F. Hengstberger

Assessment of the local supercurrent densities in long superconducting coated conductors

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.

Thickness dependence of the critical current density in superconducting films: A geometrical approach

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.

Assessing the spatial and field dependence of the critical current density in YBCO bulk superconductors by scanning Hall probes

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 thickness-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 also obtained at low fields, which is not accessible to magnetization measurements due to the self-field of the sample. It is further shown that an evaluation of magnetization loops, taking the self-field into account, results in a similar dependence in the field range accessible to this experiment.

Neutron irradiation of coated conductors

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.

Scan Techniques for Coated Conductors

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 dasiastrongpsila and dasiaweakpsila superconducting areas in the tape allowing to analyze the defect structure and the critical currents. The results show good agreement with conventional measurements.

Rotating sample magnetometer for cryogenic temperatures and high magnetic fields

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.

Critical current anisotropy in nanostructured HLPE coated conductors

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.

Magnetic measurement of the critical current anisotropy in coated conductors

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.

Current and Field Distribution in Meandered Coated Conductors for Roebel Cables

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.

Numerical extension of the power law Jc(B) to zero field in thin superconducting films

Abstract Numerical simulations of the current and field distribution in thin superconducting films are carried out for a given material law Jc(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 Jc(H) is computed for comparison with experiment, considering the geometry of transport measurements. We show that extrapolating the high-field power law J c ∝ B - α to the lowest fields results in a finite critical current at zero applied field J c ( H = 0 ) , despite the singularity of Jc(B). Moreover, particular features of the experiment, such as a low field plateau in Jc(H), are reproduced and found to be determined by the self-field.