`Electromaglev' (`active-maglev') - magnetic levitation of a superconducting disk with a DC field generated by electromagnets. Part 4: Theoretical and experimental results on supercurrent distributions in field-cooled YBCO disks

We present Part 4 results of a comprehensive theoretical study of an `electromaglev' system, in which a high-temperature superconducting bulk YBCO sample is levitated stably in a DC magnetic field generated by magnet system underneath the floating object. An electromagnetic analysis, based on a three-dimensional finite element technique (FEM) applied to the current vector potential method, has been developed to determine the supercurrent distribution in a field-cooled (and hence trapped-flux) YBCO disk that levitates stably in a magnetic field generated by the magnet system. The supercurrent distribution thus determined was in turn used to compute trapped-flux-induced field profiles of the disk and predict a `levitation current' in the magnet system at which the disk, initially resting on a support plate, begins to levitate. Agreement between computed field profiles and levitation currents and those measured in the experiment was excellent, validating the analysis itself and the method used to derive solutions. The analysis demonstrates that the supercurrent distribution within a trapped-flux disk is far more complicated than that derived from the Bean model for a long cylinder under a uniform axial magnetic field. It is used for a parametric study of the effects of disk dimensions (radius, thickness, radius/thickness ratio) and trapped-flux strength on supercurrent distribution and lift-to-weight ratio. The magnitude of the B_r component generated by the magnet system is very important for lift and it is shown that thinner disks rather than thicker disks can improve lift-to-weight ratio. Because accuracy of the analysis is disk-size independent, small disks are time-efficient for performing the analysis.