Showing papers in "Superconductor Science and Technology in 2014"

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

Abstract: The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.

Coated conductors for power applications: materials challenges

Abstract: This manuscript reports on the recent progress and the remaining materials challenges in the development of coated conductors (CCs) for power applications and magnets, with a particular emphasis on the different initiatives being active at present in Europe. We first summarize the scientific and technological scope where CCs have been raised as a complex technology product and then we show that there exists still much room for performance improvement. The objectives and CC architectures being explored in the scope of the European project EUROTAPES are widely described and their potential in generating novel breakthroughs emphasized. The overall goal of this project is to create synergy among academic and industrial partners to go well beyond the state of the art in several scientific issues related to CCs’ enhanced performances and to develop nanoengineered CCs with reduced costs, using high throughput manufacturing processes which incorporate quality control tools and so lead to higher yields. Three general application targets are considered which will require different conductor architectures and performances and so the strategy is to combine vacuum and chemical solution deposition approaches to achieve the targeted goals. A few examples of such approaches are described related to defining new conductor architectures and shapes, as well as vortex pinning enhancement through novel paths towards nanostructure generation. Particular emphasis is made on solution chemistry approaches. We also describe the efforts being made in transforming the CCs into assembled conductors and cables which achieve appealing mechanical and electromagnetic performances for power systems. Finally, we briefly mention some outstanding superconducting power application projects being active at present, in Europe and worldwide, to exemplify the strong advances in reaching the demands to integrate them in a new electrical engineering paradigm.

Progresses and challenges in the development of high-field solenoidal magnets based on RE123 coated conductors

Abstract: Recent progresses in the second generation REBa2Cu3O7 − x (RE123) coated conductor (CC) have paved a way for the development of superconducting solenoids capable of generating fields well above 23.5 T, i.e. the limit of NbTi−Nb3Sn-based magnets. However, the RE123 magnet still poses several fundamental and engineering challenges. In this work we review the state-of-the-art of conductor and magnet technologies. The goal is to illustrate a close synergetic relationship between evolution of high-field magnets and advancement in superconductor technology. The paper is organized in three parts: (1) the basics of RE123 CC fabrication technique, including latest developments to improve conductor performance and production throughput; (2) critical issues and innovative design concepts for the RE123-based magnet; and (3) an overview of noteworthy ongoing magnet projects.

Roebel cables from REBCO coated conductors: a one-century-old concept for the superconductivity of the future

Abstract: Energy applications employing high-temperature superconductors (HTS), such as motors/generators, transformers, transmission lines and fault current limiters, are usually operated in the alternate current (ac) regime. In order to be efficient, the HTS devices need to have a sufficiently low value of ac loss, in addition to the necessary current-carrying capacity. Most applications are operated with currents beyond the current capacity of single conductors and consequently require cabled conductor solutions with much higher current carrying capacity, from a few kA up to 20–30 kA for large hydro-generators. A century ago, in 1914, Ludwig Roebel invented a low-loss cable design for copper cables, which was successively named after him. The main idea behind Roebel cables is to separate the current in different strands and to provide a full transposition of the strands along the cable direction. Nowadays, these cables are commonly used in the stator of large generators. Based on the same design concept of their conventional material counterparts, HTS Roebel cables from REBCO coated conductors were first manufactured at the Karlsruhe Institute of Technology and have been successively developed in a number of varieties that provide all the required technical features such as fully transposed strands, high transport currents and low ac losses, yet retaining enough flexibility for a specific cable design. In the past few years a large number of scientific papers have been published on the concept, manufacturing and characterization of such cables. Therefore it is timely for a review of those results. The goal is to provide an overview and a succinct and easy-to-consult guide for users, developers, and manufacturers of this kind of HTS cable.

Challenges and status of ITER conductor production

Abstract: Taking the relay of the large Hadron collider (LHC) at CERN, ITER has become the largest project in applied superconductivity. In addition to its technical complexity, ITER is also a management challenge as it relies on an unprecedented collaboration of seven partners, representing more than half of the world population, who provide 90% of the components as in-kind contributions. The ITER magnet system is one of the most sophisticated superconducting magnet systems ever designed, with an enormous stored energy of 51?GJ. It involves six of the ITER partners. The coils are wound from cable-in-conduit conductors (CICCs) made up of superconducting and copper strands assembled into a multistage cable, inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the toroidal field (TF) and central solenoid (CS) coils require about 600?t of Nb3Sn strands while the poloidal field (PF) and correction coil (CC) and busbar conductors need around 275?t of Nb?Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and has called for a significant worldwide production scale up. The TF conductors are the first ITER components to be mass produced and are more than 50% complete. During its life time, the CS coil will have to sustain several tens of thousands of electromagnetic (EM) cycles to high current and field conditions, way beyond anything a large Nb3Sn coil has ever experienced. Following a comprehensive R&D program, a technical solution has been found for the CS conductor, which ensures stable performance versus EM and thermal cycling. Productions of PF, CC and busbar conductors are also underway. After an introduction to the ITER project and magnet system, we describe the ITER conductor procurements and the quality assurance/quality control programs that have been implemented to ensure production uniformity across numerous suppliers. Then, we provide examples of technical challenges that have been encountered and we present the status of ITER conductor production worldwide.

Imaging superconducting vortex cores and lattices with a scanning tunneling microscope

Abstract: The observation of vortices in superconductors was a major breakthrough in developing the conceptual background for superconducting applications. Each vortex carries a flux quantum, and the magnetic field decreases radially from the center. Techniques used to make magnetic field maps, such as magnetic decoration, give vortex lattice images in a variety of systems. However, strong type II superconductors allow penetration of the magnetic field over large distances, of the order of the magnetic penetration depth λ. Superconductivity survives up to magnetic fields where, for imaging purposes, there is no magnetic contrast at all. Static and dynamic properties of vortices are largely unknown at such high magnetic fields. Reciprocal space studies using neutron scattering have been employed to obtain insight into the collective behavior. But the microscopic details of vortex arrangements and their motion remain difficult to obtain. Direct real-space visualization can be made using scanning tunneling microscopy and spectroscopy (STM/S). Instead of using magnetic contrast, the electronic density of states describes spatial variations of the quasiparticle and pair wavefunction properties. These are of the order of the superconducting coherence length ξ, which is much smaller than λ. In principle, individual vortices can be imaged using STM up to the upper critical field where vortex cores, of size ξ, overlap. In this review, we describe recent advances in vortex imaging made with scanning tunneling microscopy and spectroscopy. We introduce the technique and discuss vortex images that reveal the influence of the Fermi surface distribution of the superconducting gap on the internal structure of vortices, the collective behavior of the lattice in different materials and conditions, and the observation of vortex lattice melting. We consider challenging lines of work, which include imaging vortices in nanostructures, multiband and heavy fermion superconductors, single layers and van der Waals crystals, studying current-driven dynamics and the liquid vortex phases.

Progress in superconducting metamaterials

Abstract: We review progress in the development and applications of superconducting metamaterials. The review is organized in terms of several distinct advantages and unique properties brought to the metamaterials field by superconductivity. These include the low-loss nature of the meta-atoms, their compact structure, their extraordinary degree of nonlinearity and tunability, magnetic flux quantization and the Josephson effect, quantum effects in which photons interact with quantized energy levels in the meta-atom, as well as strong diamagnetism.

Modelling and comparison of trapped fields in (RE)BCO bulk superconductors for activation using pulsed field magnetization

Abstract: The ability to generate a permanent, stable magnetic field unsupported by an electromotive force is fundamental to a variety of engineering applications. Bulk high temperature superconducting (HTS) materials can trap magnetic fields of magnitude over ten times higher than the maximum field produced by conventional magnets, which is limited practically to rather less than 2 T. In this paper, two large c-axis oriented, single-grain YBCO and GdBCO bulk superconductors are magnetized by the pulsed field magnetization (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetizing such samples as trapped field magnets (TFMs) in situ inside a trapped flux-type superconducting electric machine. A comparison is made between the temperatures at which the pulsed magnetic field is applied and the results have strong implications for the optimum operating temperature for TFMs in trapped flux-type superconducting electric machines. The effects of inhomogeneities, which occur during the growth process of single-grain bulk superconductors, on the trapped field and maximum temperature rise in the sample are modelled numerically using a 3D finite-element model based on the H-formulation and implemented in Comsol Multiphysics 4.3a. The results agree qualitatively with the observed experimental results, in that inhomogeneities act to distort the trapped field profile and reduce the magnitude of the trapped field due to localized heating within the sample and preferential movement and pinning of flux lines around the growth section regions (GSRs) and growth sector boundaries (GSBs), respectively. The modelling framework will allow further investigation of various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors, including inhomogeneous Jc distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimization of processing and PFM techniques for practical bulk superconductor applications.

RCE-DR, a novel process for coated conductor fabrication with high performance

Abstract: We report in detail on SuNAM's reactive co-evaporation by deposition and reaction (RCE-DR) process. We have successfully fabricated a high performance GdBCO coated conductor (CC) with high throughput by the RCE-DR process, that consists of two steps for the deposition of elemental metal oxides and the conversion of cation oxides into the GdBCO superconducting phase. Constituting metals such as Gd, Ba and Cu were first deposited on LaMnO3 (LMO)-buffered IBAD-MgO templates at low temperatures and low pressures followed by a high temperature treatment step under high oxygen partial pressure for fast phase conversion. GdBCO CCs fabricated by RCE-DR showed excellent transport properties such as a critical current of 794 A cm−1 width at 77 K in self-field. With the RCE-DR process, we have achieved an overall processing speed of more than 120 m h−1 (in terms of a real process linear tape speed equivalent). SuNAM's RCE-DR technique showed great potential as the highest throughput fabrication process compared with other methods developed previously for second generation high temperature superconducting wires, meeting the current and future need of industry in terms of price and production speed.

Development and production of second generation high T c superconducting tapes at SuperOx and first tests of model cables

Abstract: The SuperOx and SuperOx Japan LLC companies were founded with the goal of developing a cost-effective technology for second generation HTS (2G HTS) tapes by utilizing a combination of the most advanced chemical and physical deposition techniques, together with implementing original tape architectures. In this paper we present a brief overview of our production and experimental facilities and recent results of 2G HTS tape fabrication, and describe the first tests of the tapes in model cables for AC and DC power application.

Superconducting single-photon detector made of MoSi film

Abstract: We fabricated and characterized nanowire superconducting single-photon detectors made of 4 nm thick amorphous Mo x Si1−x films. At 1.7 K the best devices exhibit a detection efficiency (DE) up to 18% at 1.2 wavelength of unpolarized light, a characteristic response time of about 6 ns and timing jitter of 120 ps. The DE was studied in wavelength range from 650 nm to 2500 nm. At wavelengths below 1200 nm these detectors reach their maximum DE limited by photon absorption in the thin MoSi film.

3D modeling and simulation of 2G HTS stacks and coils

Abstract: Use of 2G HTS coated conductors in several power applications has become popular in recent years. Their large current density under high magnetic fields makes them suitable candidates for high power capacity applications such as stacks of tapes, coils, magnets, cables and current leads. For this reason, modeling and simulation of their electromagnetic properties is very desirable in the design and optimization processes. For many applications, when symmetries allow it, simple models consisting of 1D or 2D representations are well suited for providing a satisfying description of the problem at hand. However, certain designs such as racetrack coils and finite-length or non-straight stacks, do pose a 3D problem that cannot be easily reduced to a 2D configuration. Full 3D models have been developed, but their use for simulating superconducting devices is a very challenging task involving a large-scale computational problem. In this work, we present a new method to simulate the electromagnetic transient behavior of 2G HTS stacks and coils. The method, originally used to model stacks of straight superconducting tapes or circular coils in 2D, is now extended to 3D. The main idea is to construct an anisotropic bulk-like equivalent for the stack or coil, such that the geometrical layout of the internal alternating structures of insulating, metallic, superconducting and substrate layers is reduced while keeping the overall electromagnetic behavior of the original device. Besides the aforementioned interest in modeling and simulating 2G HTS coated conductors, this work provides a further step towards efficient 3D modeling and simulation of superconducting devices for large-scale applications.

The effects of co-wound Kapton, stainless steel and copper, in comparison with no insulation, on the time constant and stability of GdBCO pancake coils

Abstract: Recently, studies of partially insulated, high-temperature superconducting (HTS) coils have shown application in the design and construction of compact, stable and self-protecting HTS coils. This article presents the electrical characteristics of HTS coils based on the turn-to-turn inserted materials and conditions. Three partially co-wound pancake coils were fabricated and tested. Each coil was partially co-wound with Kapton, stainless steel and copper tape at every fourth turn of the winding. Tested coils were co-wound on every turn with Kapton, stainless steel and copper tape, and coils without turn-to-turn insulation were the control group. Charge–discharge, sudden-discharge and over-current tests were performed to evaluate the performance of the fabricated coils. The experimental results show that the properties of materials inserted into the coil can control the time constant (τ). Therefore, HTS coils can be designed for specific purposes according to the time constant control. The experimental results of the study could be useful in designing HTS coil applications.

Room temperature deposition of sputtered TiN films for superconducting coplanar waveguide resonators

Abstract: We present a systematic study of the properties of room temperature deposited TiN films by varying the deposition conditions in an ultra-high-vacuum reactive magnetron sputtering chamber. By increasing the deposition pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti1−xNx (x = 0.5) without substrate heating, the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive in-plane strain changes to weak tensile in-plane strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the target–substrate distance set to 88 mm the contaminant levels increase from ∼0.1% to ∼10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with in-plane distance from the center of the substrate and increase by roughly two orders of magnitude as the target–substrate distance is increased from 88 to 266 mm. These contaminants are found to strongly influence the properties of TiN thin films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results strongly suggest that the energy of the sputtered TiN particles plays a crucial role in determining the TiN film properties, and that it is important to precisely control the energy of these particles to obtain high-quality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 to 7 mTorr show a substantial increase in intrinsic quality factor from ∼104 to ∼106 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. Surprisingly, the films with a higher oxygen content exhibit lower loss, but care must be taken when depositing at room temperature to avoid nonuniform oxygen incorporation, which presents as a radially dependent resistivity and becomes a radially dependent surface inductance in the superconductor.

Development of superconducting links for the Large Hadron Collider machine

Abstract: In the framework of the upgrade of the Large Hadron Collider (LHC) machine, new superconducting lines are being developed for the feeding of the LHC magnets. The proposed electrical layout envisages the location of the power converters in surface buildings, and the transfer of the current from the surface to the LHC tunnel, where the magnets are located, via superconducting links containing tens of cables feeding different circuits and transferring altogether more than 150 kA. Depending on the location, the links will have a length ranging from 300 m to 500 m, and they will span a vertical distance of about 80 m. An overview of the R&D program that has been launched by CERN is presented, with special attention to the development of novel types of cables made from MgB2 and high temperature superconductors (Bi-2223 and REBCO) and to the results of the tests performed on prototype links. Plans for future activities are presented, together with a timeline for potential future integration in the LHC machine.

Effects of neutron irradiation on pinning force scaling in state-of-the-art Nb3Sn wires

Abstract: We present an extensive irradiation study involving five state-of-the-art Nb3Sn wires which were subjected to sequential neutron irradiation up to a fast neutron fluence of 1.6 × 1022 m−2 (E > 0.1 MeV). The volume pinning force of short wire samples was assessed in the temperature range from 4.2 to 15 K in applied fields of up to 7 T by means of SQUID magnetometry in the unirradiated state and after each irradiation step. Pinning force scaling computations revealed that the exponents in the pinning force function differ significantly from those expected for pure grain boundary pinning, and that fast neutron irradiation causes a substantial change in the functional dependence of the volume pinning force. A model is presented, which describes the pinning force function of irradiated wires using a two-component ansatz involving a point-pinning contribution stemming from radiation induced pinning centers. The dependence of this point-pinning contribution on fast neutron fluence appears to be a universal function for all examined wire types.

Synthesis of Bi2Sr2CaCu2Ox superconductors via direct oxidation of metallic precursors

Abstract: Bi2Sr2CaCu2Ox(Bi2212)/Ag multifilamentary wires are manufactured via the powder-in-tube process using oxide powders. After deformation, the wires undergo a partial-melt process, resulting in a complex, heterogeneous microstructure containing multiple secondary phases and porosity, limiting the wires? electrical and mechanical performance. Here, an alternative approach using the direct conversion of metallic precursors (MPs) to Bi2212 is studied. The formation of metallic precursor powders via a mechanical alloy is discussed. The MP powder is then converted to superconducting Bi2212 through a simple two-step heat treatment. By introducing oxygen at a temperature at which Bi2212 is in a stable phase, and holding at an elevated temperature for a sufficient time, the metallic precursors are oxidized and transformed into Bi2212. Several factors that impact the formation and growth of Bi2212 grains are discussed. Peak temperature, holding time and heating rate are shown to affect the MP ?Bi2212 conversion, the Bi2201 content and the Bi2212 morphology and density. It is found that Bi2Sr2CuOy (Bi2201) can be the only phase impurity after heat treatment, which is quite different from what is observed in partial-melt processed wires derived from oxide precursors. Lastly, the microstructure at the sample/silver interface suggests larger size and preferred orientation of Bi2212 grains with the aid of a silver surface. Implications for MP Bi2212 wires are discussed.

Optimized controlled-Z gates for two superconducting qubits coupled through a resonator

Abstract: Superconducting qubits are promising candidates for building a quantum computer. A continued challenge for fast yet accurate gates is to minimize the effects of decoherence. Here we apply numerical methods to design fast entangling gates, specifically the controlled-Z, in an architecture where two qubits are coupled via a resonator. We find that the gates can be sped up by a factor of two and reach any target fidelity. We also discuss how systematic errors arising from experimental conditions affect the pulses and how to remedy them, providing a strategy for the experimental implementation of our results. We discuss the shape of the pulses, their spectrum and symmetry.

Potentials of iron-based superconductors for practical future materials

Abstract: Since the discovery of high-Tc superconductivity in the REFeAs(O, F) system in 2008, studies on the development of superconducting materials using iron-based superconductors has been undertaken because of their high Hc2 and relatively high Tc. Although the cuprate superconductors exhibit much higher Tc and similar high Hc2, the small degree of electromagnetic anisotropy between the c-axis and ab-plane directions confirmed in 11, 122 and 1111 systems encouraged us to develop more versatile conductors. Single crystals and thin films deposited on single-crystalline and metal substrates have proved that the potentials of the iron-based superconductors are high enough for designing superconducting materials for high field generation. In addition, critical current properties of powder-in-tube processed tapes have been greatly improved in the past two years and are reaching the application level at 4.2?K in high magnetic field. However, the pinning mechanism and determining factors of the critical current properties of the iron-based superconductors have not been well understood. Characteristics and potentials of iron-based superconductors are discussed from various viewpoints in this paper in an effort to understand the current status and future prospects.

Band-edge BCS-BEC crossover in a two- band superconductor: physical properties and detection parameters

Abstract: Superconductivity in iron-based magnesium diborides and other novel superconducting materials has a strong multi-band and multi-gap character. Recent experiments support the possibillity for a BCS–BEC crossover induced by strong-coupling and proximity of the chemical potential to the edge of one of the bands. Here we study the simplest theoretical model which accounts for the BCS–BEC crossover in a two-band superconductor, considering tunable interactions and tunable energy separations between the bands. Mean-field results for condensate fraction, correlation length, and superconducting gap are reported in typical crossover diagrams to locate the boundaries of the BCS, crossover and BEC regimes. When the superconducting gap is of the order of the local chemical potential, superconductivity is in the crossover regime of the BCS–BEC crossover and the Fermi surface of the small band is smeared by the gap opening. In this situation, small and large Cooper pairs coexist in the total condensate, which is the optimal condition for high-Tc superconductivity. The ratio between the gap and the Fermi energy in a given band results in the best detection parameter for experiments to locate the system in the BCS–BEC crossover. Using available experimental data, our analysis shows that iron-based superconductors have the partial condensate of the small Fermi surface in the crossover regime of the BCS–BEC crossover, supporting the recent ARPES findings.

Characterization of transverse tensile stress response of critical current and delamination behaviour in GdBCO coated conductor tapes by anvil test

Abstract: Recent problems in superconducting devices operating at the high magnetic field regime include the possibility of substantial critical current, Ic, degradation due to the delamination of the coated conductor (CC) tapes. The delamination may originate from excessive tensile radial stress and cleavage stress developed due to epoxy curing, cool-down and Lorentz force generated. Therefore, it is necessary to investigate the Ic response under transverse loading and the delamination behaviour in REBCO CC tapes to predict its performance in practical device applications. In this study, using the anvil test method, the delamination strength and the transverse tensile stress response of Ic in GdBCO CC tapes were investigated. Under transverse tensile stress, the Ic degradation in GdBCO CC tapes showed both abrupt and gradual behaviours. A reversible Ic degradation behaviour was also observed which was similar to the case under uniaxial tension. The electro-mechanical and mechanical delamination strengths were determined, wherein both increased when a smaller-sized anvil was used during the delamination test. The electro-mechanical delamination strength was approximately half of the mechanical delamination strength in the GdBCO CC tapes tested. Four delamination modes were observed and well correlated with electro-mechanical delamination strength and Ic degradation behaviour. Lastly, delamination sites within the CC tapes were also correlated with the Ic degradation behaviour that had been observed.

The influence of the geometric characteristics of nanorods on the flux pinning in high-performance BaMO3-doped SmBa2Cu3Oy films (M = Hf, Sn)

Abstract: Recently, the BaHfO3 (BHO) nanorod has attracted attention as a new c-axis-correlated pinning center in REBa2Cu3Oy films. We fabricated SmBa2Cu3Oy (SmBCO) films with BHO nanorods using an alternating-targets technique with pulsed laser deposition on single-crystal LaAlO3(100) substrates, and then compared the microstructure and flux pinning properties with those of BaSnO3 (BSO)-doped SmBCO films. Transmission electron microscopy observations indicated that the BHO and BSO nanorods both grew straight, but the inclination of the BHO nanorods from the c-axis of the SmBCO was less than that of the BSO nanorods. The inclination had a strong influence on the flux pinning property. The flux pinning force of the BHO-doped SmBCO film ( with Jc = 2.0 MA cm−2 at 77 K under 1.4 T) became stronger than that of the BSO-doped SmBCO film ( with Jc = 1.4 MA cm−2 at 77 K under 1.8 T) due to the shape of the BHO nanorods without inclining.

Strong enhancement of high-field critical current properties and irreversibility field of MgB2 superconducting wires by coronene active carbon source addition via the new B powder carbon-coating method

Abstract: We report an effective carbon-containing additive, coronene (C24H12), for MgB2 superconducting wires. We used B powder coated with C24H12 to fabricate MgB2 wires using the powder-in-tube (PIT) and internal Mg diffusion (IMD) processes. The in-field critical current properties are strongly enhanced for both PIT- and IMD-processed MgB2 wires. For PIT MgB2 wires, a critical current density (Jc) value of 1.8 × 104 A cm−2 is obtained at 4.2 K and 10 T. For IMD MgB2 wires, we obtained a Jc of 1.07 × 105 A cm−2 and an engineering Jc (Je) of 1.12 × 104 A cm−2 at 4.2 K and 10 T. These Jc and Je values are similar to the highest values reported for MgB2 wires thus far. Furthermore, the irreversibility field, Birr, determined with a current density criterion of 100 A cm−2, is strongly enhanced to 25 T at 4.2 K, which is also the highest value reported for MgB2 superconducting wires thus far. Coronene is an active carbon source for MgB2 superconducting wires because (1) coronene has a high carbon content (96 wt%) with a small amount of hydrogen (impurity), (2) the decomposition temperature for coronene is near the reaction temperature between Mg and B, and (3) uniform dispersion of coronene on the B surface can be obtained due to the melting point of coronene being lower than the decomposition temperature. Carbon substitution for B caused by the coronene active carbon source is mainly responsible for the high field critical current properties and the high Birr obtained in this work.

An H-formulation-based three-dimensional hysteresis loss modelling tool in a simulation including time varying applied field and transport current: the fundamental problem and its solution

Abstract: When analytic solutions are not available, finite-element-based tools can be used to simulate hysteresis losses in superconductors with various shapes. A widely used tool for the corresponding magnetoquasistatic problem is based on the H-formulation, where H is the magnetic field intensity, eddy current model. In this paper, we study this type of tool in a three-dimensional simulation problem. We consider a case where we simultaneously apply both a time-varying external magnetic field and a transport current to a twisted wire. We show how the modelling decisions (air has high finite resistivity and applied field determines the boundary condition) affect the current density distribution along the wire. According to the results, the wire carries the imposed net current only on the boundary of the modelling domain, but not inside it. The current diffuses to the air and back to the boundary. To fix this problem, we present another formulation where air is treated as a region with 0 conductivity. Correspondingly, we express H in the air with a scalar potential and a cohomology basis function which considers the net current condition. As shown in this paper, this formulation does not fail in these so-called AC-AC (time varying transport current and applied magnetic field) simulations.

Study of second-generation high-temperature superconducting magnets: the self-field screening effect

Abstract: Second-generation high-temperature superconductors (2G HTS) have high current density in very high magnetic fields. They are good candidates for high field magnets, especially when the magnetic field exceeds the critical fields of low-temperature superconductors. However, the thin and flat geometry of these conductors allows persistent screening currents (or shielding currents) to flow in the conductors. The screening currents caused by the ramping of applied current to the coil is identified as the self-field screening effect. The screening-current-induced magnetic field changes the magnetic field distribution of the magnet, and it also generates drift. This paper employs both experimental and numerical methods to study the mechanism of self-field screening currents for 2G HTS magnets. A 2G HTS magnet was constructed and tested, and a finite element model was built based on the magnet. The comparison between calculation and measurement is presented with detailed analysis. Current distributions inside the HTS magnet are calculated to illustrate the effects of screening. The screening-current-induced magnetic field is quantified by comparing the magnetic field distribution with a baseline copper model. The model is also used to explain the mechanism of the current sweep strategy, which can be used to effectively eliminate screening currents.

Conduction cooled magnet design for 1.5 T, 3.0 T and 7.0 T MRI systems

Abstract: Main magnets for magnetic resonance imaging (MRI) are largely constructed with low temperature superconducting material. Most commonly used superconductors for these magnets are niobium-titanium (NbTi). Such magnets are operated at 4.2 K by being immersed in a liquid helium bath for long time operation. As the cost of liquid helium has increased threefold in the last decade and the market for MRI systems is on average increasing by more than 7% every year, there is a growing demand for an alternative to liquid helium. Superconductors such as magnesium-diboride (MgB2) and niobium-tin (Nb3Sn) demonstrate superior current carrying quality at higher critical temperatures than 4.2 K. In this article, electromagnetic designs for conduction cooled main magnets over the range of medium field strengths (1.5 T) to ultrahigh field strengths (7.0 T) are presented. These designs are achieved by an improved functional approach coming from a series of developments by the present research group and using properties of the state-of-the-art second generation MgB2 wires and Nb3Sn wires developed by Hyper Tech Research Inc. The MgB2 magnet designs operated at different field strengths demonstrate excellent homogeneity and shielding properties at an operating temperature of 10 K. At ultrahigh field, the high current density on Nb3Sn allowed by the larger magnetic field on wire helps to reduce the superconductor volume in comparison with high field NbTi magnet designs. This allows for a compact magnet design that can operate at a temperature of 8 K. Overall, the designs created show promise in the development of conduction cooled dry magnets that would reduce dependence on helium.

Intrinsic detection efficiency of superconducting nanowire single photon detector in the modified hot spot model

Abstract: We theoretically study the dependence of the intrinsic detection efficiency (IDE) of a superconducting nanowire single photon detector on the applied current, I, and magnetic field, H. We find that the current, at which the resistive state appears in the superconducting film, depends on the position of the hot spot (a region with suppressed superconductivity around the place where the photon has been absorbed) with respect to the edges of the film. This circumstance leads to inevitable smooth dependence IDE(I) when IDE , even for a homogenous straight superconducting film and in the absence of fluctuations. For IDE , a much sharper current dependence comes from the fluctuation-assisted vortex entry to the hot spot, which is located near the edge of the film. We find that a weak magnetic field strongly affects IDE when the photon detection is connected with fluctuation-assisted vortex entry to the hot spot (IDE ), and it weakly affects IDE when the photon detection is connected with the current-induced vortex nucleation in the film with the hot spot (IDE ).

Correlation between in-field critical currents in Zr-added (Gd, Y)Ba2Cu3Ox superconducting tapes at 30 and 77 K

Abstract: Critical current (Ic) values of 1384 A/12 mm, corresponding to a critical current density of 12.47 MA cm−2 and a pinning force of 374 GN m−3, have been achieved at 30 K, 3 T in the orientation of field parallel to the c axis (B ∥ c) in (Gd, Y)BaCuO tapes with 15 mol% Zr addition made by metal organic chemical vapor deposition (MOCVD). These tapes show pinning force levels as high as 453 GN m−3 at 30 K. An analysis of the properties of 24–28 (Gd, Y)BaCuO tapes with 15 mol% Zr addition showed a lack of correlation between their critical currents at 30 K, 3 T (B ∥ c) and Ic values both at 77 K, zero field and at 77 K, 1 T (B ∥ c). However, a strong correlation was found between the critical currents at 30 K, 3 T and at 77 K, 3 T (B ∥ c). It has also been discovered that the minimum critical current (Ic, min) value at 77 K, 3 T has no influence on the Ic, min value at 30 K, 3 T, and it in turn depends on the ratio of the Ic values in the orientations of field parallel and perpendicular to the c axis at 77 K, 3 T.

Size-controlled spontaneously segregated Ba2YTaO6 nanoparticles in YBa2Cu3O7 nanocomposites obtained by chemical solution deposition

Abstract: We present a thorough study of the nucleation and growth processes of the solution-based YBa2Cu3O7?Ba2YTaO6 (YBCO?BYTO) system, carried out with a view to controlling the characteristics of the BYTO phase to meet the requirements for specific power applications Scanning transmission electron microscopy and x-ray diffraction have been used to characterize the BYTO nucleation and phase evolution during the YBCO?BYTO conversion At high BYTO loads (>10?mol%), the nanoparticles tend to aggregate, resulting in much less efficiency for generating nanostrained areas in the YBCO matrix, and enhancement of the vortex pinning Our experiments show that by modifying the nucleation kinetics and thermodynamics of the BYTO, the nucleation mode (homogeneous versus heterogeneous), the particle size and the particle orientation can be controlled We demonstrate that YBCO?BYTO nanocomposites with high concentration of nanoparticles can be prepared in such a way as to obtain small and randomly oriented nanoparticles (ie high incoherent interface), generating highly strained nanoareas in the YBCO, with enhancement in the vortex pinning We have also observed that the incoherent interface is not the only parameter controlling the nanostrain The Cu?O intergrowth characteristics must also be a key factor for controlling the nanostrain in future tuning of YBCO vortex pinning

The effect of lattice strain on the diameter of BaZrO3 nanorods in epitaxial YBa2Cu3O7−δ films

Abstract: An elastic strain model has been applied in an effort to understand the effect of the lattice strain on the diameter of the BaZrO3 (BZO) nanorods self-assembled into aligned arrays along the c-axis in BZO-doped epitaxial YBa2Cu3O7−δ (YBCO) thin films. The calculated elastic energy of the strained BZO/YBCO composite lattice suggests that the diameter of the nanorods is approximately independent of the doping concentration of BZO as long as the density of the nanorods is sufficiently large. An experimental confirmation was carried out using transmission electron microscopy on YBCO thin films with BZO doping varying from 2% to 6% volume concentration. The diameter of the BZO nanorods was indeed found to be approximately constant in the range of 5.2–5.9 nm. The increase of the doping concentration therefore simply leads to an increase of the nanorod density, which links directly to the matching field of the effective pinning and is consistent with the transport Jc results measured for these samples.