Superconducting Levitation

In this article we review the levitation technologies based on superconductivity. To float an object in midair, the lifting force to counteract gravity is necessary. However, the upward force alone is not sufficient and stabilizing forces must act on the object in horizontal directions for a stable levitation or suspension. Herein, we discuss that stable levitation is not achieved by Meissner effect or perfect diamagnetism of superconductors, since there is no restoring force in the horizontal direction. Hence type I superconductors are not suitable for stable levitation. Furthermore, the suspension of a permanent magnet by a superconductor is achieved only by the aid of pinning of magnetic flux lines, which is important for realizing zero resistivity in type II superconductors. The flux pinning imparts the restoring force to push the levitating object back toward the equilibrium position. The levitation force of superconductors can also be understood in terms of a combination of electromagnetic induction and Lenz's law. The levitation mechanism of superconducting Maglev is different from those of superconductor pellet. Electrodynamic suspension technology is used for Maglev levitation, where superconducting magnets are simply used as a source of high magnetic field. When the train moves, the superconducting coils on the train induce electric currents in the guideway's normal conducting coils, and the train is levitated by the repulsive and attractive force between these magnetic fields. The differences between type I and II superconductors and the basic mechanism of flux pinning are reviewed in the appendix.