FIGS. 2A and 2B-2D are a perspective view of an asymmetric magnet array 200, and plots of magnetic field lines generated separately and jointly by the assemblies, respectively, according to another embodiment of the present invention. Uniformity is not evident by uniform density of the lines (as lines were drawn denser in the imaging zone for better details) rather by z-axis alignment of the lines.

As seen in FIG. 2A, an inner volume 230 is a simply-connected region at least partially enclosed by a first magnet assembly 210, which is typically an ellipsoid or a sphere. A second magnet assembly 220 of the asymmetric array, “caps” inner volume 230. As mentioned above, different rings may have different magnetization directions to optimize the uniformity and fringe field of the magnet array. For instance, one ring may have a magnetization vector in a direction substantially different (e.g., by more than 45 degrees) from another ring. For instance, the magnetization vectors of the permanent magnet segments may point primarily in the r direction in one ring, and primarily in the Z direction in another ring. Furthermore, two rings belonging to the same assembly may have substantially different magnetization directions. For instance, one ring of the first assembly may have its magnetization primarily in the r direction, another ring of the first assembly may have its magnetization in primarily the ?z direction while a third ring of the first assembly may have its magnetization at ?45 degrees in the r-z plane. In an embodiment, the two or more ring have a magnetization vector in a direction different by more than 45 degrees from one another.