Alternating currents or fields in coils, transformers or generators may lead to high losses in superconducting cables. The main contributions to these ac-losses are hysteresis losses of the superconductor and coupling losses between the strands. The hysteresis losses can be reduced by decreasing the width of the tapes. The resulting smaller cross section of the individual filaments could lead to a strong reduction in performance, if the tape is not homogeneous and the current cannot flow around local defects any longer. Hence, detecting local defects before cutting and assembling becomes very important. With our scanning techniques, we visualize the field penetration and the current flow within different two-dimensional structures like Roebel strands. The effect of different local defects on the current distribution in Roebel single strands is examined. Direct measurements of the conductor performance with respect to its geometry are important for finding the best solutions for future cable designs.

We report on the design and implementation of a rotating sample magnetometer (RSM) operating in the variable temperature insert (VTI) of a cryostat equipped with a high-field magnet. The limited space and the cryogenic temperatures impose the most critical design parameters: the small bore size of the magnet requires a very compact pick-up coil system and the low temperatures demand a very careful design of the bearings. Despite these difficulties the RSM achieves excellent resolution at high magnetic field sweep rates, exceeding that of a typical vibrating sample magnetometer by about a factor of ten. In addition the gas-flow cryostat and the high-field superconducting magnet provide a temperature and magnetic field range unprecedented for this type of magnetometer.

We determine the critical current anisotropy at maximum Lorentz force from hysteresis loops in a vibrating sample magnetometer. To eliminate the signal of spurious variable Lorentz force currents it is sufficient to cut the sample to a specific length, which is calculated from the position dependent sensitivity of the instrument. The procedure increases the resolution of the measurement and the results compare well with transport data on the same sample. As the electric field in magnetization measurements is lower than in transport experiments, the anisotropy at high currents (low temperatures and fields) can be measured without the need to make current contacts or any special sample preparation.

(LRE)BCO (where LRE is a light rare earth element such as Nd, Sm, Gd or Y) single grain bulk superconductors have considerable potential for engineering applications due to their ability to trap large, stable magnetic fields at 77 K. However, the cost of processing these materials is relatively high, and particularly so when Pt and Ag are added to the precursor powder to improve the field trapping and mechanical properties of the fully processed sample. The nature of the top seeded melt growth (TSMG) process used commonly to fabricate (LRE) BCO single grain superconductors results typically in a relatively high failure rate, particularly during optimization of the thermal process, which results in high material wastage. In this paper, we report a simple, but economical method of recycling GdBCO-Ag bulk superconductors using the TSMG technique. The microstructure and the superconducting properties of the recycled single grains before and after re-processing are reported.

Various commercial coated conductors were irradiated with fast neutrons in order to introduce randomly distributed, uncorrelated defects which increase the critical current density, Jc, in a wide temperature and field range. The Jc-anisotropy is significantly reduced and the angular dependence of Jc does not obey the anisotropic scaling approach. These defects enhance the irreversibility line in not fully optimized tapes, but they do not in state-of-the-art conductors. Neutron irradiation provides a clear distinction between the low field region, where Jc is limited by the grain boundaries, and the high field region, where depinning leads to dissipation.

We analyze the influence of the magnetic field generated by the supercurrents (self-field) on the current density distribution by numerical simulations. The thickness of the superconducting film determines the self-field and consequently the critical current density at zero applied field. We find an equation, which derives the thickness dependence of the critical current density from its dependence on the magnetic induction. Solutions of the equation reproduce numerical simulations to great accuracy, thus enabling a quantification of the dependence of the self-field critical current density with increasing film thickness. This result is technologically relevant for the development of coated conductors with thicker superconducting layers.

Numerical simulations of the current and field distribution in thin superconducting films are carried out for a given material law J(c)(B) and as a function of the applied field H, taking the sample's self-field into account. The dependence of the critical current density on the applied field J(c)(H) is computed for comparison with experiment, considering the geometry of transport measurements.We show that extrapolating the high-field power law J(c) proportional to B-alpha to the lowest fields results in a finite critical current at zero applied field J(c)(H = 0), despite the singularity of J(c)(B). Moreover, particular features of the experiment, such as a low field plateau in J(c)(H), are reproduced and found to be determined by the self-field. (C) 2010 Elsevier B.V. All rights reserved.

Although the flux density map of a bulk superconductor provides in principle sufficient information for calculating the magnitude and the direction of the supercurrent flow, the inversion of the Biot-Savart law is ill conditioned for thick samples, thus rendering this method unsuitable for state of the art bulk superconductors. If a thin (< 1 mm) slab is cut from the bulk, the inversion is reasonably well conditioned and the variation of the critical current density in the sample can be calculated with adequate spatial resolution. Therefore a novel procedure is employed, which exploits the symmetry of the problem and solves the equations non-iteratively, assuming a planar z-independent current density. The calculated current density at a certain position is found to depend on the magnetic induction. In this way the average field dependence of the critical current density Jc(B) is obtained also at low fields, which is not accessible to magnetisation measurements due to the self-field of the sample. It is further shown that an evaluation of magnetisation loops, taking the self-field into account, results in a similar dependence in the field range accessible to this experiment.

Coated conductors can be produced by hybrid liquid phase epitaxy (HLPE) with high growth rates and excellent critical current densities. Samples with a thickness of about 1 μm carrying a current of several 100 A/cm-width were reproducibly fabricated in this way. In this paper we report on the critical current densities in HLPE coated conductors focussing on the angular dependence of Jc(H,θ). Of particular interest for future technical applications is a reduction of the ratio Jc(H∥ab)/Jc(H∥c). This can be achieved by defects induced during crystal growth and correlating with the c-axis of the conductor, which therefore predominately contribute to Jc(H∥c). The correlation of the pinning sites will be discussed in terms of the Jc-anisotropy in fields of up to 6 T.

High temperature superconductors (HTS) might be applied for the magnets in a next step fusion device. The magnets could operate in the temperature range of liquid nitrogen or slightly below. Apart from the special requirements on the mechanical properties, the conductors will be exposed to radiation. We examine the influence of neutron irradiation on coated conductors. These conductors were irradiated sequentially to the ITER specification of 1022m−2. The samples were examined by magnetisation measurements and by direct transport measurements with regard to changes in their superconducting parameters. The critical current, its anisotropy and the irreversibility line were determined under different conditions. The magnetisation measurements were carried out in a vector vibrating-sample magnetometer (VSM) in a wide temperature range with a maximum field of 5T. The transport measurements were carried out in fields up to 15T. An increase of the critical current for fields parallel to the c-axis is observed in the high field region. A crossover of the critical current between the unirradiated and the irradiated state is found at lower fields. Also a peak in IC (shoulder effects) can be observed in anisotropy measurements at small angles (close to the ab-plane) in highly irradiated samples.

The development of coated conductors is now entering a stage where they become available in lengths of several meters. A technique capable of analysing the local superconducting properties of long samples within a relatively short time is therefore highly desirable. In this paper, the application of a scanning technique, initially developed for the characterisation of bulk materials [M. Eisterer, S. Haindl, T. Wojcik, H.W. Weber, Supercond. Sci. Technol. 16 (2003) 1282] to coated conductors is presented. The principle consists of spatially mapping the magnetic field (B-z) perpendicular to the top sample surface, generated by local shielding currents. The supercurrents are induced by a permanent magnet placed above the scanning Hall probe. This technique was successfully applied to a sample with a length of 10 cm, providing information on the tape homogeneity. Local disturbances of the supercurrent flow and spatial variations of the critical current density are detected in this way on the scale of <= 1 mm. (c) 2007 Elsevier B.V. All rights reserved.

We report on Hall scan and magnetoscan measurements on coated conductors. The magnetoscan technique was optimized for scanning tapes of high temperature superconductors, in order to analyze their homogeneity and current carrying capability. The principle of the magnetoscan technique consists of magnetizing the sample locally by moving a magnet over the sample surface and measuring the magnetic field of the induced supercurrents with a Hall probe. The position of the Hall probe, the permanent magnet as well as the gap between permanent magnet and sample can be changed in our setup. The measurements were done on different conductors with different architecture. The map of the magnetic field above the surface of the sample reflects 'strong' and 'weak' superconducting areas in the tape allowing to analyze the defect structure and the critical currents. The results show good agreement with conventional measurements.

The magnetoscan technique was recently introduced to investigate local inhomogeneities of large high T, superconductors without destroying them. Local currents are induced by a small permanent magnet moving slightly above the sample surface. The magnetic field of these currents, which depend on the local material properties, is measured by a Hall probe. We show that the technique can be successfully applied to coated conductors. Inhomogeneities and defects are easily detected by significant changes in the magnetoscan signal. To confirm our interpretation of the experiment, numerical simulations, based on Maxwell's equations, were carried out. We calculate the current flow and the magnetoscan signal for different experimental setups and critical current densities. Inhomogeneities were introduced by assuming a position dependent critical current density. Good agreement between experiment and theory is obtained. (c) 2007 Elsevier B.V. All rights reserved.

The authors report on measurements of the local supercurrent density in long Y1Ba2Cu3O7−δ based coated conductors by the magnetoscan technique. Significant inhomogeneities were found, which are well resolved by the resulting magnetic field map. A single central line scan along the length of the conductor reflects the inhomogeneities over the entire width of the sample, thus offering the possibility of very fast characterization. Modifying the applied field leads to different results highlighting either the overall critical current or details of the defect structure. In addition, numerical simulations of the current dynamics were carried out for a qualitative and quantitative interpretation of the results.

Hybrid liquid phase epitaxy (HLPE) is among the most promising high rate techniques for fabricating coated conductors. In this paper we report on the irreversible magnetic properties of HLPE thick films grown on single crystalline SrTiO3 at temperatures of 860degC and 890degC. Magnetic measurements in the temperature range from 5 to 90 K were carried out on these samples to determine the critical current density. Furthermore, neutron irradiation which introduces effective pinning centers of known density provides us with additional information on the field dependence of pinning in HLPE coated conductors.

Mechanical stress is introduced into the bulk of large YBCO single domains during oxygenation, mainly because of the tetragonal to orthorhombic phase transition. Due to the slow oxygen diffusion rate, full oxygenation is nearly impossible in reasonably large samples without the occurence of macro-cracks. An alternative morphology of the bulk, i.e. a periodic array of holes drilled parallel to the c-axis prior to oxygenation, could lead to a solution of this problem, because the perforation limits the wall thickness (diffusion distance) to the distance between the holes and provides additional paths for oxygen flow during processing. Conventional flux density mapping of the fully magnetised bulks showed, that the artificial holes were beneficial for the material properties, resulting in an enhancement of the trapped flux density compared to reference samples without these holes. However, variations of the flux density profile with the periodicity of the perforation are observable at higher resolution. This indicates that the array of holes influences the local current distribution. The bulks were also investigated more locally, by the magnetoscan technique. These measurements clearly display the array of holes and provide additional information about the cracks.

Melt processed superconductors containing mechanically drilled holes parallel to the c-axis were investigated by means of Hall scans of the trapped field distribution and the magnetoscan technique. We show that the remnant flux profiles are affected by the perforation in different ways. The location of the holes can be resolved by field mapping at activation fields below that for complete penetration of the bulk. The magnetoscan technique allows one to spatially localize the array of holes.

A project to measure the u.v. and visible spectral irradiance of global and diffuse daylight and direct sunlight over a one year period was undertaken at Durban. Special attention was given to the 295 to 315 nm region which is of biological interest. One of the goals of the project was to establish how well measured values agree with those obtained semi-empirically by computation. In spite of the many variables and some uncertainties, concerning the different parameters, fairly good agreement has been obtained with the results of Dave and others.