Drag torque on slowly rotating high temperature superconductor in non-axisymmetric magnetic field

To further understand the relationship between the residual loss and the deviations of the magnetic field from perfect axisymmetry in a superconductor magnetic bearing, the drag torque acting on a high temperature superconductor slowly rotating in a nonaxisymmetric magnetic field was studied. A circular YBaCuO disk is rotated about its central axis, in the presence of a nonaxisymmetric magnetic field created from a pair of coils placed in its vicinity. This nonaxisymmetric field is either a uniform, or a uniform gradient magnetic field perpendicular to the axis of rotation of the YBaCuO disk. The torque exerted by the coils on the YBaCuO disk was monitored as a function time as the rotation proceeds, for different values of the current being passed through the coil. This torque exhibits periodic variations with a nonzero mean drag component which can be related to power loss, while the periodic variations were found to be connected to initial conditions and/or nonuniformity of the YBaCuO disk itself