Design considerations and performance of a PM linear actuator in a radiation environment

This paper discusses design considerations for a water cooled high acceleration 3-phase air-cored brushless DC PM linear actuator used in a vacuum radiation environment. Radiation can cause damage to magnets, and the requirement for a vacuum chamber around the moving parts imposes additional constraints that further complicate the electromagnetic and mechanical design of the actuator. This paper discusses the selection of suitable materials and bearings that are compatible with operation in vacuum and can cope with the required millions of actuation cycles. The selection of suitable bearings with low friction and wear is discussed and the design of a low inertia shaft is described. The factors that have an influence on the susceptibility of the magnets to radiation damage are discussed. These factors include magnet dimensions, magnet material, external magnetic field, temperature and the directions of both the magnetic flux and radiation. FLUKA simulations are presented showing the fluences of protons, neutrons, electrons and gamma radiation to which the magnets are exposed. Based on these simulations, loss of magnetisation for different magnet materials can be predicted, and used to estimate the effect of magnet radiation ageing on actuator current, and increased temperature rise. The paper also presents transient electromagnet FEA computation of the force produced by the actuator when magnets are housed in a stainless steel vacuum chamber.