DESIGN AND CONSTRUCTION OF A LOW-POWER-DISSIPATION, CYLINDRICAL Z(4) SHIM COIL FOR A WIDE-BORE NMR MAGNET

The relative merits of various designs of z(4) shim coils consisting o f a small number of discrete rings, situated on a cylindrical former, are investigated. The power dissipation associated with a given z(4) h eld and deviations from the ideal z(4) behavior are determined for two types of design, namely those that generate no z(0) and z(2) componen ts and those that generate no z(2) and z(6) components. The constructi on and performance of a shim coil of the latter type are described. In stallation of this shim coil in a 1984-vintage animal-size NMR magnet has enabled the large z(4) field variation to be fully compensated wit hout excessive power dissipation, thereby greatly improving the field homogeneity. (C) 1994 Academic Press, Inc.