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The second generation (2G) high-temperature superconducting (HTS) REBCO tape and the HTS cable made by the REBCO company are considered to be alternative materials for future superconducting magnet design due to their exceptional performance. The twisted stacked-tape slotted-core (TSSC) cable-in-conduit-conductor cable, which is one of the crucial layout structures in HTS cables, has been extensively studied by numerous research groups over the years. In this paper, a 3D finite element model of the TSSC HTS cable under bending and axial tensile loads is established using the bilinear isotropic hardening model in COMSOL commercial finite element software. The mechanical behavior of the TSSC cable under bending and axial tensile loads, as well as the evolution process of overall cable performance and critical current of individual tapes inside slots, are revealed by conducting mechanical analysis and using an empirical fitting formula between the critical current density of the 2G HTS tapes and axial strain. Furthermore, optimization engineering suggestions for its structure are provided, such as reducing the twist pitch, decreasing the tape width, increasing the number of tapes, reducing the slot width while avoiding direct contact between tapes and slot walls, increasing the number of slots, increasing diameter of diversion trench and inner diameter of helical core under predominant bending loads. In the case of axial tensile loads, the aforementioned suggestions are also applicable except for the inner and outer diameters of the helical core. The critical current performance can be enhanced by augmenting the outer diameter of the helical core in this case. However, it is almost unaffected by the inner diameter of the helical core. [ABSTRACT FROM AUTHOR]

This paper presents a novel two-step procedure for measuring the critical current (I c) properties as a function of tensile stress. The proposed method completely eliminates the possible negative effect of the voltage tap used during the tensile procedure, allowing the actual I c irreversible degradation stress to be clearly determined. Six different commercial REBCO tapes from five manufacturers were tested. The I c value does not degrade until the stress reaches the 'inflection area' in the tensile curve, which corresponds to the most pronounced transformation step from elastic to plastic deformation. This allows easy estimation of the stress corresponding to I c irreversible degradation by a simple and accurate cryogenic tensile curve instead of complicated in-situ I c tensile measurement. A feasible composite-material tensile model is established to explain the phenomenon. In addition, fatigue measurements on a commercial REBCO tape from Shanghai Superconductor technology show that the tape can withstand 10 000 cycles under 580 MPa and over 5000 cycles under 695 MPa, which also is consistent with the proposed composite-materials tensile model and is confirmed by the EBSD experiments on the Hastelloy substrate. This research provides better insight and tools for designing and fabricating extremely high-field magnets. [ABSTRACT FROM AUTHOR]

The high aspect ratio of REBCO tapes has a significant impact on several characteristics in high-temperature superconducting applications, such as critical current and AC loss. Narrow filamentary technology can effectively reduce the impact of magnetic field dependence and enhance the electromagnetic performance of REBCO tapes. However, the existing methods are constrained by the trade-off between the narrow degree of REBCO filaments and high current capacity. Meanwhile, when processing REBCO tapes for large-scale magnets, there is a great possibility that local defects are lurking. A striated narrow-stacked (NS) structure is proposed based on the existing narrowing methods to address these challenges. To verify the validity of this structure, it is imperative to explore the non-uniform critical current and stacking effect on performance for multi-filament REBCO tapes with potential defects. This article introduces a magnetic extended network (MEN) model to analyze the electrical characteristics of striated NS structures with different types of potential defects. Then, by coupling with a 3D finite element method electromagnetic module, the calculation results of the MEN model are visualized and used to analyze the electromagnetic characteristics including current sharing mechanism, magnetic field distribution, and critical current compensation due to stacking effect. It is found that stack structures successfully provide the performance remedy for multi-filament REBCO tapes with potential defects. This study aims to promote the narrowing improvement of REBCO tapes in high-field magnets and high-current applications. [ABSTRACT FROM AUTHOR]

The VEGA-3 laser system at the Centro de Láseres Pulsados delivers laser pulses up to 1 PW at 1 Hz repetition rate, focused to intensities up to 2.5 × 10 20 W cm−2. A versatile and compact solid target suitable for up to 0.05 Hz is presented which can operate in the challenging petawatt laser environment close to the laser-plasma interaction. Strips are spooled in a tape target system to deliver a solid density target to the laser focus for every shot. Results are presented for different tape materials and thicknesses, and a shot-to-shot standard deviation of the cut-off energy of 1.5% was obtained for Kapton reinforced aluminium tapes of 9 µ m thickness. Experimental ion spectra are recorded by a Thomson Parabola Ion Spectrometer coupled with a scintillator screen; and an antenna array is used for the characterization of electromagnetic pulses. The results of both diagnostics show a good shot-to-shot stability of the system. [ABSTRACT FROM AUTHOR]

More than a century after the discovery of superconductors (SCs), numerous studies have been accomplished to take advantage of SCs in physics, power engineering, quantum computing, electronics, communications, aviation, healthcare, and defence-related applications. However, there are still challenges that hinder the full-scale commercialization of SCs, such as the high cost of superconducting wires/tapes, technical issues related to AC losses, the structure of superconducting devices, the complexity and high cost of the cooling systems, the critical temperature, and manufacturing-related issues. In the current century, massive advancements have been achieved in artificial intelligence (AI) techniques by offering disruptive solutions to handle engineering problems. Consequently, AI techniques can be implemented to tackle those challenges facing superconductivity and act as a shortcut towards the full commercialization of SCs and their applications. AI approaches are capable of providing fast, efficient, and accurate solutions for technical, manufacturing, and economic problems with a high level of complexity and nonlinearity in the field of superconductivity. In this paper, the concept of AI and the widely used algorithms are first given. Then a critical topical review is presented for those conducted studies that used AI methods for improvement, design, condition monitoring, fault detection and location of superconducting apparatuses in large-scale power applications, as well as the prediction of critical temperature and the structure of new SCs, and any other related applications. This topical review is presented in three main categories: AI for large-scale superconducting applications, AI for superconducting materials, and AI for the physics of SCs. In addition, the challenges of applying AI techniques to the superconductivity and its applications are given. Finally, future trends on how to integrate AI techniques with superconductivity towards commercialization are discussed. [ABSTRACT FROM AUTHOR]

This paper reports a diagnostic method for clarifying performance bottlenecks in superconducting wires and tapes for their further performance enhancements. In particular, our achievements by scanning Hall-probe microscopy (SHPM), which worked well for selecting positions for microstructure observation by SEM, TEM, and x-ray CT, are introduced. This hybrid microscopy offers the information of the direct relationship between the performance of a practical-scale superconducting wire or tape and its origin in micro-scale or nano-scale structure. As such examples, characterization results for an MgB2 multi-filamentary wire, a commercially available long RE-123 coated conductor, and a striated multi-filamentary coated conductor are reported in this paper through reviewing our past studies. [ABSTRACT FROM AUTHOR]

The repeatability of superconducting DC limiters has not been introduced by other literature. Understanding the performance degradation characteristics of high-temperature superconducting (HTS) tapes under the overcurrent is of great significance for popularizing the application of a resistive-type DC superconducting fault current limiter. In this paper, the threshold experiment and repeated DC overcurrent experiment of HTS tape were carried out. With the critical current decay of the tapes under repeated DC overcurrent recorded, the current threshold under the DC overcurrent was obtained. And the performance degradation characteristics of the tapes under different repeated overcurrents were analysed, the resistance characteristics of the samples during different currents were compared. The results show that the DC overcurrent threshold of the copper-encapsulated second-generation (2G) YBCO tapes samples of Suzhou Advanced Materials Research Institute is 2.3/4 ms/3700 A. Under the repeated overcurrent, the performance degradation of the samples is abrupt, and the critical value is between 2.3/4 ms/3500 A and 2.3/4 ms/3550 A. The overcurrent has a limited influence on the magnitude of the resistance and will not affect the change trend of the resistance. The resistance at the falling edge of the current was greater than that at the rising edge of the current. The simulation was carried out by using COMSOL Multiphysics, and the simulation results matched the experimental results. The life prediction model of the samples was established and it was found that there is a custom equation relationship between the reliability of superconducting tapes and the number of the overcurrent, and the performance decline of this kind of samples is catastrophic. [ABSTRACT FROM AUTHOR]

This paper studies the influence of the thermal disturbance process on the electro-thermo-mechanical properties of quasi-isotropic strand (Q-IS) in self-field. We first measured the heat transfer time as well as the longitudinal temperature distribution through experiments and compared them with simulated results. Subsequently, through the finite element analysis, the transversal distribution of the temperature in the center of the strand and the attenuation of the critical current during the thermal disturbance are obtained. Finally, with considering the uniformity of the temperature distribution, the internal stress of the strand under different operating currents was studied. The results show that compared to the strand formed by directly stacked tapes, the Q-IS has advantages of uniform temperature distribution, less attenuation of critical current, and lower amplitude of internal stress in the case of higher current. [ABSTRACT FROM AUTHOR]

Cable-in-conduit conductors comprised of twisted stacks of high-temperature superconducting (HTS) tapes constitute a very promising technology by virtue of their easy manufacturing process, flexibility capabilities, and high current densities. In a cable, the current distribution among tapes is one of the key parameters affecting the cable performances. The distribution of current is affected mainly by the self-field configuration (ultimately related to the cable layout) and the termination resistances. In this paper we present a 2D finite element (FE) model, based on the T-A formulation, which computes the magnetic field and current distribution in stacked tapes. This model has been used to describe the experimental V–I results obtained in cables in which different current distributions among tapes are expected. The first case refers to V–I curves of stacks of HTS tapes inserted into ducts formed in the extruded aluminium cylindrical core for a straight cable. The excellent agreement between the experimental findings and the simulation results can be explained in terms of uniform current distribution within the tapes stack, up to the superconducting to normal transition. The second sample, an Al-slotted core Cable-In-Conduit-Conductor, has been bent down to a radius of 0.15 m, and from the measured V–I characteristic of each individual tape, a different tape degradation depending on the tape position within the stack was recorded. The model is able to reconstruct the V–I of the stacks from the characteristic curves of the individual tapes with a satisfactory agreement. The finite element analysis reveals non-uniform current distribution among the tapes, which could expose the cable to a potentially irreversible damage during operation. The proposed FE model constitutes a useful tool for the analysis and predictions of HTS CIC conductor performances and represents a suitable basis for the implementation of more complex models aimed at the design of specific and large applications of this conductor in the next future. [ABSTRACT FROM AUTHOR]

Previous studies of test coils have demonstrated the high thermal and electrical stability of no-insulation (NI) high temperature superconducting (HTS) coils thanks to the presence of turn-to-turn current paths. These turn-to-turn current paths in a NI coil are significantly influenced by the contact resistivity. In practice, it is very challenging to measure the contact resistivity of a NI coil by direct experiments of short samples, since the contact resistivity of superconducting tapes is influenced by surface roughness and tolerance, stress, temperature etc. A proper simulation model is needed to investigate the contact resistivity of the NI coils with dedicated experiments. Hence, in this paper a distributed circuit model is employed. This model, implemented in Matlab 2018a, considers the local contact resistivity, self and mutual inductance, and HTS resistance, which depends on the supplied current, magnetic field and temperature. To validate the model, experimental results from literature, including sudden discharge, and charge–discharge processes, are employed and the results from simulations are consistent with experimental results. Then the model is used to investigate the equivalent contact resistivity of a 157-turn NI coil. Through the comparison of simulated and experimental results, it is found that the contact resistivity of the NI coil has an inhomogeneous distribution. When the current changes with different speeds, ramping rates or frequency, a different number of turn-to-turn contacts carries radial current. Since the turn-to-turn contacts have different contact resistivity, the equivalent contact resistivity calculated from sudden discharge cannot be used in simulations to reproduce all the experimental data. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms that limit the current transport in granular iron-based superconductors is necessary to unlock their potential for magnet applications. This paper reports the transport critical current surface IC(T,H) of powder-in-tube Ag/Ba1−xKxFe2As2 tapes densified by flat rolling and uniaxial hot pressing. We investigate the amplitude of the isothermal-IC(H)-curve magnetic hysteresis, which is a marker for the electrical connectivity between superconducting grains. This allows us to demonstrate that, in spite of an improved connectivity, the percolative path through weakly linked grains continues to play a role in the IC properties of hot-pressed conductors. The limitations to the current transport due to weak links depend on the orientation of the magnetic field and are more severe when H is on the tape plane. This finding provides a justification to the puzzling evidence that IC(H) can be smaller for magnetic fields applied on the tape plane and not perpendicular to it. Finally, we report an unexpected increase of IC upon augmenting the magnetic field from 0 to ∼2 T. SQUID measurements reveal a superparamagnetic-like magnetic background that takes place in the same range of magnetic fields. A microscopic study of the conductor-core elemental composition provides evidences that these features can be both related to the presence of Fe-rich non-stoichiometric regions in the superconducting core. On a more general note, this study points out that further advances in the in-field current transport properties of powder-in-tube Ag/Ba1−xKxFe2As2 conductors are possible and rely to a large extent on solutions to improve the inter-grain connectivity and the quality of the superconducting phase. [ABSTRACT FROM AUTHOR]

This work proposes two alternative analytical models to evaluate the ac losses of high-temperature superconducting (HTS) tapes during their hysteretic and resistive modes. These models intend to extend the application range of state-of-the-art analytical models for current values higher than the critical one, i.e. for the resistive state, and to correctly predict the ac losses during the transition between the hysteretic and resistive modes. Two analytical models are proposed, one considering an extension of the Norris model for the HTS tape's resistive mode and the other based on a sigmoid function to characterize the hysteretic losses and their smooth transition to the resistive mode. Analytical models capable of estimating ac losses of superconducting (SC) tapes are an important tool for the design of complex SC systems, such as SC fault current limiters, SC electrical machines and SC cables. The proposed models are validated experimentally, for a 1st generation BSCCO tape and a 2nd generation REBCO tape. Finite element simulation is also carried out to verify the accuracy of the proposed models. Results show that the proposed extended-Norris model presents some deviation at the transition between the hysteretic and resistive modes, while the sigmoid model presents very accurate results for the whole spectrum of applied current. Also, the parameters of the sigmoid models are independent of the tape geometry. [ABSTRACT FROM AUTHOR]

A superconducting joint for multi-filamentary Bi2Sr2Ca2Cu3O10+ δ (Bi2223) tape, which has applications in nuclear magnetic resonance and magnetic resonance imaging, has been developed based on incongruent melting and multi-junction methods. In this study, the critical current limit, optimal oxygen annealing conditions, and reproducibility of this joint are experimentally evaluated. The results indicate that the critical current limit of the joint at 77 K is determined by that of the tape. The critical current limit was estimated to be about 40 A. Additional oxygen annealing after heat treatment below 1143 K. appears to have no effect on the critical current, and that above 1153 K slightly increases the critical current. It is found that the multi-junction method effectively increases the reproducibility of obtaining a high critical current. To obtain a high critical current at 77 K, the highest temperature in the joint creation process should be lowered to near the incongruent melting temperature of Bi2223 (about 1133 K in air). Two Bi2223 coils, each with one joint (four junctions), were prepared and their persistent current at 77 K was tested. A persistent current of above 20 A was measured with a resistance of about 10 pΩ. [ABSTRACT FROM AUTHOR]

Significant development has been made toward guaranteeing the performance of high-temperature superconducting (RE)Ba2Cu3O7âˆ' x coated conductor (CC) tapes in superconducting devices such as high-field magnets and coils. To understand the superconducting behaviors of CC tapes used in such devices under various mechanical- and thermal-induced loads, their mechanical and electromechanical properties should be evaluated in consideration of their application environments. Under static or cyclic fatigue loads, critical current (I c) can degrade as a result of damage to the superconducting layer, even under loads that do not exceed the irreversible stress limits for I c degradation (σ irr). Therefore, prediction of the stress level that can degrade I c under various conditions, such as the endurance limit (stress), is significant for actual coil or magnet applications of CC tapes. A static fatigue tester for CC tapes at 77 K was used in this study to apply a static fatigue load to a 12 mm wide GdBa2Cu3O7âˆ' x CC tape specimen under simultaneous axial tension and bending stresses in a U-shaped configuration. Bending mandrels were used to superimpose various bending strains onto the applied static axial tensile strain, and I c across various voltage tap separations was measured over time, up to 100 h. The electrical static fatigue strength and endurance limit for I c degradation were determined based on the 95% I c retention criterion and 100 h of elapsed time, respectively. Results show that bending strain, dependent on mandrel diameter, can greatly influence I c degradation behaviors and that I c can drop considerably in the bent sections compared to the straight sections over time. Analyses of the combined strains in the bent sections allowed the prediction of diameter-dependent electrical static endurance limits under subcritical crack growth. The CC tape’s electrical static endurance limit was greatly affected at smaller bending diameters. When CC coils with diameters smaller than or equal to 50 mm are made using 12 mm wide CC tapes, the electrical static endurance limit is low, roughly â©˝0.63 σ irr. [ABSTRACT FROM AUTHOR]

In the critical state model, the critical current, , of a superconductor defines the upper limit of dissipation-free current flow. However conventional transport measurements of practical superconductors rely on the detection of a voltage drop along the length of the conductor. This requires that the superconductor has entered the dissipative regime, and hence inherently over-estimates the current at which geometric saturation occurs. Nonetheless, the convenience of the 1 µV cm−1 criterion means that it has become the widely adopted definition of transport. Here, we present an alternative definition for the transport critical current of a superconducting tape under self-field conditions, which is based on the concept of geometric current saturation across the full cross-sectional area. This saturation threshold can be experimentally determined through simple Hall sensor measurements of the evolving perpendicular magnetic field at the tape surface. The surface field exhibits a signature transition when the transport current increases beyond the current-saturation threshold. We present an analytical model which describes this effect and defines the critical saturated current as a function of. This definition is first validated using finite element (FE) modelling, and then experimentally demonstrated through measurements on a variety of commercial REBCO and Bi-2223 tapes of differing widths. It is found that the 1 µV cm−1 criterion consistently leads to an overestimation of the saturated critical current by ∼15% in REBCO tapes, and up to 30% in a Bi-2223 tape. FE modelling indicates that this overestimate is most prominent in superconductors exhibiting a low n-value (i.e. n ≲ 20). A key advantage of the measurement approach presented here is that it allows the unambiguous measurement of a transport value to be completed at lower currents, without entering the dissipative region in which sample damage can occur. There are also implications as to the correct choice of value which should be employed within the well-known Norris and Brandt equations for AC loss. [ABSTRACT FROM AUTHOR]

Aiming to address the computational difficulties caused by the large width ratio of superconducting tapes, especially in calculating the AC losses of 3D superconducting coils, this paper proposes to study the AC loss laws of a 3D model of racetrack superconducting coil using the T–A equation group model. Firstly, the critical current of the superconducting coil is estimated based on the structure of the coil and the technical parameters of superconducting materials used in high-temperature superconducting (HTS) motors. The transmission AC losses of the superconducting coil under different n values, different transmission currents, different frequencies and different coil structure sizes are studied using T–A equations. Finally, the critical current test system and AC loss test system for HTS tapes and coils are set up. The critical current test and AC loss test for HTS tapes and coils are carried out, and the results are compared with those calculated by finite element method. The test results show the reliability of the T–A mathematical model, which can be used as a pre-valuation in engineering applications. [ABSTRACT FROM AUTHOR]

Flux pumps have been investigated for a number of years as a means of powering large magnets. In this paper a simple system consisting of one, or two, loops under a moving magnet is analysed theoretically. It is a circuit model using only Faraday’s law of induction and the V–I characteristic. However in contrast to distributed systems with tapes or sheets, the performance can be calculated from first principles and the results expressed in a few simple formulae. With one loop and a constant Ic no significant flux pumping occurs if the n value is large. However if Ic is a function of B the saturation current is approximately Ic1–Ic2 where these are the critical currents in zero field and under the magnet respectively. Nearly all the magnet flux is transferred in each cycle so the number of cycles needed to reach saturation is about 2(Ic1–Ic2) L2/ϕ where L2 is the inductance of the load magnet and ϕ the flux of the moving magnet. The loss is ϕIc joules on each pass and the mean loss about ϕIc/T watts, where T is the time of a pass. Adding another loop makes an even better flux pump even if Ic is constant. Now the saturation current is Ic and the number of cycles about 2IcL2/ϕ. If the current starts above the saturation value it decreases to saturation over a similar number of cycles, more of a ‘flux suck’ than a flux pump. [ABSTRACT FROM AUTHOR]

Several new conductor designs for fusion applications, based on high temperature superconducting (HTS) materials, have been recently proposed worldwide. Most of them are based on the twisted-stacked-tape cable (TSTC) idea: a stack of HTS tapes is twisted, soldered and embedded in a copper tube, constituting what we identify here as a 'strand'. Few strands are then twisted around a central core to make an HTS cable. The cable is finally inserted in a stainless-steel jacket, which provides mechanical support and confinement for the forced flow of the supercritical helium coolant through the interstitials among the strands. The thermal gradients that build up in a TSTC make the 1D thermal-hydraulic models developed for low temperature superconducting magnets unsuitable to study fast transients such as quench propagation. A new model is presented here, which allows describing each strand, and separately accounting for the respective superconducting (SC) stack and copper tube. Compressible Euler-like equations describe the He flow and thermodynamic state in each interstitial fluid region. The distribution of the transport current is computed with a distributed parameter approach, self-consistently with the temperature distribution in the different so-called thermal components of the TSTC. The model is applied to the study of quench propagation in a conductor made of CroCo strands—a recent design proposed by the Karlsruhe Institute of Technology and based on the TSTC idea. A comparison with the modelling approach adopted for quench simulations in LTS conductors is carried out, showing that, when the heating is localized, the new model predicts, for the average temperature on the cross section at the hottest location along the conductor and for the voltage along the conductor, an evolution much slower than that predicted by a standard LTS-conductor model. [ABSTRACT FROM AUTHOR]

In applications employing high-temperature superconducting conductors, various cyclic loading (fatigue) conditions produced by mechanical, thermal, or periodic electromagnetic forces are inevitable. Applying coated conductor (CC) tapes under fatigue loading conditions is expected to critically affect the long-term reliability of its superconducting performance. Most studies evaluating the mechanical and electromechanical characteristics use quasi-static uniaxial tensile tests. Few have focused on the characterization of CC tapes under fatigue loading. In this study, the electromechanical property characterization of Cu-stabilized GdBa2Cu3Oy (GdBCO) CC tapes including fatigue behaviors were investigated at 77 K. High-cycle uniaxial fatigue tests were carried out on GdBCO CC tapes 4 and 12 mm in width, and the two were compared in terms of mechanical and electromechanical aspects at a stress ratio of 0.1. The mechanical and electrical fatigue limits of the CC tapes were determined at 77 K. The 4 mm wide CC tapes showed less fatigue limits when compared to the 12 mm wide ones. However, regardless of the CC tape width, the sequence in the obtained characteristic strengths at 77 K was the same: yield strength > irreversible stress limit > mechanical fatigue limit > electrical fatigue limit. Fracture surface morphologies were observed using scanning electron microscopy-energy dispersive x-ray spectroscopy and electron probe micro-analysis to clarify the fatigue fracture mechanism and to examine the influence of the architecture of the CC tapes on fatigue behaviors. Damage along the edges, caused by slitting during fabrication of the 4 mm wide CC tapes, generated a stress concentration, eventually resulting in earlier crack initiation not only on the substrate, reducing mechanical fatigue strength, but also on the superconducting layer, degrading the measured critical current. [ABSTRACT FROM AUTHOR]

In the calendering process a tape or sheet of deformable material passes through the nip between hard cylindrical rollers. Usually the rolls are driven at the same peripheral speed, but small differences in speed, often referred to as ‘creep’, can occur if one of the rolls is externally driven and the other is driven by the friction in the contact. In these circumstances it has been observed that a tape that enters the nip adhering to the driven (slower) surface may transfer at exit to the driving (faster) surface but not the other way round. The mechanics of this transfer process is examined theoretically and experimentally in this paper for the case of double sided adhesive tape. It is argued that on emerging from the nip the tape will separate from the surface at which the shear strain in the adhesive is greater and that for transfer to occur the contact load must be sufficient to cause plastic extension of the tape. [ABSTRACT FROM AUTHOR]

A solid oxide electrochemical oxygen pump was developed in tubular geometry with YSZ electrolyte and LSCF/CGO symmetrical air electrodes. The cells were manufactured based on green symmetrical porous YSZ electrode backbones cast on the green YSZ electrolyte tape, rolled into tubular geometry, co-sintered and impregnated with functional materials. The prepared cell had a thin 16 μm-electrolyte and supporting 76 μm-symmetric electrodes. At 700 °C, 2.8 A current (0.8 A cm−2 ) was obtained under 0.5 V in the single tubular cell infiltrated with 30% CGO and 45% LSCF. A four cell bundle produced theoretical 1.82 l hr−1 O2 with a power consumption of 2.18 W at 700 °C. Excellent stability of the cells was achieved during severe current and thermal cycling tests, indicating the successful fabrication of the tubular cells using the tape co-casting method. [ABSTRACT FROM AUTHOR]

The high-temperature superconductor (HTS) (RE)Ba2Cu3Ox (REBCO) conductor on round core (CORC) cable has great advantages with its high current capacity and power density. In REBCO CORC cables, current is redistributed among tapes through terminal contact resistance (TCR) when a local quench occurs. Therefore, its quench behavior is different from the single tape situation. To better understand the underlying physical process of local quenches in CORC cables, a new 3D multi-physics modeling tool for CORC cables is developed and presented in this paper. In this model, the REBCO tape is treated as a thin shell without thickness, and four models are coupled: a T-formulation model, an A-formulation model, a heat transfer model, and an equivalent circuit model. The T-formulation model is applied to the conductor shell only to calculate current distribution, which will be input into the A-formulation model; the A-formulation model is applied to the whole 3D domain to calculate the magnetic field, which is then fed back to the T-formulation model. The hot spot-induced quenches of CORC cables are analyzed. The results show that the thermal stability of the CORC cable can be considerably improved by reducing the TCR. The minimum quench energy (MQE) increases rapidly with the reduction of TCR when the resistance is in a middle range, which is about in this study. When the TCR is too low () or too high (), the MQE shows no obvious variation with TCR. With a low TCR, a hot spot in one tape may induce an overcurrent quench on other tapes. This will not happen in a cable with high TCR. In this case, the tape with a hot spot will quench and burn out before inducing a quench on other tapes. The developed modeling tool can be used to design CORC cables with improved thermal stability. [ABSTRACT FROM AUTHOR]

x-rays generated through tribological processes are emitted in fast bursts (∼10 ns) that hinder the correct measurement of the corresponding spectrum. In this work we implement a dosimetry technique based on thermoluminescent materials—impervious to pileup problems arising in solid-state active detectors—to measure the angular distribution of the tribo-generated x-rays from peeling adhesive tape. Unexpectedly, we find evidence of an isotropic energy emission on at least one meridian plane. These results may shed light on the physical processes behind the radiation emission in devices that harness the triboelectric effect to generate x-rays, and prove that dosimetry techniques can be used as an alternative characterization tool in the study of the emission from this and other relatively fast systems. [ABSTRACT FROM AUTHOR]

Copper nanowires (CuNWs) are remarkable components that can replace indium tin oxide as transparent electrodes due to their low cost, high conductivity and acceptable transmittance. However, a common coating method can cause poor electrical, optical and adhesive properties because of the creation of loosely connected junctions. In addition, the unsatisfactory thermal and environmental stabilities limit the practical applications. These problems should be overcome in CuNW-based films for reliable transparent electrodes through material and engineering approaches. In this work, a novel transparent composite electrode composed of chitosan and CuNWs on a flexible polyethylene terephthalate (PET) substrate, with synchronously strengthened adhesion, as well as heightened transmittance, reduced resistivity, improved flexibility, enhanced thermal stability and increased environmental stability, was prepared without vacuum processing and high-temperature annealing. The effects of the number of CuNW network layers and chitosan concentration on the performance of chitosan/CuNW composite transparent electrodes were studied. The resulting electrodes exhibitan excellent conductivity (sheet resistance: 15.6 Ω sq−1) and a superior optical transmittance (∼87%) at 550 nm. Calculation of the figure of merit displays a high value of 168, which is the highest among all the reported CuNW-based transparent electrodes. Meanwhile, the sheet resistance did not show great change after 10 tape tests and 10 000 bending cycles, suggesting good adhesion to the PET substrate and outstanding mechanical flexibility. Moreover, the composite transparent electrodes show good stability to resist long-term storage and temperature variation in thermal environment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Top and bottom microelectrode glass slide sandwiches a channel made of adhesive tape and packed with reduced graphene oxide to make a new non-planar interdigitated microfluidic device. Novel device integration allows packing of any transducer material at room temperature with no instrument. Electrical impedance spectroscopy signal is stable over long times, fits an equivalent circuit with well-defined circuit elements. Flow visualization concurs that electric double-layer signal shifts to high frequency due to disruption of the diffusive process from increased convective flow. Charge transfer resistance appears at higher frequencies making the device rapid with high signal to noise ratio. [ABSTRACT FROM AUTHOR]

Experimental data on dust resident in the EXTRAP T2R reversed-field pinch are reported. Mobile dust grains are captured in situ by silicon collectors, whereas immobile grains are sampled post mortem from the wall by adhesive tape. The simulation of collection asymmetries by the MIGRAINe dust dynamics code in combination with the experimental results is employed to deduce some characteristics of the mechanism of intrinsic dust release. All evidence suggests that re-mobilization is dominant with respect to dust production. [ABSTRACT FROM AUTHOR]

Next generation particle accelerators and fusion machines will greatly benefit from the development of low-inductance magnets capable of generating magnetic fields in excess of 16 T. Such magnets require high-temperature superconductors capable of carrying very high currents exceeding 5 kA at current densities of 400–600 A mm−2, such as Conductor on Round Core (CORC®) cables and wires wound from RE-Ba2Ca3O7-δ (ReBCO, Re = rare earth) coated conductor tapes. CORC® wires containing ReBCO tapes with 30 μm thick Hastelloy® substrates have previously been demonstrated as a viable high-field magnet conductor that can be produced at long lengths. Further improvement of the performance and flexibility of CORC® wires would benefit from the development of ReBCO tapes with even thinner substrates. SuperPower Inc. recently demonstrated ReBCO tapes based on 25 μm thick Hastelloy® substrates that allow the development of thinner and more flexible CORC® wires that meet the stringent performance requirements of high-field magnets. Several tapes containing 25 μm thick substrates were produced and analyzed, exhibiting critical current and cabling performance in-line with the current production level tapes with 30–50 μm thick substrates. Tape critical current was measured at 4.2 K and applied magnetic fields up to 31.2 T. Several CORC® wires incorporating these tapes were manufactured by Advanced Conductor Technologies using similar winding procedures that previously resulted in high-quality magnet-grade CORC® wires based on tapes with 30 μm thick substrates. The CORC® wires were tested in an applied magnetic field up to 12 T after bending to a 63 mm diameter. A critical current as high as 6231 A (12 T, 4.2 K) was measured with an engineering current density (Je) of 678 A mm−2, which extrapolates to over 450 A mm−2 at 20 T and is the highest current density reported in a CORC® conductor to date. The combination of ReBCO tapes produced using 25 μm thick substrates and the ability to wind them into long-length, high-quality CORC® magnet wires brings the development of low-inductance accelerator and fusion magnets that operate at magnetic fields exceeding 20 T closer to fruition. [ABSTRACT FROM AUTHOR]

In the case of using rare-earth barium copper oxide (REBCO) tape as a current-limiting element, there are three requirements: high normal state resistance, high thermal stability, and quick recovery to the superconducting state. In this study, a new type of REBCO tape with a metal porous medium as a stabilizer (a porous-stabilized REBCO tape) was proposed, which has high resistance compared to bulk material and prevents film boiling due to its strong capillary force. First, a numerical simulation was performed to evaluate the current-limiting performance of the porous-stabilized REBCO tape with a Ni–Cr porous medium. The results showed that the current-limiting performance was dramatically improved by placing indium partially on the REBCO tape to join the porous medium compared to placing indium wholly. Then, the porous-stabilized REBCO tapes with various arrays of indium were manufactured, and tested to confirm its recovery characteristics. The experimental results indicated that the partially-joined Ni–Cr porous-stabilized REBCO tape reduced the recovery time by 69% compared to that of the stabilizer-free REBCO tape. [ABSTRACT FROM AUTHOR]

In the frame of the EuCARD2 collaboration, aimed at developing the technology for 20 T class accelerator magnets, several demonstrator dipole magnets are being built using high critical current density and fully transposed ReBCO tape-based Roebel-type cables. In accelerator magnets the dynamic magnetic field quality is one of the key parameters, which is affected by the effective inter-strand resistances in the cables. For this reason, measurements of the inter-strand resistances on ReBCO Roebel cables were carried out at 4.2 and 77 K. Acquiring these data is also essential for input of cable simulation models. The cable samples are impregnated with epoxy resin to reduce the effect of transverse stress degradation due to Lorentz forces acting on the strands in the Roebel cables. The measured inter-strand resistance is used to estimate the AC coupling loss in different magnetic field orientations. Moreover, the contributions of diverse interface contact resistances to overall inter-strand resistance of Roebel cables were determined using a novel theoretical model. For validation, the AC loss of cables were examined in various orientations of applied field at 4.2 K. With three analytical models the hysteresis loss was calculated and compared to the measured data. The average inter-strand resistance of the cable samples impregnated with the unfilled epoxy CTD-101K range from 3 to 16 μΩ at 77 K and 1.5 to 9 μΩ at 4.2 K. Between the tapes the copper to copper interface resistance dominates the inter-strand resistance of impregnated Roebel cables. The calculated and measured AC loss for the CTD-101K impregnated Roebel cable lead to equivalent conclusions that the coupling loss is lower than the hysteresis loss within the range of the experiment. These observations substantially differ from earlier results extracted from a similar cable but impregnated with the alumina-filled epoxy resin CTD-101G, which showed considerable coupling loss when exposed to magnetic field parallel the wide face of the cable. [ABSTRACT FROM AUTHOR]

Nowadays, the superior electrical and mechanical properties of second generation (2G) practical coated conductor (CC) tapes make them viable options for various applications such as power cables, magnets and coils. The CC tapes are composite conductors where the constituent layers are directly affected by the mechanical and the electromechanical properties of the superconducting layer under operating conditions. It is therefore important to investigate the strain effect on the critical current (Ic) and to determine the irreversible limits for Ic degradation in CC tapes. However, standards or codes for testing the electromechanical properties of high-temperature superconductors are lacking. Therefore, the establishment of a test method or procedure for determining the electromechanical properties of 2G CC tapes in both engineering and practical aspects is needed. This study is a standardization activity, wherein the strain/stress sensitivity of Ic and the reversible strain/stress limits for Ic degradation of practical CC tapes using uniaxial tension tests were evaluated at 77 K and self-field. The reversible strain/stress limits of CC tapes were defined and the reversible recovery of Ic during loading–unloading was measured when using the 99% Ic (ε) recovery, 99% Ic0 recovery and 95% Ic0 retention criteria. The electromechanically determined irreversible limits were also compared to the mechanically determined stress and strain at the yield point. The obtained reversible stress limits showed a similar value regardless of criteria adopted, excepting Sample 2. However, the irreversible strain limit value obtained using the 99% Ic(ε) recovery criterion showed the largest value as compared to the cases using other criteria. Through repeated tests, the electromechanical test procedure of the CC tapes was evaluated based on statistical estimates. The obtained results can provide a fundamental basis for establishing test procedures comparing the criterion that should be considered in determining irreversible limits, whether normalized to the original state Ic value, Ic0 or determined using the reversible degradation of Ic(ε) at residual strain, when unloaded. [ABSTRACT FROM AUTHOR]

The effects of granularity on the angle and field dependence of the transport Jc in coated conductors (CCs) are discussed. The granularity is revealed by scanning Hall probe microscopy of the trapped field profiles in tapes with different architectures. It was found that pulsed laser deposited (PLD) YBCO film on a RABiTS NiW substrate has the most prominent granular morphology in its trapped field profiles. This is complemented by a peculiar behavior of the critical currents when the orientation of the applied field is within a certain angular range around the ab-planes: Jc(θ) becomes independent of the angle and Jc(B) exhibits a peak. These phenomena are explained by the physics of Abrikosov–Josephson vortices at grain boundaries (GB) that leads to a transition from a GB-limited to a grain-limited regime both in Jc(θ) and Jc(B). The effects of granularity are strongly suppressed by the chemical solution deposition route for growing YBCO films on NiW, and are almost absent in PLD-YBCO on ABAD-textured templates. These results identify the collective influence of the GB on the percolative current flow in CCs, which becomes most significant at low temperatures. [ABSTRACT FROM AUTHOR]

The majority of superconducting levitation systems are based on bulk superconductors. However, recently, stacks of coated conductor superconducting tapes (CC-tapes) have shown a proven potential to act as an alternative to superconducting bulks as components of a magnetic levitation system. This work primarily focuses on the influence of the critical current density on the levitation force of stacks of CC-tapes. The levitation force was measured for stacks of tapes made by different manufacturers, with different values of the critical current and substrate thickness. In our experiment, we used superconducting tapes manufactured by SuperOx, Theva and Sunam. The tapes were cut into 12 mm square pieces. The number of layers in the stack ranged from 10 to 100. Levitation forces were studied for all types of tapes and for various numbers of layers in the stack. As expected, it was found that a higher critical current corresponds to a higher levitation force. The levitation force of the stacks assembled from tapes with a higher critical current tends to saturate for lower numbers of elements in the stack. [ABSTRACT FROM AUTHOR]

Tapes of MgB2 with high critical current have a significant technological potential, but can experience operational breakdown due to thermomagnetic instabilities. Such events are routinely registered by magnetisation measurements, but were never observed by direct imaging techniques. Here we report magneto-optical imaging visualisation of a tape with reduced thickness and enhanced critical current density. The spatial structure of the thermomagnetic avalanche events was resolved, and the reproducibility and nucleation thresholds were determined. Avalanches appearing at low fields are small intrusions at the tape edges, while larger fields create more extensive avalanches having dendritic structures similar to those observed in thin-film superconductors. In contrast to dendritic avalanches in thin films, the avalanches in the tapes show reproducibility to a larger extent. [ABSTRACT FROM AUTHOR]

The surface properties of polydimethylsiloxane (PDMS) were modified by treatment with an atmospheric pressure rotating plasma jet (APPJ) and the surface modifications were studied to assess its hydrophilicity and adhesion to acrylic adhesive tape intended for medical applications. Furthermore, the extent of hydrophobic recovery under different storage conditions was studied. The surface treatment of PDMS with the APPJ under optimal conditions noticeably increased the oxygen content and most of the surface silicon species were fully oxidized. A brittle silica-like layer on the outermost surface was created showing changes in topography due to the formation of grooves and cracks. A huge improvement in T-peel and the shear adhesive strength of the APPJ-treated PDMS surface/acrylic tape joints was obtained. On the other hand, the hydrophilicity of the PDMS surface increased noticeably after the APPJ treatment, but 24 h after treatment almost 80% hydrophobicity was recovered and the adhesive strength was markedly reduced with time after the APPJ treatment. However, the application of an acrylic adhesive layer on the just-APPJ-treated PDMS surface retained the adhesive strength, limiting the extent of hydrophobic recovery. [ABSTRACT FROM AUTHOR]

We developed a new method to fabricate parylene-based flexible microfluidic platforms using commercial adhesive tape. Most of the previous methods required the preparation of parylene channels via a parylene–to–parylene bonding, which could only be achieved at high pressure and temperature. Instead, our method exploits the adherent property of commercial tape as a substrate for the parylene peel-off process. Once the parylene thin film is deposited by chemical vapour deposition (CVD) on top of the polydimethylsiloxane (PDMS) replica, prepared by conventional lithography process, an adhesive tape peels off the deposited parylene layer with the micro-scale structures from the PDMS replica. We found that the minimum size of the circle posts successfully fabricated by this process is about 10 μm in diameter and 10 μm in height; the maximum value in aspect ratio is about 2.5. In our experimental investigation, pressure in the parylene channels with different wall thicknesses, was measured to estimate the hydraulic resistance of the channel. Our results are comparable with calculated data, with a normalized deviation smaller than 5%. In addition, the hydraulic resistance of the channels on the curved surface obtained with a similar approach showed an increase when the radius of curvature was reduced. Finally, this contribution shows that our method enables a simple and relatively inexpensive fabrication of flexible microfluidic platforms. [ABSTRACT FROM AUTHOR]

This article presents a review of the finite-element method (FEM) model based on the H formulation of Maxwell's equations used to calculate AC losses in high-temperature superconductor (HTS) tapes, cables and windings for different applications. This model, which uses the components of the magnetic field as state variables, has been gaining a great popularity and has been in use in tens of research groups around the world. This contribution first reviews the equations on which the model is based and their implementation in FEM programs for different cases, such as 2D longitudinal and axis-symmetric geometries and 3D geometries. Modeling strategies to tackle large number of HTS tapes, such as multi-scale and homogenization methods, are also introduced. Then, the second part of the article reviews the applications for which the H formulation has been used to calculate AC losses, ranging from individual tapes to complex cables and large magnet windings. Afterwards, a section is dedicated to the discussion of the H formulation in terms of computational efficiency and easiness of implementation. Its pros and cons are listed. Finally, the last section draws the main conclusions. [ABSTRACT FROM AUTHOR]

CaKFe4As4 superconducting tapes are fabricated via different techniques to enhance grain connectivity. The critical current density, Jc, increases exponentially with the Vickers hardness of the superconducting core. The Jc (4.2 K, 10 T) of the hot isostatic pressed Cu/Ag-sheathed tape reaches 2.2 × 104 A cm−2, while that of the stainless steel/Ag-sheathed tapes surpasses 104 A cm−2. However, silver infiltrates from the sheath far into the center of the superconducting core and reacts with the CaKFe4As4 phase, which has significant ramifications for the transport current. We suggest that CaKFe4As4 has great potential if we can realize stable coexistence between the superconducting core and the sheath. [ABSTRACT FROM AUTHOR]

The dust collection and analysis strategy in ASDEX Upgrade (AUG) is described. During five consecutive operation campaigns (2007–2011), Si collectors were installed, which were supported by filtered vacuum sampling and collection with adhesive tapes in 2009. The outer and inner morphology (e.g. shape) and elemental composition of the collected particles were analysed by scanning electron microscopy. The majority of the ∼50 000 analysed particles on the Si collectors of campaign 2009 contain tungsten—the plasma-facing material in AUG—and show basically two different types of outer appearance: spheroids and irregularly shaped particles. By far most of the W-dominated spheroids consist of a solid W core, i.e. solidified W droplets. A part of these particles is coated with a low-Z material; a process that seems to happen presumably in the far scrape-off layer plasma. In addition, some conglomerates of B, C and W appear as spherical particles after their contact with plasma. By far most of the particles classified as B-, C- and W-dominated irregularly shaped particles consist of the same conglomerate with varying fraction of embedded W in the B–C matrix and some porosity, which can exceed 50%. The fragile structures of many conglomerates confirm the absence of intensive plasma contact. Both the ablation and mobilization of conglomerate material and the production of W droplets are proposed to be triggered by arcing. The size distribution of each dust particle class is best described by a log-normal distribution allowing an extrapolation of the dust volume and surface area. The maximum in this distribution is observed above the resolution limit of 0.28 µm only for the W-dominated spheroids, at around 1 µm. The amount of W-containing dust is extrapolated to be less than 300 mg on the horizontal areas of AUG. [ABSTRACT FROM AUTHOR]

We report the temperature, magnetic field and time dependences of magnetization in advanced Ba122 superconducting tapes. The sample exhibits peculiar vortex creep behavior. Below 10 K, the normalized magnetization relaxation rate S = dln(−M)/dln(t) shows a temperature-insensitive plateau with a value comparable to that of low-temperature superconductors, which can be explained within the framework of collective creep theory. It then enters into a second collective creep regime when the temperature increases. Interestingly, the relaxation rate below 20K tends to reach saturation with increasing the field. However, it changes to a power law dependence on the field at a higher temperature. A vortex phase diagram composed of the collective and the plastic creep regions is shown. Benefiting from the strong grain boundary pinning, the advanced Ba122 superconducting tape has potential to be applied not only in liquid helium but also in liquid hydrogen or at temperatures accessible with cryocoolers. [ABSTRACT FROM AUTHOR]

This work presents simulation studies on superconducting jointless loops known as double crossed loop coils, made from partially slit and stacked REBCO tape segments. The objective is to investigate design parameters for a novel magnetic bearing application, trying to increase the (levitation force)/(tape length) ratio. The adopted simulation strategy is to model the tapes as one-dimensional current sheets with the current density written as integral equations solved by finite elements. For this recently developed type of coil, each current path must be modeled, thus every tape should be represented in the simulation, as shall be discussed. The results demonstrate how the coils’ performance varies non-linearly to the geometric parameters and that stacking the maximum number of tapes is not the only way to increase the levitation force. Their force curves also do not show the same behavior as expected for bulk ones, and an analysis is performed to relate it to the critical current sensitivity to the magnetic field. A more detailed investigation explains this behavior from the induced currents’ point of view in every loop of the stack. Comparisons between the simulation data and prototype tests prove that the chosen model can satisfactorily reproduce the measurements. [ABSTRACT FROM AUTHOR]

We introduce an efficient method of fabricating cold, linear cathodes with carbon nanotube (CNT) emitters. A nickel wire was painted first with silver paste, then with CNT paste. After sintering under nitrogen and activation by adhesive tape, the cathode wire provided a stable and homogeneous emission density of 200 mA cm?2at 6.2 V µm?1. As an application, luminescent tubes were constructed using this CNT wire as the cathode and a transparent CNT film as the anode. These cold cathodes can replace conventional hot cathodes, which are widely used in vacuum electronic devices. The novel two-layer structure can be applied to any substrate, and is simple and inexpensive to fabricate. [ABSTRACT FROM AUTHOR]