Field mapping for characterization of superconducting tapes, as a function of the position and magnetic flux density
Résumé
We propose a useful contactless method for the characterization of superconducting tapes. It consists in imposing a constant flux density to the tape, thanks to permanent magnets. The superconductor being cooled at zero field, currents are induced in the conductor. A map of the flux density is then measured over the tape, using a Hall effect probe mounted on an x-y-z table. From these measurements and thanks to an inverse problem calculation, the distribution of the current flowing through the tape is computed. This operation is repeated for different applied flux densities, by varying the distance between the tape and the magnets, to obtain the field dependence of the current distribution. This result allows visualizing defects in the superconductor and their field dependence. Moreover, by fitting measurements by a current distribution model, we can obtain the critical current density as a function of the flux density and of the position on the tape. This method has been tested experimentally on an YBCO tape of 30 cm length. We therefore used a magnet assembly measuring 50x5x1 cm. It can impose to the tape a flux density varying from 7 to 180 mT with an inhomogeneity smaller than 3.5%. This method has the advantage of not only detect defects in superconducting tapes, but also to quantify them and to obtain a real measure of the critical current density. Moreover, its field dependence is also obtained.