Force density of magnetic bearings using superconducting coils and bulk superconductors

Stable levitation or suspension of a heavy object in mid-air can be realized using a combination of a permanent magnet and a bulk superconductor with high critical current density, in that the force density has reached 100 kN/m². The superconducting flywheel system for energy storage is attractive due to a great reduction in the rotational loss of the bearings So long as a permanent magnet is used as a magnetic source, however, the electromagnetic force (EMF) is essentially limited by its field strength. In order to overcome such a limitation, we employed a superconducting coil / magnet as a magnetic source and studied the EMF characteristics of bulk superconductors in high magnetic fields. We also measured the EMF between a bulk superconductor and a specially designed superconducting coil having a high field gradient, and confirmed that the EMF reached 9,000 N (force density: 1,190 k N/m²).