Mark J. Raine

The critical current density of grain boundary channels in polycrystalline HTS and LTS superconductors in magnetic fields

We provide evidence that a single mechanism—flux flow along channels—can explain the functional form of the critical current density (Jc) in the low-temperature superconductor Nb3Sn and in the high-temperature superconductors (HTS) YBa2Cu3O7−δ (YBCO) and (Bi,Pb)2Sr2Can−1CunOx (BiSCCO) in low and high magnetic fields. In this paper, we show that standard flux pinning theories, used for the past four decades to describe Jc in low-temperature superconductors (LTS), cannot explain the strain dependence of Jc in YBCO because Jc is a function of strain but the average superconducting properties are not. We conclude that in the polycrystalline samples presented here, the channels are grain boundaries that are narrow and metallic in Nb3Sn and YBCO but wide and semiconducting in BiSCCO. In Nb3Sn, strain alters Jc by changing the superconducting properties of the grains, whereas in YBCO, strain alters Jc by changing the properties of the grain boundaries.

DC characterization and 3D modelling of a triangular, epoxy-impregnated high temperature superconducting coil

The direct current (dc) characterization of high temperature superconducting (HTS) coils is important for applications, such as electric machines, superconducting magnetic energy storage and transformers. In this paper, the dc characterization of a triangular-shaped, epoxyimpregnated HTS coil wound with YBCO coated conductor intended for use in an axial-flux HTS motor is presented. Voltage was measured at several points along the coil to provide detailed information of its dc characteristics. The coil is modelled based on the H-formulation using a new three-dimensional (3D) technique that utilizes the real superconducting layer thickness, and this model allows simulation of the actual geometrical layout of the HTS coil structure. Detailed information on the critical current density’s dependence on the magnitude and orientation of the magnetic flux density, Jc(B,θ), determined from experimental measurement of a short sample of the coated conductor comprising the coil is included directly in the numerical model by a two-variable direct interpolation to avoid developing complicated equations for data fitting and greatly improve the computational speed. Issues related to meshing the finite elements of the real thickness 3D model are also discussed in detail. Based on a comparison of the measurement and simulation results, it is found that non-uniformity along the length exists in the coil, which implies imperfect superconducting properties in the coated conductor, and hence, coil. By evaluating the current–voltage (I–V) curves using the experimental data, and after taking into account a more practical n value and critical current for the non-uniform region, the modelling results show good agreement with the experimental results, validating this model as an appropriate tool to estimate the dc I–V relationship of a superconducting coil. This work provides a further step towards effective and efficient 3D modelling of superconducting devices for large-scale applications.

Modeling and Comparison of In-Field Critical Current Density Anisotropy in High-Temperature Superconducting (HTS) Coated Conductors

The development of high-temperature superconducting (HTS) wires is now at a stage where long lengths of high quality are commercially available, and of these, (Re)BCO coated conductors show the most promise for practical applications. One of the most crucial aspects of coil and device modeling is providing accurate data for the anisotropy of the critical current density Jc(B, θ) of the superconductor. In this paper, the in-field critical current density characteristics Jc(B, θ) of two commercial HTS coated conductor samples are experimentally measured, and based on these data, an engineering formula is introduced to represent this electromagnetic behavior as the input data for numerical modeling. However, due to the complex nature of this behavior and the large number of variables involved, the computational speed of the model can be extremely slow. Therefore, a two-variable direct interpolation method is introduced, which completely avoids any complex data fitting for Jc(B, θ) and expresses the anisotropic behavior in the model directly and accurately with a significant improvement in computational speed. The two techniques are validated and compared using numerical models based on the H-formulation by calculating the self-field and in-field dc critical currents and the ac loss for a single coated conductor.

European Nb 3 Sn Superconducting Strand Production and Characterization for ITER TF Coil Conductor

The European Union contributes around 20% of the cable-in-conduit conductor lengths needed for the ITER toroidal field (TF) magnet coils. For that purpose, 97 tons of Nb3Sn superconducting strand have been fabricated over five years, the production being completed in 2014. This superconducting strand has been manufactured by two companies, namely, Bruker EAS (Germany) and OST (USA), through the bronze route and the internal tin diffusion, respectively. This paper reports the outcomes of this strand mass production and of the strand characterization as performed by the suppliers and cross-checked on a regular basis by Durham University.

Characterization of the Low Temperature Superconductor Niobium Carbonitride

Niobium carbonitrides superior radiation tolerance , coupled with the recently reported increase in its upper critical magnetic field when made nanocrystalline, increases its potential importance in future technological high-field superconductor applications. The maximum transition temperature for the composition NbC0.3N0.7 is ~17.8 K and its upper critical magnetic field is ~11 T; this increases to ~12 T for the composition NbC0.2N0.8,. Using solid-state processing, we have fabricated microcrystalline bulk niobium carbonitride with a transition temperature of ~ 7.6 K. A comprehensive characterization of this material, which includes susceptibility, resistivity, magnetization, heat capacity and XRD measurements is provided. Comparisons between the heat-treated material and the same material subjected to hot isostatic pressing are made so that the values of the intrinsic fundamental properties can be identified and their sensitivities to different fabrication processes determined.

Superconducting Properties of Titanium Alloys (Ti-64 and Ti-6242) for Critical Current Barrels

We have measured the superconducting properties of the titanium alloy Ti-6Al-4V (Ti-64) as supplied and following two of the heat treatment schedules used for the Nb₃Sn strands in the ITER tokamak. The Ti-64 alloy is the standard choice in the superconducting community for the barrels used to make critical current (<italic>I</italic>_C) measurements in high magnetic fields at cryogenic temperatures. Ti-64, which has a two-phase alpha + beta microstructure and contains vanadium (n.b. <italic>T</italic>_C(V) ∼ 5.4 K), is superconducting at 4.2 K in fields up to 3 T. We have also measured Ti-6Al-2Sn-4Zr-2Mo-0.2Si (Ti-6242), which is in the near alpha phase and contains tin (n.b. <italic>T</italic>_C(Sn) ∼ 3.7 K). The critical temperature of Ti-6242 is 2.38 K, which is lower than the 5.12 K of Ti-64. Hence Ti-6242 is a better choice of barrel material for <italic>I</italic> _C measurements required at 4.2 K in low fields up to 3 T, because it remains in the normal state.

Round Robin Test of Residual Resistance Ratio of Nb3Sn Composite Superconductors

In this paper, a round robin test of residual resistance ratio (RRR) is performed for Nb3Sn composite superconductors prepared by an internal tin method by six institutes with the international standard test method described in IEC 61788-4. It was found that uncertainty mainly resulted from determination of the cryogenic resistance from the intersection of two straight lines drawn to fit the voltage versus temperature curve around the resistive transition. The measurement clarified that RRR can be measured with expanded uncertainty not larger than 5% with the coverage factor 2 by using this test method.