Evidence that transverse variability of critical current density can greatly mitigate screening current stress in high field REBCO magnets.

Plastic damage of REBCO (REBaCuO, where RE=rare earth) coated conductors by screening current stress (SCS) is a significant concern for ultra-high-field superconducting magnets. Indeed, the third Little Big Coil (LBC3), a REBCO magnet that generated a record, high field of 45.5 T, showed wavy plastic damage produced by excess SCS in all pancakes except two made with single-slit conductors having their slit edges pointing inward towards the magnet center.

The prospects of high-temperature superconductors.

Overcoming cost barriers could make high-temperature superconductors pervasive.

Origin of the enhanced NbSn performance by combined Hf and Ta doping.

In recent years there has been an increasing effort in improving the performance of NbSn for high-field applications, in particular for the fabrication of conductors suitable for the realization of the Future Circular Collider (FCC) at CERN. This challenging task has led to the investigation of new routes to advance the high-field pinning properties, the irreversibility and the upper critical fields (H and H, respectively). The effect of hafnium addition to the standard Nb-4Ta alloy has been recently demonstrated to be particularly promising and, in this paper, we investigate the origins of the observed improvements of the superconducting properties.

Effect of heat treatments on superconducting properties and connectivity in K-doped BaFeAs.

Fe-based superconductors and in particular K-doped BaFeAs (K-Ba122) are materials of interest for possible future high-field applications. However the critical current density (J) in polycrystalline Ba122 is still quite low and connectivity issues are suspected to be responsible. In this work we investigated the properties of high-purity, carefully processed, K-Ba122 samples synthesized with two separate heat treatments at various temperatures between 600 and 825 °C.

Process to densify BiSrCaCuOx round wire with overpressure before coil winding and final overpressure heat treatment.

Overpressure (OP) processing of wind-and-react BiSrCaCuO (2212) round wire compresses the wire to almost full density, decreasing its diameter by about 4 % without change in wire length and substantially raising its . However, such shrinkage can degrade coil winding pack density and magnetic field homogeneity. To address this issue, we here present an overpressure predensification (OP-PD) heat treatment process performed before melting the 2212, which greatly reduces wire diameter shrinkage during the full OP heat treatment (OP-HT).

High Performance Bi-2212 Round Wires Made with Recent Powders.

Multifilamentary BiSrCaCuO (Bi-2212) wire made by the powder-in-tube technique is the only high temperature superconductor made in the round shape preferred by magnet builders. The critical current density ( ) of Bi-2212 round wire was improved significantly by the development of overpressure heat treatment in the past few years. Bi-2212 wire is commercially available in multiple architectures and kilometer-long pieces and a very promising conductor for very high field NMR and accelerator magnets.

45.5-tesla direct-current magnetic field generated with a high-temperature superconducting magnet.

Strong magnetic fields are required in many fields, such as medicine (magnetic resonance imaging), pharmacy (nuclear magnetic resonance), particle accelerators (such as the Large Hadron Collider) and fusion devices (for example, the International Thermonuclear Experimental Reactor, ITER), as well as for other diverse scientific and industrial uses. For almost two decades, 45 tesla has been the highest achievable direct-current (d.c.

Fabrication and Testing of a Bi-2223 Test Coil for High Field NMR Magnets.

In 2005 the Committee on Opportunities in High Magnetic Fields (COHMAG) issued a challenge to develop a 30 T high-resolution NMR magnet. In response, the National High Magnetic Field Laboratory (NHMFL) is investigating all three commercially available high-temperature superconductors (HTS) including REBCO, Bi-2212 and most recently, a reinforced Bi-2223 conductor supplied by Sumitomo Electric, designated Type HT-NX. Recent investigations of Type HT-NX conductor at the NHMFL and by others suggest that operation at hoop stress above 400 MPa, and total strain above 0.

Evidence from EXAFS for Different Ta/Ti Site Occupancy in High Critical Current Density NbSn Superconductor Wires.

To meet critical current density, J , targets for the Future Circular Collider (FCC), the planned replacement for the Large Hadron Collider (LHC), the high field performance of NbSn must be improved, but champion J values have remained static for the last 10 years. Making the A15 phase stoichiometric and enhancing the upper critical field H by Ti or Ta dopants are the standard strategies for enhancing high field performance but detailed recent studies show that even the best modern wires have broad composition ranges. To assess whether further improvement might be possible, we employed Extended X-ray Absorption Fine Structure (EXAFS) to determine the lattice site location of dopants in modern high-performance NbSn strands with J values amongst the best so far achieved.

J (4.2 K, 31.2 T) beyond 1 kA/mm of a ~3.2 μm thick, 20 mol% Zr-added MOCVD REBCO coated conductor.

A main challenge that significantly impedes REBaCuO (RE = rare earth) coated conductor applications is the low engineering critical current density J because of the low superconductor fill factor in a complicated layered structure that is crucial for REBaCuO to carry supercurrent. Recently, we have successfully achieved engineering critical current density beyond 2.0 kA/mm at 4.

Effects of filament size on critical current density in overpressure processed Bi-2212 round wire.

BiSrCaCuO (Bi-2212) conductor is the only high temperature superconductor manufactured as a round wire and is a very promising conductor for very high field applications. One of the key design parameters of Bi-2212 wire is its filament size, which has been previously reported to affect the critical current density ( ) and ac losses. Work with 1 bar heat treatment showed that the optimal filament diameter was about 15 μm but it was not well understood at that time that gas bubbles were the main current limiting mechanism.

Bi-2223 Test Coils for High Resolution NMR Magnets.

Recently, significant improvement in the strain tolerance of Bi-2223 conductor has been achieved by lamination with high strength nickel alloy. The conductor, supplied by Sumitomo Electric and designated Type HT-NX, is now commercially available in lengths sufficient for manufacture of high-homogeneity solenoids. A program to fully exploit the improved conductor properties is now underway at the National High Magnetic Field Laboratory (NHMFL).

Experimental Study on Potential Heat Treatment Issues of Large Bi-2212 Coils.

The route to 30 T NMR endorsed by a recent National Academy report clearly still has many challenges to achieve high stability and homogeneous high temperature superconducting (HTS) magnet. As the only HTS conductor with round wire (RW) geometry, BiSrCaCuO (Bi-2212) RW conductor is very attractive for NMR magnet applications. At present, an NMR quality demonstration magnet with Bi-2212 RW wound insert coils is under development at the National High Magnetic Field Laboratory (NHMFL).

Understanding the densification process of BiSrCaCuO round wires with overpressure processing and its effect on critical current density.

Overpressure (OP) processing increases the critical current density ( ) of BiSrCaCuO (2212) round wires by shrinking the surrounding Ag matrix around the 2212 filaments, driving them close to full density and greatly increasing the 2212 grain connectivity. Indeed densification is vital for attaining the highest . Here, we investigate the time and temperature dependence of the wire densification.

Development of a Persistent Superconducting Joint between Bi-2212/Ag-alloy Multifilamentary Round Wires.

Superconducting joints are one of the key components needed to make Ag-alloy clad BiSrCaCuO (Bi-2212) superconducting round wire (RW) successful for high-field, high-homogeneity magnet applications, especially for nuclear magnetic resonance (NMR) magnets in which persistent current mode (PCM) operation is highly desired. In this study, a procedure for fabricating superconducting joints between Bi-2212 round wires during coil reaction was developed. Melting temperatures of Bi-2212 powder with different amounts of Ag addition were investigated by differential thermal analysis (DTA) so as to provide information for selecting the proper joint matrix.

Tensile properties and critical current strain limits of reinforced Bi-2212 conductors for high field magnets.

We study here the effect of axial strain on the degradation of the critical current for bare and reinforced, overpressure processed Bi-2212 conductors. We show that reinforcement markedly improves the conductor's stress limit, doubling it from ~150 MPa in the bare conductor to ~300 MPa when reinforced. We find also that certain processes used to reinforce the conductor slightly reduce the degradation strain limit from ~0.

Intrinsic and extrinsic pinning in NdFeAs(O,F): vortex trapping and lock-in by the layered structure.

Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the REFeAs(O,F) (RE1111, RE being a rare-earth element) is the family with the highest critical temperature T but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 (T ∼ 55 K). Here we focus on the pinning properties of the lower-T Nd1111 in the flux-creep regime.

Comparison of growth texture in round Bi2212 and flat Bi2223 wires and its relation to high critical current density development.

Why Bi(2)Sr(2)CaCu(2)Ox (Bi2212) allows high critical current density Jc in round wires rather than only in the anisotropic tape form demanded by all other high temperature superconductors is important for future magnet applications. Here we compare the local texture of state-of-the-art Bi2212 and Bi2223 ((Bi,Pb)(2)Sr(2)Ca(2)Cu(3)O(10)), finding that round wire Bi2212 generates a dominant a-axis growth texture that also enforces a local biaxial texture (FWHM <15°) while simultaneously allowing the c-axes of its polycrystals to rotate azimuthally along and about the filament axis so as to generate macroscopically isotropic behavior. By contrast Bi2223 shows only a uniaxial (FWHM <15°) c-axis texture perpendicular to the tape plane without any in-plane texture.

Development of very high Jc in Ba(Fe1-xCox)2As2 thin films grown on CaF2.

Ba(Fe(1-x)Co(x))(2)As(2) is the most tunable of the Fe-based superconductors (FBS) in terms of acceptance of high densities of self-assembled and artificially introduced pinning centres which are effective in significantly increasing the critical current density, J(c). Moreover, FBS are very sensitive to strain, which induces an important enhancement in critical temperature, T(c), of the material. In this paper we demonstrate that strain induced by the substrate can further improve J(c) of both single and multilayer films by more than that expected simply due to the increase in T(c).

Isotropic round-wire multifilament cuprate superconductor for generation of magnetic fields above 30 T.

Magnets are the principal market for superconductors, but making attractive conductors out of the high-temperature cuprate superconductors (HTSs) has proved difficult because of the presence of high-angle grain boundaries that are generally believed to lower the critical current density, J(c). To minimize such grain boundary obstacles, HTS conductors such as REBa2Cu3O(7-x) and (Bi, Pb)2Sr2Ca2Cu3O(10-x) are both made as tapes with a high aspect ratio and a large superconducting anisotropy. Here we report that Bi2Sr2CaCu2O(8-x) (Bi-2212) can be made in the much more desirable isotropic, round-wire, multifilament form that can be wound or cabled into arbitrary geometries and will be especially valuable for high-field NMR magnets beyond the present 1 GHz proton resonance limit of Nb3Sn technology.

Facility implementation and comparative performance evaluation of probe-corrected TEM/STEM with Schottky and cold field emission illumination.

We report the installation and performance evaluation of a probe aberration-corrected high-resolution JEOL JEM-ARM200F transmission electron microscope (TEM). We provide details on construction of the room that enables us to obtain scanning transmission electron microscope (STEM) data without any evident distortions/noise from the external environment. The microscope routinely delivers expected performance.

Artificially engineered superlattices of pnictide superconductors.

Significant progress has been achieved in fabricating high-quality bulk and thin-film iron-based superconductors. In particular, artificial layered pnictide superlattices offer the possibility of tailoring the superconducting properties and understanding the mechanism of the superconductivity itself. For high-field applications, large critical current densities (J(c)) and irreversibility fields (H(irr)) are indispensable along all crystal directions.

High intergrain critical current density in fine-grain (Ba0.6K0.4)Fe2As2 wires and bulks.

The K- and Co-doped BaFe(2)As(2) (Ba-122) superconducting compounds are potentially useful for applications because they have upper critical fields (H(c2)) of well over 50 T, H(c2) anisotropy γ < 2and thin-film critical current densities J(c) exceeding 1 MA cm(-2) (refs 1-4) at 4.2 K. However, thin-film bicrystals of Co-doped Ba-122 clearly exhibit weak link behaviour for [001] tilt misorientations of more than about 5°, suggesting that textured substrates would be needed for applications, as in the cuprates.

Suppression of the critical temperature of superconducting NdFeAs(OF) single crystals by Kondo-like defect sites induced by alpha-particle irradiation.

We report the effect of alpha-particle irradiation on the reduction of the critical temperature T{c} of a NdFeAs(OF) single crystal. Our data indicate that irradiation defects cause both nonmagnetic and magnetic scattering, resulting in the Kondo-like excess resistance Delta rho(T) proportional to lnT over 2 decades in temperatures above T{c}. The critical density of magnetic irradiation defects which suppresses T{c} is found to be much higher than those for cuprates and multiband BCS superconductors.

Template engineering of Co-doped BaFe2As2 single-crystal thin films.

Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties and evaluate device applications. So far, superconducting properties of ferropnictide thin films seem compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report new template engineering using single-crystal intermediate layers of (001) SrTiO(3) and BaTiO(3) grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe(2)As(2) with a high transition temperature (T(c,rho=0) of 21.