Guidance dynamics of mixed-mu levitators for MAGLEV vehicles

Mixed-mu levitation is the principle whereby iron is freely levitated in a magnetic field, being stabilized by the proximity of diamagnetic superconducting screen material [1]. For application to MAGLEV ground transport, superconducting field coils and stabilizing screens are mounted in on-board cryogenic levitator pods of the vehicle to give levitation about a mild steel rail [2]. The system exhibits little natural damping and the guidance damping requirements are particularly severe. Preliminary feasibility studies [3] show that reasonable ride characteristics in guidance are obtained using passive damping screens utilizing a secondary suspension and a primary system employing copper sheets attached to the cryostat walls. This paper fully investigates the damping and both static and dynamic stiffness obtained from the interactions of all mixed-mu lavitator components and, in particular, that occurring between the superconducting screens and the eddy current damper sheets. The superconducting screens are shown to provide a negative damping component whilst the damper sheets provide an enhanced dynamic stiffness. A non-linear time response analysis is presented together with predictions of both dynamic and static forces, the latter showing good agreement with magnetostatic experimental results.