Self-field measurements and current distribution in ceramic superconductors

We report on self-field measurements and calculation procedures which have yielded a methodology for the determination of critical current density distributions in superconductors. Axial components of the self-field, generated by polycrystalline superconductors of different geometries and composition, have been measured at grid points in planes above the sample. The measurements were made with a precision custom-designed and computer-controlled x-y scanning assembly at liquid-nitrogen temperature, using a Hall probe or a superconducting magnetic-field sensor developed in our laboratory. A method for calculating current distributions based on inverse matrix and iteration procedures has been developed for flat samples of arbitrary shape. The method makes possible the evaluation of persistent current patterns in thin superconducting samples, using experimental field data and ''inverted'' matrix equations relating currents and fields.