Field correction of a high-homogeneous field superconducting magnet using a least squares method

A 6th order superconducting solenoid coil has been developed for a whole body magnetic resonance imaging. Nine sets of shim coils are equipped on the solenoid coil. A least squares computer program was applied to determine shim coil currents analytically. After the field correction using shim coils, the homogeneity of 7.6 × 10^-6/30cm DSV was obtained. The effects of ferromagnetic objects on the homogeneity was studied. The error field caused by iron spheres agreed well with the theoretical results. It was found that ferromagnetic objects generated mainly the 1st order error field and this could be corrected by shim coils. The effects of heat and charge/discharge cycles of the magnet on the homogeneity was studied.