M. Carrera

Characterization of superconducting rings using an in-field Hall probe magnetic mapping system

A Hall probe magnetic imaging system that works in magnetic fields in the range -1 T to 1 T has been implemented, and it has been used to characterize the superconducting behavior of single domain melt textured YBa/sub 2/Cu/sub 3/O/sub 7/ rings. We show that in addition to the analysis of the evolution of the local magnetic field distribution when the external magnetic field is cycled, the hysteretic behavior of the magnetic moment can also be investigated after integration of the local magnetic field. The critical current density has been determined through the critical state model and it has been compared to that calculated by inversion of the Biot-Savart law. A remarkable agreement is achieved with both methods.

A new method of computation of current distribution maps in bulk high-temperature superconductors: analysis and validation

We present a test and analysis of our method of computation of the distribution of currents in bulk superconducting samples, which is sensitive to current contribution in the deep layers of the sample. The procedure is based on measurements of the magnetic field with a Hall probe, inverted by linearization and orthogonal triangularization, known as QR decomposition, of the matrix in the resulting linear system. No assumptions on the number or geometry of domains are required. The only constraint on the method is that the critical current must be homogeneous along the c-axis. Our method is applied to real 3d samples with size in the cm range and different geometries of technological interest. The propagation of errors in the general case is analysed, and we also supply a method to estimate the error in every computation, which is applied to the computed J(Jx, Jy) in the above samples.

In-field Hall probe mapping system for characterization of YBCO welds

Artificial welding of melt-textured YBCO blocks opens the door to the fabrication of large, complex-shaped pieces required for applications. In order to evaluate the superconducting quality of the welds, we have developed a Hall probe mapping system, able to record the local magnetization at 77 K under dynamic applied fields in the range of -1 to 1 T. The system was used to characterize welded samples prepared with a new Ag induced surface melting joining technique. The magnetization maps of unwelded and welded samples of various qualities are compared and discussed. The current distributions associated to the Hall maps were calculated using the Caragol software. The magnetization and current distribution maps over the joint show that good quality welds can be reached with this joining method.

Computation of Current Distribution in YBCO Tapes With Defects Obtained From Hall Magnetic Mapping by Inverse Problem Solution

The development of superconducting devices based on long-length HTS tapes often requires of these tapes high homogeneity along its length as well as across its width. This implies the absence of significant local defects. Non-destructive characterization techniques to examine critical current distribution for defect detection are of great interest, specially if they could be applied in situ for real-time testing of large lengths of tape. In this work, we continue the adaptation of our method for the computation of critical current maps from Hall measurements of the magnetic field over the tape. We compute the current density distribution in a stretch of a commercial YBCO tape which contains defects by using a specifically designed fast inverse problem solver. The 2-dimensional current map meshes with the current distributions in a cross-section of the tape that we previously computed in real time, so that a map of the critical current circulating on the entire surface of a tape with isolated defects may be obtained, regardless of its length, by running a Hall probe over it. This method is applied to a series of Hall mappings corresponding to several magnetization regimes, produced by applying different current intensities to the tape. Details of the experiments and the calculation method are reported and the applicability to detect the impact of the defects in the tape over the current distribution is discussed.

Current distribution maps in large YBCO melt-textured blocks

Abstract We report on the progress on computing the current maps from the magnetic induction in bulk samples of YBCO melt-textured compounds. Typical sample size is in the cm range and induction maps Bz(x,y,z) are obtained by Hall probe measurements. Currents are obtained by solving the inverse Biot–Savart problem. No assumption is required on the number of domains or geometry of the sample; the only constraint in the model is that the current circulation should be confined in the basal plane and uniform in the sample thickness. A current map with a resolution of 0.25–0.5 mm is typically obtained. Critical tests of the stability and accuracy of the current flow solutions have been performed. We have applied the algorithms to remanent flux maps obtained after both FC and ZFC magnetization processes. The algorithms can be readily used and thus can constitute a helpful tool for superconductivity engineers. The use of our software implementation is available free of charge at http://jaumetor.upc.es:8080/caragol .

Current distribution in HTSC tapes obtained by inverse problem calculation

The development of SC devices based on HTSC tapes requires a deep knowledge of the current distribution in both pre-saturation and post-saturation regimes. Magnetic measurements have shown the possibilities to derive the current distribution by Inverse Problem Solution in finite sized bulks, based on a non destructive measurement of the magnetic field created by the own current flowing in the SC. In this work, the QR inversion strategy is extended to non finite systems by considering the effect of the boundaries. We present a method to derive the current distribution in a cross-section of a tape based on Hall magnetic mapping by using a specifically designed inverse problem solver. This method is applied to a series of Hall measurements corresponding to a full magnetization cycle of a commercial tape, produced by applying a set of the currents applied to the tape of several intensities. Details of the experiments and the calculation method are reported and the applicability as homogeneity test and losses studies is discussed.

Critical current determination of artificially welded HTS samples by in-field Hall mapping technique

The direct observation of the magnetic field at the surface of SC samples when the field is applied or in the remanent state, allows the observation of the current distribution along the magnetization loop by using inverse problem solvers. Furthermore, the mean value of the field reflects well the magnetization of the sample obtaining the magnetization loop taken in account both possibilities, the In-Field Hall Mapping technique, thus, has revealed as a powerful characterization technique. This technique can be improved by including the critical state simulation, giving so a very complete way to characterize artificially welded superconducting samples, thus allowing the identification of the critical current flowing through the surface between domains as is the case of the effect of welded bulks. Some examples of the characterization procedures are reported.

Twins, electron-phonon coupling and fluctuations in Y0.5Sm0.5Ba2Cu3O7−y

We report electrical resistivity measurements of Y0.5Sm0.5Ba2Cu3O7−y samples with well defined particle size, oxygen content and twin density. From the analysis of the resistivity data we show that the electron-photon coupling is weak in these materials (λ<0.2). The upper limit of the coupling constant appears to be weakly sensitive to the oxygen content (for y<0.25) or to the twin density and it is independent on the RE. The analysis of the paraconductivity for the sample with the lowest twin density (if not zero) reveals three well defined critical regions. These regions may correspond to a mean field, crossover and full critical dynamical region. The full critical region is reached at higher reduced temperature (ln t<−5) than ever previously reported. We discuss the relevance of particle size, oxygen vacancies and twins on the enhancement of t and on the fluctuations.

An effective model for fast computation of current distribution in operating HTS tapes from magnetic field measurements in non-destructive testing

Computation of the current distribution of superconducting devices is an important topic in the understanding of the behavior of superconducting materials, in the development of their applications and in testing the quality of the materials. The most successful technique to measure the currents is based on mapping the magnetic field in the surface of the superconducting sample during or after a magnetization process. Models for solving the inverse problem have been developed based on different techniques. The inversion technique based on considering the currents as the result of a distribution of magnetic moments has been tested for many years in the description of the currents in bulks, giving a useful tool for their characterization, including junctions and porous samples. The restriction of this technique to closed loops of currents does not allow its application to the exploration of superconducting wires. Specific considerations developed for this technique have, however, allowed extension of the technique to the study of HTS tapes in a simple and fast way that can be applied to the exploration of long tapes to determine the current distribution ?on the fly?. In this work we report on the model and the procedure to perform the calculation of currents on tapes carrying current. The specific application to the detection of defects that do not affect the ability to carry current, such as longitudinally centered scratches, is experimentally tested. Details of the application of the method for long tapes are also reported.

Computation of critical current in artificially structurated bulk samples from Hall measurements

High-temperature superconducting artificially structured bulk samples, such as bulks with slanted drills or welds of different pieces along the c-axis with holes drilled in different asymmetric positions, pose a challenge for the computation of critical current maps able to spot the position of inhomogeneities, as they combine a current map in every ab-plane adapted to the drill/welding structure with the dependence of this structure on the position along the c-axis. We improve and adapt our technique of Hall probe field measurement, discretization and QR inversion to this problem by measuring the magnetic field on top and bottom of the bulk sample to obtain some spatial resolution of the current. To test our technique we apply it to both simulations with asymmetrical geometry and a real artificially structured bulk samples.