A magnet structure includes a permanent magnet assembly and first and second pole assemblies. The permanent magnet assembly includes an enclosure having an open portion and a magnetic mass disposed within the enclosure. Each of the first and second pole assemblies includes a base portion, disposed within the enclosure, a pole extension, extending from the first base portion outward from the enclosure, and a pole on the pole extension, having a pole face. The first and second pole assemblies are arranged such that the pole face of the first pole opposes the pole face of the second pole.