Magnetic enclosure and method

A magnetic enclosure providing a uniform magnetic field in a test chamber and comprising substantially identical first north magnetic structure and second south magnetic structures that are disposed in geometric mirroring symmetry opposite to each other and separated by an air gap forming the test chamber. Each magnetic structure includes a cap-like permanent magnet assembly covering a convex ferromagnetic pole piece, a ferromagnetic cap-like shield covering the magnet assembly, and a ferromagnetic ring. The ring is in contact with and intermediate the shield of the first and of the second magnetic structure, and has at least one aperture allowing access to the test chamber.