Use of thin films in high-temperature superconducting bearings

In a permanent magnet/high-temperature superconductor (PM/HTS) bearing, a thin-film HTS positioned above a bulk HTS has been expected to maintain the large levitation force provided by the bulk while a low rotational drag is provided by the very high current density of the film. For the low drag to be achieved, the thin film must shield the bulk from inhomogeneous magnetic fields. However, measurement of rotational drag in a PM/HTS bearing that used a combination of bulk and film HTS showed that the thin film is not effective in reducing rotational drag. Subsequent experiments, in which an AC coil was placed above the thin-film HTS and the magnetic field on the other side of the film was measured, showed that the thin film provides good shielding when the coil axis is perpendicular to the film surface but poor shielding when the coil axis is parallel to the surface. This is consistent with the lack of reduction in rotational drag being due to a horizontal magnetic moment of the permanent magnet. The poor shielding with the coil axis parallel to the film surface is attributed to the aspect ratio of the film and the three-dimensional nature of the current flow in the film for this coil orientation.