A ring magnet assembly has a generally cylindrical magnet body defining an air gap having an upper end and a lower end. Upper and lower face plates dispose respectively at an upper portion of the ring magnet and lower portion of the ring magnet. The face plates preferably have a high magnetic permeability. A mass analyzer may be disposed within the air gap. An ion generator may be disposed within an air gap of a ring magnetic assembly of the present invention. A pair of vertically-stacked magnetic ring assemblies may be provided. In that embodiment, a mass analyzer may be disposed within one air gap and an ion generator within another.