Cryogenic Engineering Conference, Montreal, Canada, 12 - 15 July 1999, vol.45, pp.1707-1714
We have investigated the effect of high-temperature superconductor (HTS) films deposited on substrates that are placed above bulk HTSs in an attempt to reduce rotational drag in superconducting bearings composed of a permanent magnet levitated above the film/bulk HTS combination. According to the critical state model, hysteresis energy loss is inversely proportional to critical current density Jc, and because HTS films typically have much higher Jcvalues than those of bulk HTSs, the film/bulk combination was expected to reduce rotational losses by at least one order of magnitude in the coefficient of friction, which in turn is a measure of the hysteresis losses. We measured rotational losses of a superconducting bearing in a vacuum chamber and compared the losses with and without a film present. The experimental results showed that contrary to expectation, the rotational losses are increased by the film. These results are discussed in terms of flux drag through the film, as well as of the critical state model.