As at 20 May 2019

A Hybrid Trapped Field Magnet Lens (HTFML): concept and realization Hiroyuki Fujishiro, Iwate University The concept of a Hybrid Trapped Field Magnet Lens (HTFML) is described, which exploits two different characteristics of superconductors: the “vortex pinning effect” of an outer superconducting bulk cylinder, which acts as a trapped field magnet (TFM) using field-cooled magnetization (FCM), combined with the “diamagnetic shielding effect” of an inner bulk magnetic lens [1, 2]. The HTFML can reliably generate a concentrated magnetic field in the centre of the lens that is higher than the trapped field from the cylindrical bulk TFM and the external magnetizing field, even after the externally applied field decreases to zero. We predicted numerically a concentrated field of 13.5 T under an external magnetizing field of 10 T using the all-GdBaCuO cylinder and lens design using independent temperature control. We report the experimental verification of the HTFML, which could prove useful to enhance the magnetic field in a bulk NMR/MRI system to improve its resolution. Effect of Gd-211 on superconducting properties of GdBCO on a macro & micro-scale Devendra Kumar Namburi, University of Cambridge Bulk (RE-Ba-Cu-O) high temperature superconductors fabricated in the form of large single grains have great potential to trap large magnetic fields as required for several practical and technological applications. Two key parameters that strongly influence the figure-of-merit (i.e. trapped field ability ‘Bt’) in these materials are: (i) the size of single grain ‘d’ and (ii) the engineered microstructure of the material, which should contain an optimum number of flux-pinning centers (leading to large critical current density 'Jc'). As a result, Bt ∝ Jc*dIn the present work, a set of 25 mm diameter GdBCO bulk single grains of composition Gd-123+X mol% Gd-211 (X: 10-100) were fabricated and studied in detail with respect to their superconducting properties, including trapped field, levitation force and Jc. Correlations have been made between the microstructural parameters (including RE-211 content) and the superconducting properties both on a macro and micro-scale, and the results will be presented and discussed in detail. Stoichiometry inhomogeneity and intergrain connectivity in K-doped Ba122 bulks Fumitake Kametani, Florida State University, National High Magnetic Field Laboratory Full understanding of the current liming mechanisms at the grain boundaries (GBs) is the foundation for considering the effective architecture and wire form for K-doped BaFe2As2 (K-Ba122). However it is concerned that the intergrain connectivity in present state-of-art K-Ba122 is still governed by the extrinsic factors. The recent studies suggested that even very high quality K-doped Ba122 polycrystalline samples can still develop large extrinsic GB blocking factors caused by strong stoichiometric variations and oxygen segregation. Recently we could reduce the O2 and H2O concentration below 0.005 ppm and 0.05 ppm in our synthesis route, respectively, revealing K concentration at the GBs as the new chemistry issue. By using the atomic resolution analytical STEM, we extensively evaluated the nanostructural correlations to several chemical and mechanical-milling parameters during the high pressure synthesis process. In this presentation, we will discuss what parameters determine the stoichiometry homogeneity and intergrain connectivity in polycrystalline K-Ba122 bulks. Cross-field demagnetization of 2G HTS stacks for high number of cycles Anang Dadhich, Institute of Electrical Engineering, Slovak Academy of Sciences Stacks of REBCO tapes can trap large magnetic fields, being interesting for the rotor of motors and generators. However, transverse alternating fields reduce the trapped field. Here, we aim to understand the influence of all parameters in cross-field demagnetization after many cycles (above thousands) by means of numerical modeling, such as the superconductor layer thickness (1 um to 10 um), the number of

As at 20 May 2019

tapes (up to 100), and their separation. We obtain useful semi-empirical formulas for design. 2D and 3D modeling is based on MEMEP [1], using Jc(B,theta) in some cases, and agreeing with FEM. 3D modeling using the real superconductor thickness agrees with measurements. We found that for this simple configuration demagnetization is avoided for ripple field amplitudes below the penetration field of one tape, demagnetizing exponentially otherwise. Increasing the number of tapes increases the relaxation time. This work will ease the design of applications with stacks of tapes." Improved critical current density in bulk MgB2 with two pinning additions Muralidhar Miryala, Shibaura Institute of Technology We introduce the production route for preparation of high performance MgB2 bulks by a simple sintering solid state reaction technique useful for batch production. We prepared several sets of MgB2 bulks from commercial high-purity powders of Mg metal and amorphous carbon coated B using a single-step reaction process. To improve the critical currents and flux pinning, two additions were applied, namely silver and extra Mg, silver and nanoscopic diamond, silver and a low content of carbon encapsulated Boron. All samples were characterized by x-ray diffraction, SEM and superconducting performance, Tc, and Jc. The sample with Ag combined with carbon coated B exhibited the highest Jc of 5.2x105 A/cm2 at 20 K and self-field. This Jc value is twice as high as that of the pure sample, the best value reported so far sintered MgB2. Our results demonstrate a strong correlation between the microstructure achieved and the resulting pinning performance. Engineering a nano-scale dispersion of artificial pinning centres in MgB2 bulks Guillaume Matthews, University of Oxford Magnesium diboride (MgB2) superconductors have potential applications in moderate temperature and moderate magnetic field applications owing to the relatively low materials cost and ease of manufacture compared to cuprate HTS materials. Carbon doping has shown the best improvement in flux pinning and critical current density (Jc) at low temperatures. However, this comes at the cost of a reduced critical temperature which decreases Jc at higher temperature. In this study, a new and innovative approach to improve flux pinning based on the oxide dispersion strengthened (ODS) concept is developed to manufacture ODS MgB2 bulks showing significant improvement in Jc at high fields. Advanced microscopy has been used in combination with X-ray diffraction and magnetic measurements on powders and bulks manufactured using the Field-Assisted Sintering Technique and ultra-high pressure hot pressing to fully characterise the microstructure and properties of the new ODS MgB2 materials. High field superconductivity in C-doped MgB2 prepared by a rapid synthesis route Davide Matera, DQMP, University of Geneva The upper critical field (Bc2) of MgB2 polycrystals is still far below the values up to 60T measured in highly-disordered C-doped films. With the goal of filling this performance gap in view of practical applications, we explored the potential of a rapid-synthesis route. Rapid heating (up to 1000 °C/min) followed by quenching at the end of reaction was applied in order to limit the grain growth and freeze the system in configurations with high structural disorder, as in films. We prepared a series of C-doped MgB2 polycrystals by Internal Magnesium Diffusion, using a Design-of-Experiment approach to explore the synthesis parameter-space: dwell-temperature, dwell-time, ramp-rates and pressure. Based on extrapolations of resistive measurements in magnetic fields up to 19T, Bc2 values of ~25T at 10K were obtained reproducibly with distinct sets of synthesis parameters. To confirm these record-high values the characterization is being extended down to 4.2K in the 35T-facility of EMFL-LNCMI.

Progress in development of high-performance REBCO tapes and wires Venkat Selvamanickam, University of Houston

As at 20 May 2019

Combining thick films (4 – 5 µm) with optimized distribution of nanoscale defects, we have fabricated REBCO tapes with high critical currents over a wide range of temperatures and magnetic fields. Critical currents of our tapes have reached 6200 A/12 mm (engineering current density (Je) = 5200 A/mm2) at 4.2K, 15T which is 7X the Je of commercial REBCO tapes and 5.4X the Je of best Nb3Sn tapes. At 65K, 1.5T, critical currents exceeding the milestone of the U.S. Department of Energy Next Generation Electric Machines program of 1440 A/cm have been demonstrated. We have also developed a Symmetric Tape Round (STAR) wire technology to fabricate round REBCO wires 1.3 mm to 1.9 mm in diameter with Je of 600 A/mm2 at 4.2 K, 20 T. These wires exhibit excellent tolerance to bend strain, retaining their high critical currents even when bent to a radius of 15 mm. Can we reach fast growth methods for Coated Conductors at competitive cost? Teresa Puig, Institut de Ciència de Materials de Barcelona, ICMAB-CSIC HTS coated conductors have become powerful materials for high temperature but also for low temperature and high magnetic field applications. However, their cost is still too demanding for large market penetration. We need to decrease the cost/performance ratio by developing fast growth, simpler processing, higher processing yield, lower capital investment methods. At the same time, we should foster higher critical current densities, thicker films, thinner substrate and robust and homogeneous material.I will concentrate in fast growth processing and its compatibility with high current HTS films and nanocomposites. Presently, several technologies may envisage ways towards fast processing. I will discuss these initiatives and the most disruptive advancements.I will also analyze the case of transient liquid assisted growth combined with chemical solution deposition (TLAG-CSD) which enables ultrafast growth, at 100 nm/s, of nanocomposites compatible with additive manufacturing scalable techniques. Fabrication of 1 meter long multi-layered conductor for high Je Hongsoo HA, Korea Electrotechnology Research Institute In order to apply for the high field magnets such as large accelerator and fusion reactor, cable conductor made by bundling of coated conductors to increase transport current have been studied. Various types of cable conductors have been developed using coated conductors such as CORC cable, Roebel cable, TSTC and round wire. But, these cable conductors do not have higher engineering critical current(Je) to improve the performance of large devices.In this study, multi-layered coated conductors have been fabricated to increase the critical current and Je. Low c-axis strength coated conductors are used to exfoliate easily superconducting layer from buffer layer for making multi superconducting layers. Only silver layer is provided on superconducting layer to make multi-superconducting layers by diffusion bonding heat treatment. 2- and 4-superconducting layered coated conductor were made and Je were measured at 77K, 0~6Tesla. One method to improve the delamination strength of the 2G HTS Tapes Xingyi Zhang, Lanzhou University For the 2G wires, the thermal mismatch between different components and electro-magnetic force exerted in superconducting layer often cause the delamination behavior. To improve the delamination strength, we reported how different depositing temperatures of Ag layer influence the delamination strength of YBCO tapes. Using the special designed anvil method, we found that surface energy of the YBCO depends on temperature, thus different temperatures during the deposition determine the bonding force of the interface, delamination strength between Ag and YBCO increases from 4.4MPa to 100Mpa with temperatures elevated from 303K to 338K. Besides, we obtained the delamination strengths as 47Mpa of YBCO/buffer and 112Mpa of Buffer/substrate interfaces by combing anvil and XPS techniques. These results can explain the reason of discretizing data of delamination strength from other research groups. Finally we suggested that the whole delamination strength can be improved by elevating that of interface between Ag and YBCO layer. Scalable fabrication process for low AC loss striated HTS tapes Joseph Prestigiacomo, US Naval Research Laboratory

As at 20 May 2019

A scalable process has been developed to fabricate low AC loss cable from 2G HTS coated conductors with an emulated Rutherford twisted conductor topology. The process uses an indexed tape design to align separate YBCO tapes into a single tape structure making it compatible with reel-to-reel production and involves a combination of three methods: laser lithographic striation of 2G HTS tapes into a pattern of isolated diagonal filaments, the precise alignment and decal bonding of a pattern of solder preforms to the filament edge contact areas, and the alignment and bonding of a top and bottom HTS tape with a thin intervening adhesive layer that provides mechanical adhesion and electrical isolation between adjacent bonds and the interior where filament transposition occurs. The observation of a resistive critical current transition in the fully fabricated conductor indicates that all bonds were well formed and the superconducting current path was restricted to the filaments. 50m long multifilamentary coated conductor for high field magnets Anders C. Wulff, Technical University of Denmark Spatial magnetic field stability is a key parameter for many high magnetic field applications and the use of high temperature superconducting (HTS) coated conductor (CC) tape requires filamentization parallel to the tape length to mitigate time varying screening effects. In this work, we demonstrate commercial fabrication of a 50 m multifilamentary CC starting from a two level undercut-profile substrate (2LUPS) produced at Subra Substrates A/S. The CC stack (YSZ/CeO2/YBa2Cu3O7-x with a Ag capping) was deposited directly on the 2LUPS via alternating beam assisted deposition and pulsed laser deposition at Bruker HTS GmbH. Superconducting filaments self-formed during deposition of the HTS layer due to the substrate profile. Transport and magnetic properties were conducted at 77 and 4.2 K, and critical parameters were correlated to local tape geometries. Conclusively, we establish that multifilamentary HTS coated conductors suitable for high field applications can be commercially produced in long lengths using industrial ABAD-PLD-2LUPS technology.

Transient dynamic resistance waveforms from isolated ReBCO coated conductors Justin Brooks, Robinson Research Institute, Victoria University of Wellington Dynamic resistance in a superconductor, due to an oscillating magnetic field, is caused by the interaction between a DC transport current and moving fluxons1,2. This is a source of heat dissipation relevant to superconducting components such as HTS magnets and flux pumps, as well as in rotor coils of motors/generators.Here we present experimental measurements of the transient voltage response across isolated ReBCO tapes from differing manufacturers exposed to an oscillating perpendicular magnetic field. We compare these results with numerical simulations using an H-formulation model3 which incorporates measured data for the Jc(B,θ) dependence of the wire. Data has been obtained for various ReBCO tapes, and we highlight the variation in the voltage response observed between different samples. This arises from the differing Jc(B,θ) dependence of each coated conductor wire. Frequency dependent demagnetisation rate of shielded HTS tape stack Bartlomiej Glowacki, University of Cambridge This work presents results of investigation of crossed-field demagnetization in 2G high temperature superconducting stacks at temperatures in the range of ~ 77 - 20 K and in a variable frequency, corresponding to the particular rotor application.We propose a method to reduce the demagnetization rate for a given stack configuration necessary for the superconducting rotor operating at a cryogenic temperature. This technique involves 3-D wrapping the stack of tapes with perpendicular layers of similar superconducting properties.Previous ‘proof of concept’ studies documented some improvement in flux demagnetization reduction for basic configuration [1]. In the present study more advanced approach based on magnetic flux shielding [2] is adopted. The presented results provide an important contribution to development for design solutions that aim to increase the operational time before remagnetization of the stacks would be required.

As at 20 May 2019

Simulations of the Effect of Surface Roughness and Coatings on Critical Currents Alexander Blair, Durham University The critical current density (Jc) that a superconductor may carry in a magnetic field can depend strongly on the local conditions at its surface. Surface roughness is known to reduce the surface barrier for vortex entry into a superconductor relative to an ideal smooth surface and disrupt dissipationless current flow in the superconducting sheath. As a result, understanding the effect of surface roughness on Jc is necessary for optimising materials for applications. In this work, we carry out a systematic study of the effect of surface roughness on the critical current densities of thin film superconducting systems, using 2D numerical simulations based on time-dependent Ginzburg Landau theory. Results are obtained for a range of applied magnetic fields and different surface resistivities. We shall present our most interesting results. Optimization of the oxygenation process in CSD-grown REBa2Cu3O7-x films Pablo Cayado, Karlsruhe Institute of Technology The REBa2Cu3O7-d (REBCO) Coated Conductors have emerged as best candidates for high-field and power applications due to their high current carrying capacity in applied magnetic fields. Their properties are strongly determined by the oxygen content. Consequently, the study and optimization of the oxygenation process is crucial for obtaining the desired high performances.In order to optimize the oxygenation process, we have performed electrical conductivity relaxation measurements on CSD-grown REBCO films to monitor the oxygen diffusion and to obtain information about the diffusion kinetics. The oxygen diffusion in these compounds happens in two distinct regimes with different activation energies which, at the same time, differ for every RE. A detailed XRD analysis revealed that the transition temperature between these two regimes is compatible with the tetragonal/orthorhombic transition temperature of the compounds. This information was used to design an oxygenation process suitable for each compound allowing the improvement of their superconducting properties. Critical-Current Surface Ic(T,B) of Densified Ag-Sheathed Ba1-xKxFe2As2 Tapes Marco Bonura, University of Geneva Practical conductors made out of iron-based superconductors have rapidly evolved in recent years, making this technology promising for high-field applications. This work presents the first experimental investigation of the transport critical current surface IC(T,B) of densified Ba1-xKxFe2As2/Ag tapes, performed in the range of temperatures 4.2 K – TC with magnetic fields up to 19 T. We show that the uniaxial-hot-pressing technique allows increasing IC by more than ~70 %, over the entire range of fields, with respect to flat-rolled conductors. Regardless of the densification technique, the isothermal IC(B) curves exhibit a hysteretic behaviour that presents peculiarities compared to what observed in weakly-linked granular cuprates. In particular, IC significantly increases upon augmenting B from 0 to ~2 T in the parallel orientation. This feature correlates with the presence of a superparamagnetic-like background, detected by SQUID measurements, whose intensity varies in the same range of fields 0 – ~2 T. Development of MVT-MgB2 bulks assisted by combined defect detection & simulation Akiyasu Yamamoto, Tokyo University of Agriculture and Technology We report (i) synthesis and properties of dense MgB2 bulks by newly developed Mg vapor transport (MVT) process, and (ii) investigation of simple, non-destructive technique for detecting internal defects in MgB2 bulk prepared by MVT process. (i) Disk-shaped boron pellets as a precursor of the disc-shaped MgB2 bulk and the Mg source were separately placed, and pure vapor evaporated from the Mg source was transported, diffused into and reacted with boron. The packing factor of MVT MgB2 bulk is more than 80 %, which is largely improved compared to the in situ method. Jc of the sample by the MVT method is twice higher than that of the in situ method. (ii) To detect internal inhomogeneities, a method of measuring electrical resistance at surface by changing the direction of applied current and the positions of voltage terminals was examined. The potential distribution of the bulk surface with internal defects was estimated by simulation.

As at 20 May 2019

2D Strain Dependent Jc for (RE)BCO Tapes in High Fields for Fusion Applications Jack Greenwood, Durham University (RE)BCO coated conductors have the potential to replace low temperature superconductors in the superconducting magnet systems of future magnetic confinement fusion reactors. In this work, the critical current density (\(J_{\textrm{C}}\)) of a (RE)BCO coated conductor has been measured as a function of magnetic field, field angle and in-plane biaxial strain (\(J_{\textrm{C}}(B,\theta,\varepsilon_{xx},\varepsilon_{yy})\)) in fields up to 15 T to assess its suitability for fusion devices [1-3]. For the first time, we have shown both parabolic and linear strain behaviour for \(J_{\textrm{C}}\) in the same tape sample by changing the angle between the applied strain and the twin boundary orientation in the (RE)BCO layer. We will present our most recent results at the conference. Biaxial stress measurements in a GdBCO-Ag bulk during field-cooled magnetization Kai Yuan (Danny) Huang, University of Cambridge With the current state-of-the-art processing techniques and resultant superconducting properties Jc(B, T), trapped fields of over 20 T are considered achievable in (RE)BCO bulk superconductors [1, 2]. However, most samples fail mechanically well below 20 T as a result of the large tensile stresses stemming from the Lorentz force.In this study, the hoop and radial strains at multiple positions along the radius of a 40 mm diameter GdBCO-Ag bulk were measured using strain gauge rosettes as the bulk was field-cooled magnetized with applied fields ranging from 5 T to 9.6 T at 64 K. From the strains, the hoop and radial stresses were subsequently calculated. Furthermore, by including the realistic, inhomogeneous distributions of the Jc(B) and Young’s modulus, which arise from the seeded melt growth process required to fabricate a large single grain, into numerical simulations, distinct features observed in the evolution of the strains were explained.

Recent advances in high field performance of Nb3Sn conductors Chiara Tarantini, National High Magnetic Field Laboratory, Florida State University The construction of the Future Circular Collider (FCC) has very stringent requirements for Nb3Sn conductor with a target non-Cu Jc ­(16T, 4.2K) of at least 1500 A/mm2. Nowadays the best commercial Nb3Sn strands can only reach 1300 A/mm2, consequently a significantly increase of high-field Jc is needed. To meet this challenge requires we developed new Nb-Ta-Zr, Nb-Ta-Hf and other alloys to introduce additional pinning centers while maintaining a high HIrr in Nb3Sn. The introduction of a new pinning contribution more than doubles the maximum of Fp and shifts its peak from 4.6 to 5.8 T with respect to Ta-only doped wires. This leads to a layer Jc(16T, 4.2K) of about 3710 A/mm2, corresponding to a potential non-Cu Jc(16T, 4.2K) of 2230 A/mm2. We will discuss the sensitivity of HIrr and Fp to heat treatment with respect to standard Ta/Ti-doped conductors and the recent high-field characterizations of newly alloyed wires. Tube type strands with and without artificial pinning center (APC) in Hyper Tech Xuan Peng, Hyper Tech Research Inc. Hyper Tech has developed the tube type strands with and without artificial pinning center (APC). For the regular tube type strands, our standard conductors with 217-filament have non-Cu Jc of 2400-2500 A/mm2 at 12T/4.2K with filament size of 35 micros while our 547-filament conductors have non-Cu Jc of 2000-2200 A/mm2 at 12T/4.2K with filament size of 25 micros. Our Ta doped ternary APC Nb3Sn wires with ZrO2 pinning center demonstrated substantial grain refinement and significantly increased Jc,nonCu, while retaining the high Bc2 values of the best ternary Nb3Sn conductors. Their non-Cu Jcs is achieving the current CERN FCC target of 1500 A/mm2 at 16 T/4.2 K. Their layer Jc reaches 4700 A/mm2 at 16 T/4.2 K - more than double the present best ternary Nb3Sn conductors. This strand has been made to 61-filament restack strands getting filament size of 45 micros at the 0.5 mm strand. Binary and Ta-doped Nb3Sn wires with internally oxidized ZrO2 particles

As at 20 May 2019

Florin Buta, University of Geneva Nb-1wt%Zr, Nb-7.5wt%Ta-1wt%Zr and Nb-7.5wt%Ta-2wt%Zr were investigated as precursor alloys for the fabrication of Nb3Sn wires with fine grains and enhanced superconducting properties. Small diameter (0.22–0.5 mm) mono-core wires consisting of a Nb-alloy tube in contact with a metal oxide (SnO2 or CuO) and successive layers of Cu and Sn were reacted at 650 °C for durations exceeding 100 h. The addition of Ta to the Nb-Zr precursor alloy leads to an increase in the irreversibility field from 14.5 T to 15.9 T at 10 K, and a 20% improvement in critical current density (from inductive measurements) at 7 T. The internal oxidation of Zr leads to a finer grain structure than in Nb3Sn based on Nb-7.5wt%Ta, with the lowest average grain sizes being close to 50 nm. Results of critical field measurements at 4.2 K from the 35 T facility at EMFL-LNCMI Grenoble will also be presented. Achieving the FCC target with APCs: microstructural and magnetic investigation Stephan Pfeiffer, USTEM, TU Wien The Future Circular Collider (FCC) target critical current density of 1500 A/mm2 at 16 T and 4.2 K in Nb3Sn has recently been achieved through grain refinement and pinning enhancement by the introduction of artificial pinning centres (APCs). This has been accomplished by two approaches: one revolves around a novel manufacturing technique based on internal oxidation and formation of ZrO2 nano-precipitates, while the other relies on the introduction of defects by fast neutron irradiation. The microstructure resulting from both methods was investigated by means of transmission electron microscopy (TEM), where information about the nature and density of pinning sites was obtained. In addition, the grain size distribution was determined by transmission Kikuchi diffraction (TKD). A correlation with the superconducting properties was established, which were assessed by scanning Hall probe microscopy (SHPM) and SQUID magnetometry. Similarities and differences in microstructure and superconducting properties between the two approaches are discussed. Searching in the Nb3Sn Irreversible Strain Regime for Useful Information Najib Cheggour, University of Colorado Boulder, National Institute of Standards and Technology For decades, the reversible strain regime of Nb3Sn wires was investigated extensively to understand scaling of transport critical-current Ic as a function of strain, temperature, and magnetic field. With the advents of the ITER and LHC high-luminosity upgrade projects, the focus became more on the irreversible strain limit εirr to determine the factors governing it and to find ways to improve it. In this presentation, we look beyond εirr, and search in the irreversible strain regime itself. Data in this regime, de facto, had traditionally been considered worthless, only used for determining εirr. We will present a large set of irreversible data obtained on four RRP® Nb3Sn wires, doped either with Ta or Ti, and having either a standard or reduced Sn content. We will show how the irreversible degradation of Ic depends on doping, Sn content, and heat treatment, and that information contained in the irreversible strain regime is valuable. Phase and volume changes during reaction heat treatment of Nb3Sn wires Christian Scheuerlein, CERN Nb3Sn magnet coils are wound of multifilament wires that contain the ductile precursors Nb and Sn. During the subsequent Nb3Sn formation heat treatment, the volume of the pre-cursor wire changes. We compare the diameter changes of different Nb3Sn wire types (Internal Tin, Restacked Rod Process, Powder in Tube) with the phase transformations and associated void volume evolution, all measured in situ during the reaction heat treatment. Wire diameter changes were measured by dilatometry, and phase changes were monitored by differential scanning calometry and by high energy synchrotron X-ray diffraction. Unlike the Nb3Sn wire length changes, the wire diameter evolution is characteristic for each Nb3Sn wire type, and the wire diameter changes are closely related to the void space evolution examined by high energy synchrotron micro-tomography and by FIB nano-tomography. Field dependence of the irreversible stress limit in high-Jc Nb3Sn wires Carmine SENATORE, University of Geneva

As at 20 May 2019

The present magnet designs for the 16T-dipoles of the Future Circular Collider entail transverse stresses up to 150-200MPa on the Nb3Sn Rutherford cables. Assessing the stress tolerance of a full-size Rutherford cable is indeed an extremely complex and involved task. This paper presents the results of an activity of the h2020-project EuroCirCol on the electromechanical properties of high-performance Nb3Sn wires. The scope was to predict the degradation of a Rutherford cable under stress from a single-wire experiment. Tests were performed at 4.2K and 16Tcunload/Ic0, depends on the magnetic field and this implies that irreversibility is dominated by residual stresses on Nb3Sn arising from the plastic deformation of the matrix. This observation allowed us to compare and reconcile our results with the irreversible limits measured on Rutherford cables, typically tested at B

Long length, high performance HTS coated conductors for ultra-high fields Ulrich Betz, Bruker HTS GmbH Recent achievements in the fabrication and characterization of non-slit HTS coated tapes aimed for ultra-high field applications are presented. Stainless steel and Hastelloy substrates pre-coated with an Yttria-stabilized Zirconia buffer layer have been subjected to the pulsed laser deposition of YBCO followed by Ag-metallization and Cu-encapsulation. With it’s unique double-disordered (DD) YBCO superconducting films, excellent in-field performances with critical currents of ~750A/cm-width (4.2K, B//c) at 30T have been achieved routinely in 4mm and 12mm wide HTS tapes as well as in wider HTS sheets. Advanced process diagnostics has been applied when processing 4mm wide HTS tapes with a single piece length exceeding 600m to investigate Ic uniformity effects along the HTS tapes at 77K with respect to their superconducting in-field performances at 4.2K. The key PLD process features have been analyzed in a viewpoint of their influence on local and integral critical currents and resulting lift factors. Industrial scale 40mm production technology at Deutsche Nanoschicht Michael Baecker, Deutsche Nanoschicht GmbH A production technology for wide HTS tapes is the key for an industrial-scale production of HTS tapes with the best price/performance ratio. Chemical solution deposition (CSD) for the complete layer architecture provides the ultimate wide tape approach for realizing the cost reductions mandatory for large scale applications.Deutche Nanoschicht has developed a 40mm wide all-CSD technology for HTS wire production which simultaneously increases the capacity and reduces production cost. Ic(77K)>300A/cm values which are homogeneous over length and 40mm width have been demonstrated in multiple long production runs. The wide tapes are subsequently slitted and stabilized according to customer specifications.In parallel Deutsche Nanoschicht has developed a large-scale supply of nonmagnetic metal substrate tapes suitable for AC applications with unprecedented lot sizes up to 10t. Combined with in-house know-how for designing and constructing industrial manufacturing and measurement devices these developments complete a unique setup for production readiness at Deutsche Nanoschicht. Recent progress of 2G HTS wires at Fujikura Masanori Daibo, Fujikura Ltd Fujikura has developed 2G HTS wires for a few decades.We introduce the typical properties of Fujikura's conventional 2G HTS wires. Furthermore, we introduce the recent progress of 2G HTS wires, which wires with artificial pinning technique, and their in field Ic, mechanical properties and the topics of delamination stress. Progress in Mass-production and R&D for 2G HTS Wires in Shanghai Superconductor Yue Zhao, Shanghai JiaoTong University Shanghai Superconductor Technology Co., Ltd. (SST) as an emerging 2G-HTS manufacturer is aiming to contribute significantly to the wire availability on the market. In this paper, we introduce the mass-production and R&D results in SST. Based on the PLD+IBAD technique, we have successfully fabricated km-

As at 20 May 2019

class high-performance superconducting tapes. For power applications, average Ic value of the 500 meter long tape is higher than 500 A/cm (S.D less than 10%) at 77 K, s.f.. In order to achieve higher critical engineering current (Je), 30 mm thick C276 Hastelloy substrate was used, and 4 mm wide tapes with 60 mm thick in total are obtained. And Je value of the tape is over 2.3 kA/mm2 at 4.2 K, 10 T. Electricomechanical properties of the tape were also be characterized. This work demonstrates the high potential of using such high Je 2G HTS wires for high- field magnet and high-current conductor applications. Present Directions for 2G HTS Wire Development at SuperOx Alexander Molodyk, SuperOx Many 2G HTS wire manufacturers focus their efforts on better satisfying the demands of specific wire applications and addressing common issues, for instance, improving reproducibility and mechanical strength.Key development directions at SuperOx are: (1) to increase the wire Ic at liquid nitrogen temperature in self-field beyond 800 A/12 mm for application in FCL and cables, and (2) to increase the Je at low temperature in high magnetic field for application in magnets, in particular, to go beyond the Je of 700 A/mm2 at 20 K, 20 T.We adopt into production the approaches successfully demonstrated at lab-scale, such as: to increase the HTS layer thickness with minimum degradation of Jc, to modify the HTS layer composition for enhanced pinning, to use thinner substrate for higher Je, and to use laser slitting instead of mechanical slitting for better reproducibility and mechanical properties of narrow wire strips. Progress of 2G HTS (RE)BCO Conductor Development at SuperPower Drew Hazelton, SuperPower Inc. Developments in 2G HTS conductor performance continue to drive the design and operating limits for a broad range of demanding applications. The design, testing and fabrication technology of 2G HTS (RE)BCO conductors is presented, highlighting the ability of 2G HTS wire to function under a wide range of operating conditions. SuperPower continues to address 2G HTS conductor development and production methods to improve characteristics and performance of the wire and provide technical support in its use. In particular, extensive studies on wire properties have been carried out and processing upgrades implemented to improve both the base performance of the conductor, as well as its functionality by enhancing key characteristics such as piece length, mechanical properties and uniformity of critical current and performance in magnetic fields. Updated measurements on recent production material are presented and plans for future performance targets discussed 2G HTS Coated Conductors: Process Control Improvements at STI Jeong-Uk Huh, superconductor Technologies Inc Superconductor Technologies Inc. is manufacturing 2G HTS coated conductors using RCE-CDR of ReBCO films for various industrial applications. STI has improved the yield of the production process through enhancing repeatability of the deposition chamber. HTS electrical performance is highly dependent on film growth temperature. However, it has been challenging to achieve uniform thermal profile throughout the large drum that holds 1 km x 10 mm tape. STI has implemented a high precision temperature monitoring tool which can control the multi-zone heaters to achieve highly uniform process temperatures.STI has increased the critical current by optimizing flux pinning properties. Extrinsic pinning has been tested with a variety of dopants in the film. Also intrinsic flux pinning has been enhanced through HTS composition optimization; varying the growth conditions to deposit mixed axis growth and growing thick HTS films (> 6um). STI has produced Conductus™ coated conductors carrying minimum critical current of 470 A/cm at 65K and 1.5T. Recent improvements of ReBCO wire production at THEVA Werner Prusseit, THEVA Dünnschichttechnik GmbH THEVA is a producer of second-generation HTS wire using a unique all PVD approach. Up to 500 m long wires are manufactured at our pilot production plant in Germany routinely. Driven by the request of customers in the high field magnet area, we recently developed a new type of CC on 50 µm thick substrates. Switching our production from previously 100 µm to 50 µm thick substrates was possible

As at 20 May 2019

without changes to the equipment and the performance on the thinner substrates is already very similar to the one on thick substrates. Taking advantage of the broad PVD coating experience within THEVA a new PVD copper plating process was developed, too. Typical for PVD processes, a very high uniformity of the thickness of the copper layer is achieved. We will give an overview on the wire properties including most recent results.

As at 20 May 2019

Broad Studies for Applications Wide range studies of Jc(H,T,angle) in Bi-2212 and REBCO coated conductors. David Larbalestier, The National High Magnetic Field Laboratory, USA Quench protection of HTS magnets is significantly more troublesome than for LTS magnets and one of the issues of greatest importance is to have a predictive understanding of Jc(H,T,angle), especially for REBCO magnets where the temperature domain of interest even in fields of 30 T extends up to almost 70 K. We have recently been measuring many kinds of REBCO and the much simpler round wire Bi-2212 which is macroscopically isotropic. A recent study of Bi-2212 shows that even samples with Jc varying by more than a factor of 4, have the same functional dependence of Jc(H,T). REBCO is much more complex because of the huge variation in pinning mechanisms and the very anisotropic electronic and vortex pinning it contains. We will present our studies of these matters in forms useful for those interested in vortex pinning and magnet design and protection. The Magnetization of HTS cables for Particle Accelerator Applications Mike Sumption, The Ohio State University High temperature superconductors, such as YBCO, are being considered for high magnetic field magnets to be used in particle accelerators. Here we present analytic and FEM modelling of HTS coated conductor cables and compare these results to measurements of HTS CORC and Roebel cables. Measurements were made with two devices, (i) a hall probe type device (short sections of cable at 4.2 K and up to 12 T), and (ii) a 3 T dipole susceptometer (longer segments of cable at 4.2 K to 3 T). In addition, the time decay (creep) of the magnetization was also measured for these cables. Overall, the work indicates that a significant amount of both static and dynamic field errors will be present in precision magnets wound with such conductors, requiring some consideration of mitigation schemes. Design, Performance and Cabling Analysis of Nb3Sn Wires for the FCC Study Simon C. Hopkins, CERN The conceptual design for the Future Circular Collider (FCC) envisages a staged programme implemented in a new 100 km tunnel: firstly, an intensity-frontier lepton collider (FCC-ee), followed by construction of a hadron collider (FCC-hh) with 100 TeV centre-of-mass energy. This energy-frontier machine is based on 16 T Nb3Sn dipole magnets. CERN is coordinating a conductor development programme aiming to obtain Nb3Sn wire with a non-copper critical current density of 1500 A mm-2 at 16 T and 4.2 K, in lengths suitable for manufacturing 15-metre-long magnets, and able to withstand cabling without significant degradation. Here we report the superconducting characterisation and quantitative microscopy of recently-developed Nb3Sn wires with novel layouts and compositions, and evaluate their suitability for Rutherford cabling based on cabling trials and rolling studies. An analysis of the influence of wire layout, materials and mechanical characteristics on cabling performance is presented, to support recommendations for future wire designs. Coated Conductor technology for the beam screen of CERN FCC-hh Joffre Gutierrez Royo, Institut de Ciencia de Materials de Barcelona - CSIC Within this consortium we have identified the possibility of using high temperature superconductors, like REBa2Cu3O7-x coated conductors (CCs), as potential candidates to substitute Cu in the FCC beam-screen. We are committed to develop knowledge of CCs and their performance under the extreme conditions found in the FCC-hh, including fields of 16 T, high RF currents and high loads of synchrotron radiation.We present the consortium goals and identified challenges for using CCs’ in the FCC-hh, and the promising results already obtained on commercially available CCs. We have found that the surface resistance of REBa2Cu3O7−x at 50 K and up to 9 T is lower than that of copper and we have found a compatible coating that decreases the secondary electron yield of REBa2Cu3O7−x close to unity, a mandatory requirement for

As at 20 May 2019

the beam-screen of the FCC, showing the potential for CCs to substitute copper as the beam-screen in circular colliders. Development of REBCO tapes on dielectric flexible substrates for RF applications Jarek Wosik, University of Houston Technology of epitaxial REBa2Cu3O7-x thin films grown by metal organic chemical vapor deposition (MOCVD) on buffered nickel alloy substrates is well developed for dc applications. However, because of the metallic substrate used, these tapes are not suitable for radio-frequency (RF) applications.1 In this work we report on the development of a method to fabricate high-quality REBCO tapes on dielectric substrates. This method is based on direct growth the REBCO film on a flexible 40 μm thick low RF loss and low thermal conductivity YSZ substrate2. Using an intermediate buffer layer of MgO made by Ion Beam Assisted Deposition (IBAD), we have grown epitaxial REBCO films on flexible YSZ tapes. We will show examples of REBCO/YSZ flexible tapes used to make a magnetic resonance imaging (MRI) 300 MHz high-Q ring resonators for rat/mouse brain imaging integrated with tuning/matching electronics.3 Signal-to-noise ratio (SNR) gain from using superconducting ring resonators over conventional state-of-the art copper MRI resonators will be shown and discussed. Durham’s Contribution to ITER’s TF and PF Strand Verification Process Mark Raine, Durham University Beginning in 2011 the Durham Reference Laboratory performed verification measurements on Nb3Sn strands for ITER’s toroidal field (TF) coils, prepared and measured Witness samples (used for additional pancake coil heat-treatment quality checks) and performed verification measurements on Nb-Ti strands for poloidal field (PF) coil 6. In 2018, and over 13,000 measurements later, this work was successfully completed. In this talk we present an overview of Durham reference laboratories’ role in the ITER project and provide summary and statistical analysis of the numerous test results – along with manufacturer data (where available). We describe how we maintained high quality measurements and large quantity throughput. We expect that similar reference laboratories will be required as more large superconducting applications are built - including fusion energy tokamaks. We will also provide some useful hints for anyone else who ventures on setting up a reference laboratory for large numbers of superconducting measurements.

Coated Conductors - Properties Examining vortex creep via electrical transport measurement of coated conductors Mayraluna Lao, Karlsruhe Institute of Technology The use of coated conductors (CC) for applications is moving towards operation below 77 K. CC manufacturers are improving the in-field properties of their tapes by engineering the pinning landscape. However, not all defects are effective in increasing the critical current, Ic, over all temperatures. To optimize the pinning properties for Ic-enhancement, vortex creep must also be well-accounted for. In this work, we show that vortex creep can be reliably investigated in CCs through the exponent of the transport V(I)-curve, the N-value. Although the N-value is influenced by inhomogeneities and sample heating, its overall behavior reflects vortex creep as the temperature, field and orientation of the tape are varied. The transport measurements are complemented by magnetic relaxation data obtained from superconducting quantum interference device magnetometer. This study gives more insight on the pinning mechanism especially at intermediate temperatures, which aids in the development of commercially produced CCs. Analysis of angular dependent Jc of REBCO coated conductors with APCs Kaname Matsumoto, Kyushu Institute of Technology In current REBCO coated conductors, it is necessary for the effective control of Jc to introduce linearly grown nanorod-APCs. When growing slowly, nanorods grow linearly, but in high-speed deposition,

As at 20 May 2019

nanorods tilt or break up, then, various variations occur in angular dependent Jc. In this study, when forming BHO-containing REBCO thin films, two types of samples with different film growth rates were prepared, and the angular dependence Jc of these samples were examined under 4-40 K and 3-25 T. According to the results, the influence of the nanorods is strong near B||c, but with any sample, intrinsic pinning dominates near B||ab. In order to understand these behaviors, the stable structure of the quantized vortices was determined from the viewpoint of the energy minimum, and then Jc obtained by applying Lorentz force in this state could well explain the experimental results. The presentation will report these details. Pinning Properties of REBCO-CCs with Domain Control by High-T-Bending Annealing Tatsunori Okada, Institute for Materials Research, Tohoku Univ. It is well known that REBa2Cu3O7-δ coated conductors (REBCO-CCs) show a complicated strain dependence of critical current density, Jc(T,B,θ,ε), due to the coexistence of domains where a- or b axis is aligned in the longitudinal direction (a twin structure) [1]. In this study, as a part of studies to understand such a complicated Jc(T,B,θ,ε) of "twinned" REBCO-CCs, we tried to control the ratio of domains by annealing under a uniaxial bending strain applied at high temperature, and investigated Jc(T,B,θ) of "domain-controlled" REBCO-CCs.We succeeded in obtaining REBCO-CCs with various domain ratios (maximum domain ratio is about 75% at present) and observed that Jc(θ~B//c) at high temperature becomes lower as the domain control progresses. This suggests that the reduction of twin boundaries by the domain control resulted in a weakening of pinning by twin boundaries.In the presentation, we would like to report more-detailed information described above. Hot spot creation in coated conductors used for fault current limitation Fedor Gömöry, Institute of Electrical Engineering, Slovak Academy of Sciences Coated conductors used in resistive fault current limiters use only few micrometers of silver as metallic stabilization. However industrially produced tapes usually exhibit a fluctuation of critical current along their length. Then the concentration of voltage and heat release on short quench sections can lead to dramatic local heating and burning of the tape.Multi-tap measurement system has been used to map the evolution of electric field along the tape during DC transport. These experiments confirmed huge differences in the local electric field values as consequence of Ic fluctuations. As a complement to experiments the numerical model combining electrical, thermal and circuit equations has been developed in the Comsol Multiphysics environment. The difference in the temperature of the hot spot and the rest of the tape is used to distinguish the hot spot regime from the complete quench of the whole tape. We analyse how the low-Ic region and the tape architecture affect the evolution of hot spots. Ic properties of D-Nano HTS production wires at variable T, H, and angle Ron Feenstra, Deutsche Nanoschicht GmbH Deutsche Nanoschicht (DN) has recently completed a transition from 10 to 40 mm wide production platform for HTS wire manufacturing. DN employs CSD techniques for all layers of the wire architecture, including buffer, YBCO, and Ag contact layer. Here we report Ic properties of 40mm-w production tapes in the range 25 K to Tc and magnetic fields H to 5 T. Significantly, Ic properties on the 10mm-w and 40mm-w platforms coincide over wide ranges of temperature, field, and field angle. These properties include an Ic(77.3K, sf) of 350-400 A/cm, which increases to 2500-3000 A/cm at 25 K for H=0. The correspondence, which was a desired outcome of the upscaling, is nonetheless remarkable considering the implemented process changes. These and other comparisons attest to a degree of robustness for the in-field Ic properties of CSD YBCO tapes. In-field performance of long-length DD-YBCO coated conductors at 77 and 4.2 K Johannes Gnilsen, Bruker HTS GmbH, TU-Wien Coated conductors with double disordered (DD) YBCO structure have not demonstrated any typical correlation of critical currents at 77 K in self-field and at 4.2 K, 5 T (B//c). This study is aiming at exploring a feasible characterization concept for long length tapes that “predicts” helium temperature behaviour from

As at 20 May 2019

measurements at 77 K. Tape samples have been characterized at 77 K in (i) homogeneous fields up to 6 T, in setups with (ii) different assemblies of permanent magnets within 0.5-2 T and (iii) a pulsed split 3T-electromagnet. Field dependences of critical currents and characteristic n- and alpha- values were evaluated at 77 and 4.2 K in fields 0-6 T and 3-30 T, respectively. “Zooming” effect that gains sensitivity to tape defects occurring by characterization at highest field zone was found and studied. Field dependence of differently defined lift factors is investigated and further opportunities for tape characterization are demonstrated. 2G HTS wire with high Je at low temperature in high magnetic field Anton Markelov, SuperOx Recent practical activities in fusion systems with high field HTS magnets drive the development of commercial 2G HTS wire with improved engineering current density, Je, at low temperature in high magnetic field.The three commonly recognised approaches to enhance Je, best when used together, have been to (1) increase the HTS layer thickness, (2) reduce the substrate thickness, and (3) improve pinning in the HTS layer.In this talk we will report SuperOx’s progress along these three paths towards making high Je wire a commercially available product. In particular, whereas the previous standard production SuperOx wire exhibited average Je at 20 K, 20 T of about 300-350 A/mm2, the newly developed product achieves over 700 A/mm2.

YBa2Cu3O7-x film with Ba2Y(Nb,Ta)O6 nanoinclusions for high field applications Giuseppe Celentano, ENEA, Frascati Research Centre Structural and transport properties of YBa2Cu3O7-x films obtained by pulsed laser deposition with incorporated Ba2YTaO6 and mixed Ba2YTaO6 and Ba2YNbO6 (Ba2Y(Nb/Ta)O6) double perovskite secondary phases are investigated in an extended temperature and magnetic field range. The effects of the different doping and the film growth kinetics on the microstructure were studied. The Ba2YTaO6 truncated nano-sized columns are modified into continuous columns in case of mixed Ba2Y(Nb/Ta)O6 incorporation. High growth rates promote a peculiar microstructure characterized by the formation of Ba2Y(Nb/Ta)O6 splayed columns decorated by Y2O3 nanoparticles, very effective for flux pinning in a wide temperature and magnetic field range. Based on both dc and microwave measurements, the vortex pinning behaviour of Ba2Y(Nb/Ta)O6 films are discussed and compared with the one of BaZrO3 doped films showing the high potentiality of mixed Ba2Y(Nb/Ta)O6 inclusions in the perspective of the applications of REBa2Cu3O7-x coated conductors in a broad range of conditions. Effective Pinning Angular Range by 1D-APC inYBa2Cu3O7-xNanocomposite Films Judy Wu, University of Kansas One-dimensional artificial pinning centers (1D-APCs) in YBa2Cu3O7-x nanocomposite films provide strong collective pining at magnetic field H//c-axis. In this work, we reveal a direct impact of the 1D-APC/YBa2Cu3O7-x interface on the pinning efficiency of individual 1D-APCs. Through engineering the 1D-APC/YBa2Cu3O7-x interface to minimize the interfacial strain induced defects, we have obtained a record high pinning force density Fp,max ∼ 182.0 GN m−3 at Hmax > 9.0 T (H//c-axis) and 65 K in 1D-APC/YBCO nanocomposite. In addition, we have also revealed the effective angular range of the 1D-APCs when H is away from H//c-axis (q=0) correlates to their pinning efficiency. By plotting Fp,max, Hmax and a values normalized to those of the undoped YBa2Cu3O7-x film as functions of q, we show most 1D-APCs can provide enhanced Hmax over q =0-60° while only those 1D-APCs with high individual pinning efficiency can have normalized Fp, max > 1 over an angular range >40o at 65-77 K. Pinning enhancement in CSD YBCO nanocomposite films with preformed nanocrystals Javier Díez Sierra, Ghent University

As at 20 May 2019

Bimetallic oxide nanocrystals with perovskite structure and 2-10 nm in diameter were synthesized via microwave-assisted heating of bimetallic alkoxide precursors, stabilized in polar solvents and added to the YBCO precursor solution. Superconducting YBCO nanocomposite films deposited on LaAlO3 single crystal substrates with 300 nm thickness and Jc of around 5 MA/cm² were obtained. These nanocomposites showed a smoother decay of the Jc with increasing magnetic field and the pinning force was increased by a factor of 4-5 compared to undoped samples.The challenging upscaling of these results to long industrial tapes was carried out as a part of the Horizon2020 project SynFoNY (www.synfony.eu). Different types of coated conductor nanocomposites were deposited for the first time in full-CSD process in up to 100 m long tapes reaching high self-field currents at 77 K comparable to undoped commercial tapes and showing a reduced decay of the Jc with an increasing magnetic field. Pinning enhancement in CSD-REBCO-nanocomposites towards high-field applications Manuela Erbe, Karlsruhe Institute of Technology (KIT) Chemical solution deposition (CSD) is a cost-effective method for the production of long REBCO-based tapes and allows an easy adaption of the CC performance to the demands of an application by simple modifications of the precursor solutions, e.g. through RE mixing or secondary-phase addition for pinning enhancement. We present recent advances on performance improvements of CSD-grown REBCO films. We investigated several single-RE-BCO compounds on different substrates to clarify their Tc values when grown by CSD, to find the most-suitable candidates for high performance at low temperatures and to develop a deeper understanding of their processing windows (Tgrowth, p(O2)). All systems have been thoroughly investigated as pristine phases but also as BHO-nanocomposites for further pinning enhancement. A comparison of the resulting physical (Tc, Jc(B)T, Jc(Θ)B,T) and a correlation to their structural properties (XRD, SEM) will be discussed. The most-promising candidate is DyBCO+BHO with Fp(30 K) > 200 GN/m³ beyond 14 T. New opportunities to enhance vortex pinning in solution derived YBCO thin films Xavier Obradors, ICMAB, CSIC Creating artificial pinning centers in superconductors is a key issue to achieve high critical currents which is being intensively investigated for YBa2Cu3O7 (YBCO) coated conductors. Much progress has been recently made through Chemical Solution Deposition (CSD) by using preformed BaMO3 (M= Zr, Hf) nanoparticles where the size can be better controlled. In this presentation we will show how to use these preformed nanoparticles to prepare YBCO nanocomposites following two novel approaches: a Flash Heating (FH) BaF2 route and the Transient Liquid Assisted Growth (TLAG). In the case of FH the nanoparticles remain randomly oriented while in TLAG they are essentially epitaxial. This modified nanostructure, therefore, generates a new opportunity to deeply analyse the key influence of defect structure on vortex pinning of nanocomposite superconductors. Atomic-scale aberration-corrected STEM and angular dependent in-field transport measurements have enabled us to underpin the most favourable pinning centers for each temperature and magnetic field. Vortex pinning improved by stacking faults in YBCO films containing nanorods Tomoya Horide, Kyushu Institute of Technology Nanorods significantly improve critical current density (Jc) in YBCO films, and further structural control is needed to realize the Jc higher than that in the films. One possible strategy is to combine the nanorods with atomic scale lattice defects. In the present study, long stacking faults spreading across nanorods over 500 nm were formed in the YBCO films containing nanorods, which were prepared using pulsed laser deposition. X-ray diffraction, high angle annular dark field scanning transmission electron microscopy, and density functional theory calculation showed that atomic plane shift along the b-axis at stacking faults varied the interface structure of nanords [1]. As a result of this structural variation, the stacking faults improved the Jc in low temperatures and suppressed the Jc anisotropy. The formation mechanism of stacking faults and the vortex pinning mechanism will be discussed to control the elementary pinning force and hybrid pinning.

As at 20 May 2019

Landscape tuning of PLD YBCO-BYNTO films for improved vortex pinning efficiency Francesco Rizzo, ENEA Among the nano-engineered artificial pinning centres (APCs) introduced for improving the flux pinning capabilities of YBa2Cu3O7-x (YBCO) films, in recent years several studies pointed out the efficiency of Nb and Ta based double-like perovskite structures. Ba2YTaO6 (BYTO) and Ba2YTaO6+Ba2YNbO6 (BYNTO) compounds showed outstanding pinning features which extend to the mid-to-low temperature and mid-to-high magnetic field regime of the T-H phase diagram [1]-[2]-[3].In this work we study the landscape tuning of pulsed laser deposited YBCO-BYTO and YBCO-BYNTO films by acting on easily adjustable deposition parameters (i.e. growth temperature and laser repetition rate) and focusing on the fast growth rate regime. The resulting peculiar pinning landscapes are characterized by extended secondary phase nanocolumns (1-d defects), rare-earth oxide nanoparticles (3-d defects) and local oxygen vacancy (0-d defects). Corresponding transport properties, evaluated by means of in-field and angular D.C. electrical measurements in the temperature range from 77K down to 4.2K, show remarkable features.

Ten years of Fe-based Superconductors: are they living up to early expectations? Valeria Braccini, CNR - SPIN Fe-based superconductors (FBS) appeared very promising for applications at 4.2K and high fields since the beginning. Yet, such optimistic expectations need a boost from technological developments to become true. Two technologies are advancing towards realistic applications, namely Powder-In-Tube (PIT) and Coated Conductors (CCs). PIT tapes of the 122 family attained critical current densities Jc of 105A/cm2 at 15T with hot pressing, while CCs on commercially available IBAD and RABiTS templates showed Jc up to 105A/cm2 at 30T for both 122 and 11 families. Current efforts are focused on the development of scalable techniques for PIT conductors and the identification of simpler substrate architectures for CC. In this scenario, the ten years from the discovery of superconductivity in FBS are too short a time for a balanced answer to the question of the title. I will present the state-of-the-art of FBS conductors with an open-minded focus on challenges, results and expectations. Jc in Fe-based superconducting films: dimensionality, anisotropy, microstructure Jens Hänisch, Karlsruhe Institute of Technology In the past decade of research in Fe-based superconductors, films in 5 different crystal classes, commonly called 11, 1111, 122 as well as 111, and 11111 according to their stoichiometry have been realized. These films are investigated both for fundamental investigations of the superconducting state and regarding possible applicability in microelectronics [1] or even as coated conductors [2] due to their superconducting properties, which nevertheless also depends on other factors like competition to other superconductors, acceptance of poisonous elements, manufacturing costs, stability etc.This talk will give an overview of recent studies on growth mechanisms and electrical transport properties of Fe-based superconducting films with focus on possible tunability of pinning forces and anisotropic critical current densities by microstructure and dimensionality as well as on fundamental aspects of the vortex matter in these systems. Exemplarily, data on Ba(Fe,Co)2As2, NdFeAs(O,F), and (Li,Fe)OHFeSe films will be discussed. Improvement of the critical current of NdFeAs(O,F) by vicinal substrates Kazumasa Iida, Nagoya University LnFeAs(O,F) (Ln=Sm or Nd) has the highest superconducting transition temperature Tc(~58 K) among the Fe-based superconductors. Hence a lot of efforts have been devoted to improving the critical current density Jc of this material for conductor applications. However, Jc-H properties of LnFeAs(O,F) reported so far is inferior to those of doped BaFe2As2 (Ba-122) systems. Here, we report on a successful improvement of Jc for NdFeAs(O,F) by using vicinal MgO and CaF2substrates ([001] direction is tilted 5° toward the [100] direction). The self-field Jc exceeded 10 MA/cm2 at 4.2 K, the highest value ever reported for Fe-based superconductors. Additionally, Jc-H properties measured at 10 K for H||c were almost comparable to that

As at 20 May 2019

of the pinning enhanced Co-doped Ba-122 at 4.2 K [1]. An asymmetric behavior of the angular dependence of Jc(Jc(\(\theta\))) was observed, and the angle of the maximum Jcdepended on both temperature and applied magnetic field. The relationship between the asymmetric Jc(\(\theta\)) and the microstructure will be discussed in the talk. Superconductivity at 48 K in heavily hydrogen-doped SmFeAsO epitaxial thin films Jumpei Matsumoto, Tokyo Institute of Technology SmFeAsO exhibits the highest Tc of 55 K among Fe-based superconductors through electron doping by replacing O sites with F [1] or H [2] ions. Although F-doping concentration is limited to Tc superconductivity in 1111-type iron-oxypnictide compounds. However, the detailed researches on Fermi surfaces e.g., using angle-resolved photoemission spectroscopy, have been a challenging issue because of technical difficulty in fabrication of large-size single crystals. In this study, we successfully fabricated H-doped SmFeAsO epitaxial thin films via chemical topotactic reaction between undoped epitaxial films and a binary hydride, and examined their superconducting properties. The resulting H-doped SmFeAsO epitaxial films exhibited superconductivity at 48 K. This is the first demonstration of superconducting H-doped 1111-type epitaxial thin films. Influence of epitaxial strain on the phase diagram of Ni doped BaFe2As2 films Ruben Hühne, IFW Dresden Biaxial strain induced by epitaxial growth influences the electronic properties of iron based superconductor thin films and therefore enables studying the interplay between structural and functional properties as well as opens new opportunities for optimization. It has been shown, that the electronic phase diagram for Co-doped BaFe2As2 films is shifted with the induced strain to lower or higher doping content depending on the strain direction [1]. We investigated now if similar effects occur for nickel doped films.Accordingly, Ba(Fe1-xNix)2As2 thin films with different Ni content were deposited on CaF2 and MgO single crystalline substrates. Whereas all films are grown epitaxially, the lattice constants showed a distinct dependence on the substrate used. The observed maximum Tc of 21.4 K on CaF2 is among the highest values reported for the Ni doped system. Finally, we will discuss the electronic phase diagram in comparison to single crystals and Co doped BaFe2As2 thin films. Microwave properties of Fe(Se,Te) films in magnetic field: pinning and flux flow Nicola Pompeo, Università Roma Tre Microwaves are a powerful tool to access fundamental parameters of vortex physics, such as the flux flow resistivity, the Labusch parameter (pinning constant) and the depinning frequency [1]. HTC [1] and LTC [2] superconductors have been extensively studied, but no report exist on Fe(Se,Te) despite its interesting perspective for applications. We present here a microwave (10 GHz range) investigation of the complex surface impedance in a dc magnetic field (up to 1 T) of Fe(Se,Te) thin (~200 nm) films, for 5 K T Tc. We derive the vortex parameters: the flux-flow resistivity yields information on the dynamics of the quasiparticles, the Labusch parameter yields a measure of the steepness of the pinning potential wells, and the depinning frequency assesses the frequency range where the material is suitable for high-frequency applications in a dc magnetic field. We compare the results to the data obtained in YBCO, Nb and Nb3Sn.

As at 20 May 2019

Absence of Weak-Link Signature in Severely Underdoped Bi-2212 Round Wires Yavuz Oz, Applied Superconductivity Center - NHMFL - FSU Recently, densification of filaments has enabled very high Jc in Bi-2212 round wires, long thought to lack the macroscopic texture cuprate tapes require. One explanation is that high Jc is enabled by Bi-2212’s ~15° FWHM quasi-biaxial texture. A second hypothesis is that Bi-2212 can be significantly more overdoped than other cuprates, potentially allowing for more transparent grain boundaries. To elucidate these conjectures, we compared the properties of standard, optimally processed Bi-2212 round wires after progressively underdoping them. Transport Jc, hysteretic magnetization and Hirr all diminished with underdoping. Surprisingly, significant Jc-B hysteresis, a weak link signature, was absent in all samples. We conclude that Bi-2212’s biaxial texture enables growth of large grains that easily merge into each other, facilitating both ab-plane and c-axis grain-to grain supercurrent flow. The much larger grain size of Bi-2212 compared to Bi-2223 enables high c-axis Jc, removing the weak link evidence common in all Bi-2223 conductors. Effects of precursor powder on the performance of Bi-2212 round wires Jianyi Jiang, National High Magnetic Field Laboratory, Florida State University Bi-2212 round wire made by powder-in-tube technique is a very promising conductor for high field NMR and accelerator magnets. Critical current density (JC) of Bi-2212 wire has been improved significantly in the past few years. Short samples of wire made with recent nGimat powder showed JC(4.2K,15T) = 6640 A/mm2 and engineering critical current density JE(4.2K, 15T) = 1320 A/mm2 with overpressure processing. An essential component of Bi-2212 wire is the precursor powder because it affects the wire performance significantly. Bi-2212 powder is being further developed by US companies, nGimat and MetaMateria. We are investigating Bi-2212 powders with different compositions made by MetaMateria and nGimat, and are doing overpressure heat treatments (OP-HT) on multifilamentary round wires and monocore tapes made from these powders by Bruker OST. Details of the performance improvements in powder, wire and heat treatment in recent Bi-2212 wires will be presented. Could we do without the OP process? Bi-2212 wire development at CNR-SPIN Andrea Malagoli, CNR-SPIN The Over Pressure (OP) process led to the realization of Bi-2212 wires with Jc performance well beyond the minimum application requirements. While several efforts are under way to demonstrate that it is possible to apply such a process to real coils, researchers at CNR-SPIN are developing a process based on mechanical deformation (the GDG process) to realize denser Bi-2212 wires with superconducting properties satisfying application requirements through an easily industrially scalable process. Initial evidence of the effectiveness of the process has already been reported, but a demonstration on longer wires is needed. Here we present a comparison between the Je results obtained for short and long samples wound on different barrels. We also report a detailed analysis of the evolution of microstructural properties during the partial-melt process to obtain insights to support Jc improvement. Finally, we address the question of whether the OP process is actually needed. Strategies for Improving Ic of Superconducting Joint Connecting Bi2223 Tapes Jun-ichi Shimoyama, Aoyama Gakuin University Recently, our group succeeded in the fabrication of superconducting joints connecting Ag-sheathed Bi2223 multi-filamentary tapes with high Ic ~100 A at 77 K and ~400 A at 4.2 K [1] using Bi2223 thick films as the intermediate layer. These high Ic values indicate that superconducting magnets with the persistent current circuit can be designed using Bi2223 tapes, however, further enhancement of joint Ic is needed for development of the magnets applicable at high temperatures and/or operated under very large current.

As at 20 May 2019

Based on the background, improvement of critical current properties of intermediate Bi2223 layer are attempted in this study by careful control of chemical compositions mainly through post-annealing processes and grain alignment of Bi2223 crystals by magneto-scientific method. The latter is a quite simple method, in which only a permanent magnet (< 0.4 T) and ethanol were used. Development of MgB2 superconductor wire and coils for AC and DC applications Mike Tomsic, Hyper Tech Research Hyper Tech Research will report on progress that has been made on developing magnesium diboride superconductor wires, coils and magnets for commercialization efforts, with a specific emphasis on conduction cooled MRI and AC motor/generator applications. Development of high-Jc MgB2 wires fabricated from mechanically millied powder Motomune Kodama, Research & Development Group, Hitachi Ltd. Mechanical milling of magnesium and boron powders generates precursor particles with metal-matrix-composite structure, in which boron particles are dispersed in a magnesium matrix. We found that coronene addition and roll drawing caused the precursor particles to deform effectively in cold working. As a results, we successfully manufactured iron-sheathed monofilamentary MgB2 wires with lower porosity and higher critical current density, Jc, than typical in situ-processed MgB2 wires [1]. Now, we have been developing copper-stabirized monofilamentary and multifilamentary wires. Their Jc was almost equivelant to that of iron-sheathed monofilamentary MgB2 wires. The uniformity of critical current, Ic, was evaluated over the length of an 100-meter-long coppre-stabilized monofilamentary wire with a diameter of 0.78 mm: the average Ic(4.2 K, 7.5 T) was 141 A, the minimum Ic was 125 A, and the standard deviation of Ic was 6.6 A. Reverse AIMI to produce dense MgB2 bulks and high Jc wires through HIP treatment Gianmarco Bovone, SPIN Institute Recent improvements in the Advanced Internal Magnesium Infiltration (AIMI) technique have achieved very high Jc at high magnetic fields compared to conventional Powder-In-Tube (PIT) methods. Here we report the development of an inverse architecture to the AIMI route, replacing a Mg rod surrounded by B with a Mg tube filled with B, inside a Ti sheath. This permits an easier tube-filling procedure and avoids issues with Mg rod centring. Heat treatment yields a very dense MgB2 core with a Jc exceeding 106 A/cm2 at 20 K in self-field, with good in-field behaviour. For ambient pressure reaction, MgB2 core is detached from the sheath due to volume reduction, efficiently synthesising long MgB2 bulks. High-pressure heat treatment prevents this detachment, creating a wire with high core density and connectivity, and enabling transport Ic measurements. In this study, results are reported for wires produced from commercial and nano-scale lab-made B powders. Occurrence of Flux Jumps in MgB2 Bulk Magnets during Pulse-Field Magnetization Naomichi Sakai, Shibaura Institute of Technology Magnetic flux-capturing of MgB2 bulk magnets has been precisely estimated to clarify the mechanism of flux motions during pulsed-field magnetization processes. In the process, parameters as the field penetration ratio Bp/Ba and the field trapping ratio BT/Bp were defined as key parameters for evaluating the field trapping ability in corporation with heat generation by rapid flux motion in samples. The evolutional profiles of magnetic flux density revealed actual flux motion which penetrated the samples. The flux-trapping after the peaks of penetration field Bp exhibited three elemental behaviors, showing no flux flow, fast flux flow and flux jumps according to heat generation and transfer. The times when the flux jumps happened varied widely from 70ms to 420ms after showing Bp. Considering the heat transfer speed of MgB2, this long delay of flux jumps occurrence should be attributed to the macroscopic barriers against the heat transfer to the center of bulk magnets.

As at 20 May 2019

Nanoengineered high temperature superconducting films with novel functionalities Anna Palau, Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC) The individual and collective behaviour of vortices in a superconductor represents one of the most relevant physical aspects of these materials and it is of enormous practical significance for applications. This has sustained the study of different methods for engineering the vortex pinning landscape in order to infer in vortex dynamics of superconductors. In this talk, I will discuss the opportunities of high temperature superconducting cuprates for applications in novel multifunctional electronic devices. In particular, I will present different strategies to design nanostructured high temperature YBa2Cu3O7-x superconducting films, with controllable artificial pinning centres, able to manipulate vortex motion for fluxtronic device applications [1]. In addition, I will present novel functionalities associated to field effect tuning of the metal-insulating transition and spin-texture manipulation in superconducting hybrids with the vision of energy-efficient electronics [2,3]. On the versatility of growing REBCO films through transient liquids Juri Banchewski, Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) We are developing a novel growth approach, entitled Transient Liquid Assisted Growth (TLAG), that aims to overcome throughput and cost constraints of current state-of-the-art growth techniques for REBCO tape production. TLAG combines the cost-effectiveness of Chemical Solution Deposition with ultrahigh growth rates of liquid-mediated approaches.Here, we use fast in-situ X-Ray imaging (100 ms/frame) under synchrotron radiation to explore the growth process of YBa2Cu3O7-x (YBCO) pristine and nanocomposite films through transient liquids. Insights into single nuclei growth behaviour, nucleation density changes and reorientation mechanisms reveal key factors to tune crystallization kinetics towards high current percolation. In return, YBCO films are demonstrated to grow at rates of 100 nm/s reaching a high epitaxial quality (Δω YBCO nanocomposite films prepared by fluorine-free polymer assisted deposition Mircea Nasui, Technical University of Cluj-Napoca Polymer assisted deposition (PAD) has been reported as a novel chemical solution deposition approach for thin film growth with improved homogeneity and long stability of coating solution. The ex-situ core-shell (magnetic core and non-magnetic shell) chemical solution deposition approach for YBCO nanocomposite film growth ensures multiples functions: magnetic pinning, better grain orientation and a precise control of Nps cation concentration.This work presents the preparation of an eco-friendly YBCO solution obtained by PAD followed by the subsequent multideposition of ex-situ nanocomposite YBCO films. Preformed core-shell colloidal solutions of Fe2O3@Ag NPs are added to the water-soluble polymer based precursor, and the NPs’ stability is confirmed up to 20 mol%.To study the influence on microstructure and superconductor performance of the as-processed YBCO films, XRD, TEM and AFM measurements have been carried out. The superconducting transport properties have been determined by means of superconducting transport measurements (R(T) and Jc(B, θ)). Ultrafast growth of YBCO films by low-pressure Transient Liquid Assisted Growth Silvia Rasi, University of Girona In this communication we present the synthesis of YBCO films based on the so-called transient liquid assisted growth (TLAG) process. TLAG is based on the capability of the ternary system BaCuO2, CuO, and Y2O3 to achieve ultrafast growth of YBCO. The eutectic reaction between the first two compounds is able to form a transient liquid in the region of the phase diagram where solid YBCO is the equilibrium phase. Our approach involves decoupling BaCuO2 from YBCO formation. The decoupling parameter is the oxygen partial pressure. BaCO3 decomposition is carried out by heating the films at an oxygen partial pressure where YBCO is not a stable phase. Once BaCO3 is decomposed, the oxygen partial pressure is increased to

As at 20 May 2019

achieve a region where YBCO is the equilibrium phase in order to obtain epitaxial YBCO. Additionally, the mechanisms underlying precursor pyrolysis as well as BaCO3 decomposition are described. Correlations between the structure and superconducting properties of MT-YBaCuO Tetiana Prikhna, V. Bakul Institute for Superhard Materials of the National Academy of Sciences of Ukraine MT-YBaCuO is promising for application in magnetic bearings, MAGLEV transport, superconducting electric motors, fault current limiters, etc. The results of comprehensive experimental study of the fully oxidized (up to YBa2Cu3O6,9-7) materials (1) with increased density of dislocations but detwinned (after high temperature treatment at 2 GPa) and (2) with increased density of twins, but free from dislocations and staking faults (after high temperature oxygenation at 10-16 MPa) show that for attaining high critical current densities and fields of irreversibility (jc(H||c,0T)=9×104 A/cm2, Hirr=9.7 T at 77 K) the formation of high twin density in YBa2Cu3O6,9-7 matrix of MT-YBCO is of high importance. The density of twins in fully oxidized materials to high extend depends on distances between Y2BaCuO5 inclusions and are higher if the distances are shorter. The effect of phase composition of the initial powdered mixtures, pressure-temperature-time conditions on the distances between Y2BaCuO5 inclusions and twin density will be discussed. The effect of the addition of silver on the fracture of Y-Ba-Cu-O Jasmin Congreve, University of Cambridge The widespread use of ceramic-like (RE)Ba2Cu3O7 bulk superconductors , where RE=Y, Gd or Sm, is hindered generally by their poor mechanical properties. The mechanical properties of these inherently brittle ceramic materials are limited due to weak regions within the microstructure such as cracks and pores. The Lorentz forces generated within a bulk superconductor in the presence of a large magnetic field create complex stress fields within the bulk single grain. The flaws within the microstructure induce local stress concentrations which then lead to internal fracture of the single grain. The addition of BaCuO (liquid phase) and Ag prior to growth by TSMG has been demonstrated to be effective in improving the mechanical properties of single grain bulk superconductors. The differences in the fracture mechanisms will provide a better understanding of how the provision of additional liquid phase and the addition of silver produces YBCO single grains with better mechanical properties. Tl-1223 superconducting films, coatings and tapes: feasibility and perspectives Alessandro Leveratto, CNR-SPIN The FCC-hh study is calling for an unprecedented collision energy of 100TeV. One of the issues, related to the beam screen absorbing the synchrotron radiation at 50K and 16T, is the surface resistance Rs of this screen that has a strong impact on beam stability. Theoretical calculations and first experiments shows that the adoption of high temperature superconductors coating may drastically reduce Rs. Tl-cuprates and in particular TlSr2Ca2Cu3Ox with a Tc of about 120K and very high Hc2 and Hirr, is a good candidate for such a purpose.The work at CNR-SPIN is devoted to develop new routes for the synthesis of Tl-based superconductors, from pellets to films (electrochemical and Pulsed Laser Deposition, ink techniques) up to tapes. We will show the progress on Tl1223 synthesis done in Genoa and preliminary measurements of Rs at high field and high temperature performed in Barcelona (ICMAB-ALBA).

Neutron irradiation: introduced defects and effects on various superconductors Michael Eisterer, Atominstitut, TU Wien Irradiation techniques offer the unique possibility to create pinning centers in a superconductor and to investigate the resulting changes in the superconducting properties of the very same sample. In particular, neutron irradiation introduces various defects ranging from single displaced atoms to nanometer-sized defects. The resulting defect structure depends on the irradiated material and the neutron energy distribution. The influence of neutron irradiation on the superconducting properties in various materials

As at 20 May 2019

will be compared. Pinning is only moderately enhanced in MgB2 because of its comparably large coherence length. The largest effect is obtained, if the coherence length matches the defect size, such as in the iron or cuprate superconductors, where about 3-5% of the depairing current density can be achieved. However, also small defects can contribute to pinning quite efficiently as observed in Nb3Sn wires or Gd-123 coated conductors. The results will be discussed in terms of defect size and density. Critical current anisotropy in Fe(Se,Te) films irradiated by high-energy protons Gaia Grimaldi, CNR - SPIN Institute Irradiation effects are used to improve the effective pinning in superconductors, thus enabling higher critical currents by introducing random or correlated additional defects. The study of pinning energy in Fe(Se,Te) thin film irradiated by 3.5 MeV protons reveals an anisotropic behavior in favor of a stronger pinning in the ab-planes. Here, we study the pinning forces for the magnetic field applied parallel and perpendicular to the sample surface. There is an overall weak anisotropy of this superconductor probed by an anisotropy factor gammaH J is always quite low. The Fe(Se,Te) behavior can be ascribed to its intrinsic anisotropy due to its layered crystallographic structure, on which the irradiation effects act only by slightly increasing this anisotropy. These results are supported by the angular field dependence of the critical currents, pointing towards implementing this material for high-field applications. Power law correlation between Tc and Jc in neutron irradiated Ba-122 crystals Daniel Kagerbauer, Atominstitut, TU Wien A power law correlation between the depairing current density, Jd, and the critical temperature, Tc, is predicted for Ba-122 single crystals based on the Uemura relation and a correlation between the thermodynamic critical field and Tc. Furthermore, the critical current density, Jc, is assumed to be proportional to Jd with the pinning efficiency as proportionality factor. Experimental results indicate that the pinning efficiency of pristine Ba-122 depends on the dopant concentration. Thus, a constant pinning efficiency for all doping concentrations is necessary to access the correlation between Jd and Tc by measuring Jc. We performed neutron irradiation to introduce a more efficient pinning landscape than the pristine defects. Our previous results on the potassium doped Ba-122 follow the predicted behavior. In this study we extended our investigation to the phosphorus and cobalt doped systems. We will discuss the universality of and possible deviations from this power law correlation. Intrinsic anisotropy in nanostructured YBa2Cu3O7-x from microwave measurements Enrico Silva, Università Roma Tre Nanostructuration of YBa2Cu3O7-x may significantly change the effective anisotropy, due to the presence of directional pinning, whereas the intrinsic electron mass anisotropy is expected to be independent of it. To clarify this issue, we measured the microwave (48 GHz) field-dependent complex surface impedance in YBCO films with different types of secondary phases and thus different nanostrain. We derive the free-flux-flow resistivity and the Labusch parameter (pinning constant) as a function of the field (μ0H ≤ 0.8 T) and field orientation. We show that the intrinsic, anisotropic electron mass scaling (Blatter-Geshkenbein-Larkin scaling) applies to the free-flow resistivity, and we find a nearly constant anisotropy parameter γ=(5.5 ± 1.0) for the three films investigated, in contrast with the Jc effective anisotropy, decreasing with the nanostrain. We also show that the directional pinning comes instead from a complex interplay between the anisotropic electron mass and the directional extended defects. Optimized Vortex-Pinning in Superconducting Wires Amit Goyal, University At Buffalo Engineered nanoscale defects within REBa2Cu3O7-δ (REBCO) based coated conductors are ofgreat interest for enhancing vortex-pinning, especially in high-applied magnetic fields. We haveconducted extensive research to optimize vortex-pinning and enhance Jc via controlledintroduction of various types of nanoscale defects ranging from simple rare-earth oxides and Babasedperovskites to double perovskite rare-earth tantalates and niobates (Ba2RETaO6 andBa2RENbO6). This talk will provide an overview on how density, morphology, and compositionof these engineered nanoscale defects affects vortex-pinning in different

As at 20 May 2019

temperature, field andangular regimes. Detailed microstructural and superconducting properties coated conductors withthese engineered defects will be presented. It will be shown that certain nanodefect configurationsthat provide the best performance at high-operating temperatures also provide the optimalproperties at low operating temperatures out to high-applied magnetic fields. The talk willdiscuss routes to enhance vortex-pinning in both in-situ films and ex-situ films. Resistivity of HTS tapes in overcritical current regime: impact on SFCL modeling Nicolo Riva, Ecole Polytechnique Federale Lausanne (EPFL) A detailed knowledge of the resistivity of high-temperature superconductors in the over-critical current regime is important to achieve reliable numerical simulations of applications such as fault current limiters (SFCL). We have previously shown that the combination of fast-pulsed current measurements and finite element analysis allows accounting for heating effects occurring during the current pulses. In this way, one can retrieve the correct current and temperature dependence of the resistivity data points of the superconductor material [1]. In this contribution, we apply this method to characterize the resistivity vs. current and temperature of commercial conductor tapes based on YBCO in the over-critical current regime, between 65 K and 90 K and in self-field condition. Then, we determine by simulation whether and to what extent using such curves instead of the widely utilized power-law resistivity affects the thermal and electromagnetic performance of the tapes in the practical case of a SFCL. Growth of CSD low fluorine YBCO superconducting layers on sapphire substrates Cornelia Pop, Materials Science Institut of Barcelona (ICMAB-CSIC) Due to its excellent properties such as high thermal conductivity and good mechanical strength, sapphire is an appealing candidate as a substrate for YBCO superconducting films growth, especially for applications as fault current limiters. To increase the critical current it is required, however, to grow thicker YBCO layers.We will report here an investigation of a conductor architecture highly promising for being used as element of resistive Fault Current Limiters where large area production is required: CSDYBCO/CSDCZO/sputt.YSZ/unpolished sapphire. In order to succeed with this architecture we first optimized the growth of both thin and thick YBCO layers on MODCZO/YSZ single crystals. We have been able to achieve high performances in both cases (JC(77K)sf = 4.5 MA/cm2 for 200 nm and (JC(77K)sf = 3.7 MA/cm2 for 1µm). The knowledge acquired was transferred to the growth of YBCO on polished and unpolished sapphire based architectures. The microstructure and superconducting properties were analyzed.

Recent progress of iron-based wire development for high-field applications Yanwei Ma, Institute of Electrical Engineering, Chinese Academy of Sciences In this talk, the overview of the recent progress on iron-based superconducting wires will be provided. Firstly, the highest transport Jc values have achieved 0.15 MA/cm2 (Ic = 437 A) at 4.2 K and 10 T in densified and textured 122 tapes. The transport Jc measured at 4.2 K under high magnetic fields of 27 T is still on the level of 55 kA/cm2. More importantly, transport Jc of 100-m-class 122-type IBS wires has been doubled, compared to the first one. Secondly, in order to reduce costs and improve the mechanical strength, high strength Monel/Ag and Cu/Ag 122 composite conductors have been fabricated. Thirdly, reversible critical current under a large compressive strain of ε = –0.6 % for 122 tapes has been observed. Finally, the latest results of the first performance test of a 30 mm IBS single pancake coil under a 24 T background field will be presented. Large Critical Current Density in HIP-processed (Ba,Na)Fe2As2 Round Wire Tsuyoshi Tamegai, The University of Tokyo Iron-based superconductors (IBSs) have high Tc and Hc2 with low electromagnetic anisotropies. These excellent characteristics stimulated developments of tapes and wires of IBSs, and a practical level of Jc of

As at 20 May 2019

1x105 A/cm2 even at 10 T has already been achieved in pressed tapes of (AE,K)Fe2As2 (AE =Sr, Ba) [1]. Although Jc in round wires of (AE,K)Fe2As2 is lower, steady progress has been achieved, leading to Jc = 3.8x104 A/cm2 at 10 T [2]. In the present study, we have performed exploration and improvements of fabrication processes of round wires of another 122-type IBS, (Ba,Na)Fe2As2. We have achieved a large Jc of 4.0x104 A/cm2 at 10 T when the wires were processed under high pressure. Details of the fabrication process and comparison with (Ba,K)Fe2As2 round wires will be presented. Comprehensive microwave study of BaFe2As2 with K, Co, Rh, and P substitutions Gianluca Ghigo, Politecnico di Torino Reaching a comprehensive understanding of the basic properties of the Ba122 family of iron-based superconductors (IBS) is a prerequisite of any practical application employing these materials. Our approach towards this goal combines experimental and theoretical analyses of BaFe2As2 single crystals with different substitutions (K, Co, Rh, P), with the aim to establish a general picture that could be a reference for the materials choice for applications. Characterizations were performed by a microwave resonator technique, yielding critical temperature, penetration depth, and conductivity [1]. Controlled introduction of nanoscale defects, as a tool to investigate the fundamental properties of IBS through their dependence on disorder, was obtained by ion irradiation [2,3]. A multi-band s± wave Eliashberg model is able to explain in a self-consistent way the whole set of data. We also show evidence of disorder-driven s± to s++ order parameter symmetry transition [3], as a further confirmation of this picture. Unique defect structure and advantageous vortex pinning properties in CaKFe4As4 Shigeyuki Ishida, AIST The enhancement of critical current density (Jc) is one of the key issues towards superconductivity applications. After the discovery of iron-based superconductors (IBSs), which are considered as candidate materials for high-field applications, high Jc values have been achieved by various techniques to introduce artificial pinning centers, while a further improvement of Jc is desired. We report unprecedented vortex pinning properties in the CaKFe4As4 (CaK1144) system arising from the inherent defect structure. Scanning transmission electron microscopy revealed the existence of nanoscale intergrowths of the CaFe2As2 phase, which is unique to CaK1144 formed as a line compound. The Jc properties in CaK1144 are found to be distinct from other IBSs characterized by a significant anisotropy with respect to the magnetic field orientation as well as a novel pinning mechanism significantly enhanced with increasing temperature. We propose a comprehensive explanation of the Jc properties based on the unique intergrowths acting as pinning centers. Upper critical fields of the stoichiometric iron-based superconductor CaKFe4As4 Matt Bristow, University of Oxford The recently discovered stoichiometric iron-based superconductor CaKFe4As4 is an excellent candidate for practical applications. It has a superconducting transition temperature of Tc=35 K and an exceptionally high critical current density of ~2x107 with H||c. We have studied the evolution of the upper critical field, μ0Hc2 of single crystals using electrical transport in pulsed magnetic fields up to 90 T for different orientations. We find that Hc2 is unusually large reaching 86 T at 4.2 K, well exceeding the Pauli limit. Hc2 is rather anisotropic near Tc, but becomes isotropic at low temperatures, useful for high-magnetic field applications. From temperature-dependent transport studies up to 16 T we characterise the vortex phase, its activation energy and the pinning force. We find an extremely high activation energy, implying the presence of strong vortex pinning that can support high critical current densities, as found from previous magnetization studies [1]. The high critical current density together with the large and isotropic upper critical fields makes CaKFe4As4 exciting for future applications. Local magnetization of EuRb-1144: superconductivity and magnetic ordering Sigrid Holleis, Atominstitut, TU Wien The coexistence of superconductivity and magnetic ordering is of great theoretical interest. The recently discovered iron-based EuRb-1144 system is an ideal platform for studying the intriguing interplay between

As at 20 May 2019

these two phenomena since long-range magnetic order of the Eu2+ local moments occurs about 20 K below the superconducting transition. This allows an investigation of the superconducting properties with and without magnetic ordering in the same sample. We mapped the magnetic field of single crystals above the sample surface at various fields and temperatures by means of Scanning Hall Probe Microscopy with micrometer resolution. The contributions from magnetism and superconductivity could be clearly distinguished in the obtained field profiles. A striking difference in the local magnetization above and below the magnetic ordering transition was observed. Moreover, the Bean-profile heavily distorts near zero field in the magnetically ordered state of the single crystal, which suggests a strong interaction between superconductivity and magnetic ordering. Hidden Antiferromagnetic Phase in Electron over-doped SmFeAsO Soshi Iimura, Tokyo Institute of Technology Hot issue in unconventional superconductors is why the 2nd highest-Tc = 56 K after cuprates is accomplished in the electron-doped iron-oxyarsenides SmFeAsO1-xFx. However, its phase diagram, especially its magnetic aspects, is not well understood, due to the prohibitively large neutron absorption cross sections of Sm, which are 2 or 3 orders of magnitude larger than that of La. Here we show results of neutron powder diffractions on isotopically substituted 154SmFeAsO1−xDx samples at x = 0, 0.56, 0.67, and 0.73. We discovered a new antiferromagnetic (AFM) phase in the electron-overdoped regime with x ≥ 0.56, in which the magnetic moment on Fe reaches the largest value of 2.73 μB/Fe in all the iron arsenides reported so far. Our theoretical calculations reveal that the AFM2 originates not in the Fermi surface nesting but in the kinetic frustration of the Fe-3dxy orbital whose nearest-neighbor hopping parameter becomes zero.