A 4-tesla superconducting whole-body magnet for MR imaging and spectroscopy

The magnet, weighing 10 tons, is part of a whole-body magnetic resonance imaging (MRI) device devoted to in vivo research studies at magnetic fields significantly higher than 2 T. The stored energy of the adiabatically stable six-coil system is 39 MJ at 4 T. The rated current is 376 A. Superconducting and resistive shim-coils together with iron shims reduce the bare magnet inhomogeneity of 100 p.p.m. down to less than 5 p.p.m. in a 50-cm diameter spherical volume. The magnet operates in persistent mode with the current leads retracted. The field drift is less than 3*10^-8/h. A small helium refrigerator is used both to cool the two radiation shields of the cryostat and the current leads and to reliquify the evaporating helium gas. Results of coil design, analysis of forces and stresses, quench protection, cryostat design, and magnet operation are reported.