New technique for the measurement of the intrinsic drag torques of high temperature superconductor bearings

A new technique providing a non-intrusive means for the isolation and accurate measurement of the intrinsic drag torques of high temperature superconductor bearings is reported. It involves measuring the rotational acceleration and deceleration of a rotating symmetric object suspended in vacuo on a bearing formed by the combination of a magnet and an HTS pellet. Accelerations are produced by exploiting molecular drag torque in medium Vacuum while deceleration from low starting speeds in ultra high vacuum, in which gaseous drag and eddy current effects are minimized enables intrinsic drag torques to be accurately determined. Experiments have been carried out on YBa2Cu3O7 pellets and improvement in pellet grain structure is shown to decrease the size of the intrinsic drag torque. Theoretical estimates are in reasonable agreement with measured values. The effect of the magnetic field intensity of the suspended magnet and the suspension height is also reported. A threshold type starting phenomenon was observed in which the torque required to accelerate the suspension was two to three orders of magnitude higher than the intrinsic drag torque obtained from deceleration measurements. Observations of spontaneous reacceleration and the potential for using the device to measure vacuum pressures is also discussed.