Development of superconducting split-coil magnets generating high and homogeneous fields

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Abstract
High and homogeneous magnetic fields can easily be generated by superconducting solenoids. Superconducting split-coil magnets are both suitable for fields up to about 4.77*106 A m-1 (60 kOe) and sufficiently homogeneous over the sample (1 cm3) to permit wide-line NMR experiments. Moreover, the magnets are limited in size, i.e. smaller than 20 cm in diameter. The magnetic field is calculated for a general system of coils that possesses cylindrical symmetry and that also has a mirror plane perpendicular to the cylindrical axis. An expression for the homogeneity of the central field is derived. Two models of split-coil magnets are treated in detail: the L-coil and the N4-coil. Due to the number of available free coordinates the homogeneity of the field can be corrected up to eighth order for these magnets. The results of the calculations for L- and N4-coils are given graphically. Finally, the properties of an actually built magnet of the N4 type are discussed.