retention of more than 97% when bent to a radius of 15 mm. A 1.6 mm diameter REBCO STAR wire made with six 2.5 mm wide symmetric tapes reached an engineering current density (Je) of 454 A mm−2 at 4.2 K in a background field of 15 T at a bend radius of 15 mm. Such superior performance at a small bend radius can enable fabrication of future accelerator magnets, operating at magnetic fields above 20 T.

AC Loss Measurements in a Hybrid REBCO/BSCCO Coil Assembly

Wire cost and ac loss are paramount issues for the future uptake of high-temperature superconductor (HTS) ac machines. In particular, new approaches which reduce the total manufacturing cost of HTS coils are critical to the future commercial uptake of HTS technology. Here, we present the results of ac loss measurements of a hybrid HTS coil assembly comprising end windings (EWs) wound with BSCCO wire and a central winding (CW) wound with REBCO wire. As a cost mitigation measure, we have chosen to use REBCO wire which possesses a relatively low self-field $I_{{\rm{c}}}$ at 77 K, equating to approximately half that of the BSCCO wire. Transport ac loss measurements were made on the total coil assembly at both 77 K and 65 K, using a recently developed lock-in amplifier approach. We show the ac losses in the assembly at these temperatures scale with the overall transport critical current threshold of the hybrid coil assembly. The results are then compared with a reference coil assembly having similar geometrical dimensions, and in which both the EWs and CW were wound with identical BSCCO wire. Surprisingly, we observe that the ac loss of the hybrid coil is lower than that of the BSCCO coil, despite the CW of the hybrid coil employing wire a lower overall average $I_{{\rm{c}}}$ . We attribute this effect to the lower ac loss of coated conductor REBCO wire under parallel field, which is the dominant field orientation in the CW region. Based on this observation, we conclude that the hybrid assembly design is a good design strategy for HTS coil assemblies, enabling both wire cost and ac loss to be balanced.

Study on the Persistent Current Switch in HTS Coils Wound with 2G Wire for Compact NMR Magnets

High temperature superconducting (HTS) magnets wound with REBCO wires are used in many applications, and the superconducting magnet which is operated in persistent current mode has many advantages. Therefore, we have been developing compact NMR relaxometry devices using HTS coils operated at liquid nitrogen temperature. The required strength and homogeneity of magnetic field of proposed NMR relaxometry devices are 1.5 T and 150 ppm/cm3 respectively. The proposed HTS magnet for NMR relaxometry device consists with stacked HTS double pancake coils wound with REBCO wires and operated by persistent current mode (PCM) using superconducting joint between REBCO wires. In PCM operation, the ability of persistent current switch (PCS) is very important, so, in this study, the thermal properties of YBCO wire against the various thermal inputs by heater were investigated experimentally to design the PCS for PCM HTS magnets. The thermal behaviors of the YBCO wires were measured as functions of amount of heat input using two types of epoxy resin. The current bypassing properties on the YBCO loop coil with developed PCS were studied experimentally.

A Novel REBCO Wire Structure That Improves Coil Quality Factor in MHz Range and its Effect on Wireless Power Transfer Systems

We have developed a structure consisting of two conventional REBCO wires that increases the quality factor of coils, and have simulated its effect on to wireless power transfer (WPT) systems. The proposed structure is used to reduce the conductor loss of the Hastelloy substrate. The conductor losses of the skin effect of the wires were reduced by not using a copper stabilizer and a silver overlayer. We investigated the effect of embedding the proposed structure in styrene foam on the coil quality factor and found that the foam reduced the quality factor due to its dielectric loss. We fabricated several different wire type spiral coils embedded in styrene foam and measured their quality factor at 300 and 77 K. The quality factor of the coil using the proposed structure was about 3.2 times that of one using copper wire at 300 K. We also simulated the power transfer efficiency of a WPT system using two coils with different quality factors for different distances between the coils. Use of a high-quality factor coil enabled the system to achieve a longer transfer range.

Numerically Thermal Stability Analysis on a Geometrically Symmetrical Strand Fabricated by 2G Wires in Liquid Helium Temperature

Due to its excellent j c (B) performance at low temperatures, the second generation (2G) high-temperature superconducting (HTS) REBCO wire is promising for high-field applications. The thermal stability of a strand consisting of 2G wires operating at low temperatures is an important issue with increasing current density at high magnetic field. The paper focuses on the thermal stability of geometrically symmetrical strands under background direct current (dc) magnetic field of 6 T in liquid helium temperature by numerical method. The strand with copper sheath consists of 80 stacked 2G wires. The thermal stability of the strand is accurately analyzed based on the analytical dependence of the critical current density on temperature and magnetic field as well as its orientation to wide surface of 2G wire. By using the helium heat transfer coefficient, the minimum quench energy and the quench propagation velocity of the strand in liquid helium temperature are calculated and compared with those in the adiabatic conditions, the results are useful for application of the strand in high magnetic field at low temperature.

2G HTS wires made on 30 μm thick Hastelloy substrate

REBCO (RE = rare earth) based high temperature superconducting (HTS) wires are now being utilized for the development of electric and electromagnetic devices for various industrial, scientific and medical applications. In the last several years, the increasing efforts in using the so-called second generation (2G) HTS wires for some of the applications require a further increase in their engineering current density (Je). The applications are those typically related to high magnetic fields where the higher Je of a REBCO wire, in addition to its higher irreversibility fields and higher mechanical strength, is already a major advantage over other superconducting wires. An effective way to increase the Je is to decrease the total thickness of a wire, for which using a thinner substrate becomes an obvious and attractive approach. By using our IBAD-MOCVD (ion beam assisted deposition-metal organic chemical vapor deposition) technology we have successfully made 2G HTS wires using a Hastelloy® C276 substrate that is only 30 μm in thickness. By using this thinner substrate instead of the typical 50 μm thick substrate and with a same critical current (Ic), the Je of a wire can be increased by 30% to 45% depending on the copper stabilizer thickness. In this paper, we report the fabrication and characterization of the 2G HTS wires made on the 30 μm thick Hastelloy® C276 substrate. It was shown that with the optimization in the processing protocol, the surface of the thinner Hastelloy® C276 substrate can be readily electropolished to the quality needed for the deposition of the buffer stack. Same in the architecture as that on the standard 50 μm thick substrate, the buffer stack made on the 30 μm thick substrate showed an in-plane texture with a Δϕ of around 6.7° in the LaMnO3 cap layer. Low-temperature in-field transport measurement results suggest that the wires on the thinner substrate had achieved equivalent superconducting performance, most importantly the Ic, as those on the 50 μm thick substrate. It is expected the 2G HTS wires made on the 30 μm thick Hastelloy® C276 substrate, the thinnest and with the highest Je to date, will greatly benefit such applications as high field magnets and high current cables.

AC loss measurements in HTS coil assemblies with hybrid coil structures

Both AC loss and wire cost in coil windings are critical factors for high temperature superconductor (HTS) AC machinery applications. We present AC loss measurement results in three HTS coil assemblies at 77 K and 65 K which have a hybrid coil structure comprising one central winding (CW) and two end windings (EWs) wound with ReBCO and BSCCO wires with different self-field Ic values at 77 K. All AC loss results in the coil assemblies are hysteretic and the normalized AC losses in the coil assemblies at different temperatures can be scaled with the Ic value of the coil assemblies. The normalised results show that AC loss in a coil assembly with BSCCO CW can be reduced by using EWs wound with high Ic ReBCO wires, whilst further AC loss reduction can be achieved by replacing the BSCCO CW with ReBCO CW. The results imply that a flexible hybrid coil structure is possible which considers both AC loss and wire cost in coil assemblies.

Reversible stress and strain limits of the critical current of practical REBCO and BSCCO wires

Practical REBCO and BSCCO-2223 tape-shaped wires are now manufactured on an industrial scale. They are a typical composite material consisting of superconducting layer/filaments together with functional components. These functional components affect directly the stress and strain dependences of the critical current. When applying an external stress R, the critical current Ic was measured. Then the external stress was reduced to R = 0 and the recovered critical current Icr was again measured. The tensile stress and strain dependences of both normalized critical currents divided by the original value, Ic/Ic0 and Icr/Ic0 were investigated. In general Icr/Ic0 recovered close to unity when the applied stress was low, but its recovering level decreased gradually with increasing applied stress. The definition of the reversible stress and strain limits was investigated and its validity was proved using the cyclic loading test. The original definition of reversible stress and strain limits of critical current relates to: (1) when releasing the applied stress and strain, the Ic shall recover to the original value, and (2) when applying the cyclic stresses, the Ic shall keep the original value. Here, as a practical definition for the reversible stress and strain limits, the tensile stress and strain at 99% recovery of Ic have been proposed. On the other hand, it was made clear that the stress and strain at Ic 95% retention are not valid for use commonly as a criterion of reversible stress and strain limits for both practical REBCO and BSCCO-2223 wires.

R&D Project on HTS Magnets for Ultrahigh-Field MRI Systems

An RD $\text{RE}=\text{rare earth}$ ) wires for ultrahigh-field (UHF) magnetic resonance imaging (MRI) systems is described. Our targets are 9.4-T MRI systems for whole-body imaging and brain imaging. REBCO wires are promising components for UHF-MRI because REBCO wires have high critical current density in high magnetic fields and high strength against hoop stresses, which allows MRI magnets to be smaller and lighter than conventional ones. The aim of the project is to establish basic magnet technologies for adapting REBCO coils for UHF-MRI. The project term is three years, and this year is the final year. We have already demonstrated the generation of an 8.27-T magnetic field with a small test coil composed of 22 REBCO pancake coils. A magnetic field spatial distribution with inhomogeneity of several hundreds of parts per million within 100-mm diameter spherical volume (DSV) was demonstrated with a 1-T model magnet. A stable magnetic field of a few parts per million per hour was also demonstrated with the 1-T model magnet. The targets of the project, to be achieved by March 2016, are to demonstrate the generation of a 9.4-T field with the small REBCO coil, and to demonstrate a homogeneous magnetic field in 200-mm DSV with a 1.5-T magnet having three pairs of split coils. Imaging will be performed with the 1.5-T magnet.

Development of a Quasi-Isotropic Strand Stacked by 2G Wires

This paper presents a concept of quasi-isotropic high-temperature superconducting (HTS) strands with round and square cross sections made from REBa2Cu3O7-x (RE=rare earth, REBCO) coated conductor, the so-called second-generation (2G) HTS wires under development at North China Electric Power University. Considering several key issues for fabricating the quasi-isotropic strands, for instance, in-plane bending performance of 2G wire, isotropic-strand bending performance, strand termination method, and critical current in liquid nitrogen (LN2 ) temperature and self-field, a feasible manufacturing method by using mature laser welding is suggested and investigated experimentally. After fabricating several short-length strand samples consisting of 72 REBCO wires with an outer diameter of 7.5 mm, critical currents in straight and bending and under direct current (DC) magnetic fields with different orientations were tested in LN2 temperature. The critical current in straight strand is in agreement with the simulated one; the anisotropy of the critical current is less than 8% in the orientation range of $0^{\circ}{-}90^{\circ}$ with a DC magnetic field of 0.5 T. After successfully finishing a 10-m-length strand, it was demonstrated that the laser welding technique is an attractive and feasible method for manufacturing practical long strands which have potential applications in large-scale high-field magnet and energy transmission.

Stress–Strain Relationship, Critical Strain (Stress) and Irreversible Strain (Stress) of IBAD-MOCVD-Based 2G HTS Wires Under Uniaxial Tension

The mechanical behavior of a REBCO-coated conductor wire under uniaxial tension is largely determined by the two thickest component layers in the architecture, namely, the substrate and the stabilizer. A rather complicated stress-strain relationship is often observed when the composite conductor is under uniaxial tension, which is the result of the differences in elastic modulus and yield stress between the Hastelloy substrate and the electroplated Cu stabilizer. In this paper, the stress-strain relationships of a free standing Cu stabilizer and a bare REBCO wire, i.e., a tape without any stabilizer, were measured. The results are utilized for the calculation of the stress-strain relationships of electroplated Cu stabilized wires using a two-component composite model. Given the thicknesses of the substrate and stabilizer, the calculated results agree well with the measured stress-strain curves, which can be well fitted with the Ramberg-Osgood equation. The tensile strain (stress) dependence of the critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) was measured at 77 K in liquid nitrogen. Depending on whether the critical current was measured with the wire under a stress or after unloading, a critical strain (stress) and an irreversible strain (stress) were determined, respectively.

Experiment and Numerical Analysis on the YOROI Structure for High-Strength REBCO Coil

To achieve a high current density and a compact REBCO coil, a reinforcing structure is essential to prevent coil degradation and deformation caused by the large Lorentz force. A new reinforcing structure for the REBCO coil called the Y-based oxide superconductor and reinforcing outer integrated (YOROI) coil was proposed in the previous study. The YOROI coil, which exhibited no degradation after an excitation test with a maximum hoop stress of 1.7 GPa at 4.2 K in 8-T backup fields, has great ability to reduce the stress and strain acting on the coil winding to maintain the coil shape and prevent degradation in the REBCO wire. In this study, the hoop stress in the winding and the stress shared by the reinforcing structure of a YOROI model coil were measured at 40 K in 10-T backup fields to clarify the reinforcing mechanism of the YOROI coil. A three-dimensional structural analysis, which simulated the experimental condition, was performed on the model coil to determine the distribution of stress and strain in the winding and reinforcing structure.