Reduced hysteresis loss in superconducting bearings

Spin-down results are reported for a permanent magnet (PM) ring levitated in vacuum over an array of high-temperature superconductors (HTSs). A method was developed that uses a small amount of soft-ferromagnetic shims to reduce the azimuthal inhomogeneity of the PM. The spin-down results show that this method reduces low-velocity hysteresis losses but does not change velocity-dependent losses. These results are consistent with the interpretation that velocity-dependent losses are primarily due to eddy currents induced in the PM by inhomogeneity of the field produced by the magnetized HTS array and that the velocity-independent losses are a result of hysteresis in the HTSs; this, in turn, is due to inhomogeneity of the PM magnetic field. The low-speed results suggest that surface pinning, as opposed to bulk pinning, may be the dominant mechanism in hysteresis losses for PMs with low inhomogeneity. Residual gases in the vacuum chamber are shown to contribute negligibly to rotational losses. We measured a low-speed coefficient of friction of 6 x 10(-8).