As seen, by way of example, array 400 comprises ten magnet rings 411-420 which are coaxial with a central Z-axis passing through an inner volume 430. The different rings are located at different positions along the Z-axis and, in general, have different transverse dimensions, radial thicknesses, and axial thicknesses. As seen, magnet array 400 has reflectional asymmetry with respect to the longitudinal axis (i.e., is asymmetric with respect to Z-axis inversion). In the context of the present disclosure and in the claims, the term “reflectional asymmetry with respect to the longitudinal axis” means that no plane perpendicular to the longitudinal axis is a plane of symmetry for the magnet array. In other words, the magnet array is not symmetric under flipping with respect to the longitudinal axis at any point along the axis. Reflectional asymmetry is also referred to as point asymmetry or mirror-image asymmetry. For brevity, any reference to “asymmetry” of the magnet array in the description below means the reflectional asymmetry defined above.

In addition, inner volume 430 may be interior (as shown) or at least partially extending exterior in the z-direction (not shown) to magnet array 400. Furthermore, the disclosed magnet array may or may not be combined with a yoke.