Superconductor-Magnetic Levitation: Investigation of the Restoring Force

Stable levitation of systems consisting of a magnetic source and a superconductor is the root of some promising applications of superconductors, especially Maglev trains and superconducting bearings. However, instability can occur in some circumstances. In this contribution, we firstly report measurements of the lateral force of two different axisymmetric set-ups. The first one is measured at 20 K and consists of a 70 mm diameter MgB2 disc and a 80 mm NdFeB magnet. The second is measured at 77 K and comprises a 56 mm diameter YBCO disc and a 45 mm NdFeB magnet. For both investigated set-ups, the superconductor was cooled down at distance Zcp from the magnet. In the case of the first set-up levitation was stable in the vicinity of Zcp while unstable far from Zcp. Levitation was stable with the second set-up, whatever the Z value. In a second part of this work, we present an analytical mean field model permitting to reproduce all the measurements and we state the condition required to secure the stability of axisymmetric superconducting magnetic levitation systems.