Abstract
Stacks of large width superconducting tape can carry persistent currents over similar length scales to bulk superconductors, therefore giving them potential for trapped field magnets and magnetic levitation. 46 mm wide high temperature superconducting tape has previously been cut into square annuli to create a 3.5 T persistent mode magnet. The same tape pieces were used here to form a composite bulk hollow cylinder with an inner bore of 26 mm. Magnetic levitation was achieved by field cooling with a pair of rare-earth magnets. This paper reports the axial levitation force properties of the stack of annuli, showing that the same axial forces expected for a uniform bulk cylinder of infinite Jc can be generated at 20 K. Levitation forces up to 550 N were measured between the rare-earth magnets and stack. Finite element modelling in COMSOL Multiphysics using the H-formulation was also performed including a full critical state model for induced currents, with temperature and field dependent properties as well as the influence of the ferromagnetic substrate which enhances the force. Spark erosion was used for the first time to machine the stack of tapes proving that large stacks can be easily machined to high geometric tolerance. The stack geometry tested is a possible candidate for a rotary superconducting bearing.
Highlights
Stacks of high temperature superconducting (HTS) tapes have proven potential to act as composite superconducting bulks, for either trapped field magnets or as passive components of a magnetic levitation system
The cylindrical geometry has previously been investigated by using HTS tape in the form of a coil to create a hollow cylinder with a bore of 35 mm [10]
Wide width HTS tape can be machined into annuli and used for magnetic levitation suited to a cylindrical rotary bearing geometry
Summary
Stacks of high temperature superconducting (HTS) tapes have proven potential to act as composite superconducting bulks, for either trapped field magnets or as passive components of a magnetic levitation system. American Superconductor produce 46 mm wide tape which was used to create square annuli with a 26 mm hole These annuli were stacked to form a stack with a 26 mm bore capable of generating a uniform persistent field when magnetized using field cooling [3, 4]. The cylindrical geometry has previously been investigated by using HTS tape in the form of a coil to create a hollow cylinder with a bore of 35 mm [10]. Over 300 N of axial force was measured in this case for a pair of rare-earth magnets This approach has the advantage of creating composite bulk cylinders of potentially unlimited diameter, but the currents induced in such a cylinder can be complex due to no directly circulating current paths around the bore. The main advantage of using the stack of commercial HTS tape annuli for superconducting levitation is the predictability and uniformity of the superconducting properties related to available commercial tape
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