NMR magnet technology at MIT

Two key issues in the construction of high field NMR (nuclear magnetic resonance) magnets are discussed: field drift due to the index of the superconductor and the shimming of large high order gradients. It is noted that the index of the conductor as measured at the critical current gives a conservative guide to the drift rate of the magnet. With the assumption of a Gaussian distribution of critical current the effective index increases as the current decreases. The use of ferromagnetic shimming to improve the quality of the field generated by a nonuniform winding is a relatively simple procedure, particularly if linear programming is used to calculate the shim array. A commercial device maps the field along a helical path. Those field measurements are then used for a least squares fit to the harmonics. From that a shim array is designed by linear programming