Magic-Angle Gradient-Coil Design

Abstract

The target-field approach to gradient-coil design has been applied to the production of coils which will produce a linear gradient in the z component of the magnetic field, in a direction oriented at the magic angle to the z axis. The resulting coils, which consist of wires wound on the surface of a cylinder whose axis is parallel to the z direction, produce a pure linear gradient over a large fraction of the volume enclosed by the coil. Such magic-angle gradient coils may be usefully employed in two-dimensional NMR experiments requiring magnetic-field gradients, since magic-angle gradients effectively eliminate the long-range dipole coupling between spins, which can give rise to "intermolecular multiple-quantum coherences." An appropriate target field has been developed and this was used to produce a magic-angle gradient-coil design. A coil of 20 mm inner diameter was constructed to this design and tested using a 500 MHz NMR microscope. The resulting coil had an efficiency of 0.032 T m−1A−1 and an inductance of 3.2 μH. Using the magic-angle gradient coil, it was possible to largely eliminate the multiple spin echoes which occur in two-pulse NMR experiments applied to highly polarized liquids as a manifestation of the long-range dipolar coupling between spins.