Patent ID: 7898257

Claim:
A yoked magnetic structure providing a predetermined volume of substantially uniform magnetic field in a region having an axis of symmetry parallel to the direction of said magnetic field, said yoked magnetic structure comprising: a. an open ferromagnetic yoke having a first yoke surface and a second yoke surface, said first and second yoke surfaces being spaced apart along said axis of symmetry, said first and second surfaces shaped as a mirror images of each other; b. a first permanent magnet assembly attached to said first yoke surface, said first permanent magnet assembly comprising a plurality of first annular permanent magnets; and, c. a second permanent magnet assembly attached to said second yoke surface, said second permanent magnet assembly comprising a plurality of second annular permanent magnets, the relative positions of said plurality of first annular permanent magnets and said plurality of second annular permanent magnets chosen so as to provide a volume of substantially uniform magnetic field; wherein at least one of the said annular permanent magnets has a non-rectangular cross -section.