QUANTITATIVE CHARACTERIZATION OF THE EDDY-CURRENT FIELDS IN A 40-CM BORE SUPERCONDUCTING MAGNET

The temporal and spatial dependence of the eddy current fields, genera ted by switching off x, y, and z gradients in a 40-cm bore Bruker supe rconducting magnet, have been studied by measuring the offset frequenc y of the proton FID obtained from a small spherical sample. The measur ements were made with the pre-emphasis unit deactivated. The data obta ined at each location were well fitted to a sum of four exponentially decaying components. The shortest decay is, in each case, associated w ith the decay of the current in the gradient coil itself, while the ot her decays are associated with three eddy currents with decay times ra nging from 13 to 480 ms. Each of the three eddy currents generates an essentially uniform gradient for at least 4 cm on either side of the i socenter. However, when the eddy currents are generated by either the vertical transverse gradient or the longitudinal gradient, the eddy cu rrent gradients are also accompanied by a field shift.