Adjustable passive shims for dipole NMR magnets

Abstract

Two new methods are presented for shimming dipole magnets intended for NMR. Both methods allow for very fine scale adjustment of the passive shim strength, in one case while the shim remains in the magnet. The first method utilizes magnetic ink printed with a standard ink-jet printer. First and second order shim fields are produced, and the shimming of a 1.4 T magnet to a variation of less than 3 ppm over an 8 mm diameter sphere is demonstrated. The RMS difference between the measured and expected fields is less than 0.1 ppm. The passive shim requires less than 0.25 mm of gap space per pole. The second method involves orienting pairs of fixed-strength shims in a manner that produces correction fields that continuously span a range of strengths and directions. The adjustment of the fields produced by these shims is demonstrated for first and second order corrections. The shims are used to correct the field errors in a 0.5 T magnet.

Keywords: Adjustable passive shims; Dipole NMR magnet; Passive shims; Permanent magnet.