STABILIZATION TECHNIQUES FOR SPINNING ROTOR GAUGE RESIDUAL DRAG

The pressure-independent residual drag on a spinning ball requires an offset correction for the accurate use of a spinning rotor vacuum gage (SRG) at low pressures. Instabilities in the residual drag (RD) can cause significant errors in low-pressure SRG measurements. The instabilities usually occur as discontinuous shifts when a ball is levitated by the magnetic suspension, or as continuous changes as the ball slows down. The discontinuous shifts in residual drag have been found to be caused by changes in the orientation of the ball's magnetic moment, induced by the suspension field or the inductive drive circuit. Premagnetizing the ball in a strong field has stabilized the orientation of the magnetic moment and the RD value. The continuous changes are caused by competition between magnetic and inertial forces, which result in a frequency-dependent orientation of the rotational axis of the ball. Etching a spot on the ball alters its shape and defines an inertial axis. Magnetizing the ball in a preferred direction with respect to this axis minimizes both the residual drag and its frequency dependence. © 1990, American Vacuum Society. All rights reserved.