REDUCED HYSTERESIS LOSS IN SUPERCONDUCTING BEARINGS

Spin-down results are reported for a permanent magnet (PM) ring levita ted in vacuum over an array of high-temperature superconductors (HTSs) . A method was developed that uses a small amount of soft-ferromagneti c 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 consi stent with the interpretation that velocity-dependent losses are prima rily due to eddy currents induced in the PM by inhomogeneity of the fi eld produced by the magnetized HTS array and that the velocity-indepen dent 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 s uggest that surface pinning, as opposed to bulk pinning, may be the do minant mechanism in hysteresis losses for PMs with low inhomogeneity. Residual gases in the vacuum chamber are shown to contribute negligibl y to rotational losses. We measured a low-speed coefficient of frictio n of 6 x 10(-8).