Magnetic structure.

摘要

Magnetic shielding of a magnet such as a first set of superconducting coils (20, 22, 24) as used in nuclear magnetic resonance (NMR) applications includes a second set of coils (26, 28, 30) around and spaced from the first set of coils. The magnetic flux of the second set of coils opposes and offsets the magnetic flux of the first set of coils in the region outside of the second set of coils but is less than the magnetic flux of the first set of coils in the region within the first set of coils. The uniformity of magnetic flux within the first set of coils is not adversely affected by the magnetic flux produced by the second set of coils. In a preferred embodiment the coils of the first set are independently energized to establish a uniform field within the coils having suppressed lower order derivatives of flux with respect to position within the uniform field. The external field of the first set of coils comprises a dipole which is offset by the field of the second set of coils. Higher order multipoles beyond the dipole are also suppressed by the geometries of the two sets of coils.