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The principles of active screening of time dependent magnetic fields, introduced by Mansfield and Chapman (1986), have been extended to include two screens. Magnetic screening is especially useful in NMR when rapidly switched gradients are required within the close confines of a superconductive magnet. However, single screens reduce and spatially distort the field from the primary coil. With two active screens it is possible to fully screen the primary coil far field without impairing its near field spatial characteristics.
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Chapman et al. (1986) studied this question.
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