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.
By using a split-pair superconducting magnet with cross-bore, one can implement a solenoidal radio frequency (RF) coil for solution nuclear magnetic resonance (NMR). We expect the solenoidal RF coil to give us higher sensitivity than an ordinary saddle-shaped RF coil. We have developed both room temperature and cryogenic NMR probes with a solenoidal RF coil for a split-pair superconducting magnet. The probes and the solenoidal RF coils were evaluated using a sensitivity test with standard 0.1% ethylbenzene. The sensitivity tests of the room temperature probes indicated that the signal-to-noise ratio ( ) of the solenoidal RF coil was higher than that of an ordinary saddle-shaped RF coil. In the sensitivity test of the cryogenic probe, we compared a four-turn solenoidal RF coil with a two-turn one. As a result, the of the one of two-turn RF coil was higher than that of the one of the four-turn one. It is believed that the reason is that the depends on the quality factor of the specimen and, furthermore, depends on the shape of the RF coil.Index Terms-Nuclear magnetic resonance, quality factor, signal to noise ratio, solenoidal RF coil, split-pair magnet.
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