Abstract: A method and apparatus is provided for quickly shimming a tubular supermat to function in several different environments which create unique impurity gradients to interfere with uniformity of the electromagnetic field from the supermagnet. Non-magnetic tubes are secured to the supermagnet casing for receiving non-magnetic shim supporting rails. The shim supporting rails carry magnetic material masses, usually steel, for precise location on the periphery of the supermagnet casing. Holes may be cut into the supermagnet shroud for introducing and withdrawing the shim support rails. Arcuate aesthetic covers for the shroud holes are provided to restore the aesthetic appearance of the shroud.