Development of shimming system for a. highly sensitive NMR spectrometer with a superconducting split magnet

A highly sensitive NMR spectrometer with a superconducting split magnet and a solenoid-type probe coil is being developed. One of the-most challenging tasks is to achieve high homogeneity of magnetic field in a measuring volume with this configuration, because inevitable structural asymmetry and limited production accuracy of the magnet cause larger inhomogeneity of magnetic field compared with a conventional solenoid-type NMR spectrometer. Numerical estimation revealed that field correction capability of conventional shim coils, i.e., superconducting solenoid and saddle coils placed outside main coils, is insufficient for the split NMR magnet. Therefore, we have designed and constructed superconducting shim coils with sufficiently high capability by adopting the following three novel ideas: (1) superconducting shim coils are placed not only outside main coils but also inside them in the vicinity of a measuring volume; (2) superconducting shim coils with "periodically wavy" shape are utilized to correct non-axial inhomogeneous magnetic field; (3) the current of each superconducting shim coil is independently controlled to correct plural modes of magnetic field simultaneously.