A magnet structure produces a field within a magnet gap. The field is provided at least in part by a pair of permanent magnets that are fixed in place by a frame. The frame fixes a magnet assembly that is adapted to hold the magnetic material composing the permanent magnets, such that the quantity of the magnetic material can be adjusted to suit the particular application. The magnetic material can be provided in the form of discrete magnetic elements, such as magnetic “bricks”. The frame also functions as the flux collector and return. Accordingly, the general geometry of the magnet structure is fixed, and the amount of magnetic material, and therefore the magnetic field strength, is adjustable.