Superconducting Magnetic Levitation

Abstract

Some possible applications of ceramic superconductors in magnetic bearings have been addressed in the introduction. One important quantity in these applications is the levitation force which is exerted on a permanent magnet by a superconducting ceramic sample. The basic form of levitation is caused by the Meissner-Ochsenfeld effect. Owing to the complete expulsion of magnetic flux out of a type I superconductor a permanent magnet is levitated above it. This effect is used as an experimental proof of superconductivity in HTSCs. However, it can only be demonstrated in type I superconductors or in type II superconductors below H _c1 . Melt-processed YBCO ceramics, which are used for magnetic-bearing application, are extreme type II superconductors. That means that magnetic flux can enter the sample as soon as H _ext δ; H _c1 . Good-quality melt-grown ceramics are hard superconductors; thus flux is pinned. This leads to a further physical effect, magnetic suspension, when a permanent magnet is suspended below a superconductor. An important consequence is:The Meissner-Ochsenfeld effect cannot be demonstrated with a high-quality melt-grown YBa ₂ Cu ₃ O _7-δ sample above H _c1 !The physical reason for this will be discussed below.