Initial magnetic field decay of the superconducting magnet in persistent current mode

The initial magnetic-field decay in the persistent current mode of a magnetic resonance imaging magnet is greater than the steady field decay due to joint resistances of conductors. Imaging experiments cannot be carried out during periods which last ten or more hours. The current distribution in the multifilamentary conductor is nonuniform just after energization. It is suggested that the change of the current distribution causes the initial magnetic-field decay. A sixth-order superconducting magnet was prepared for experiments (central field=0.35 T, inner diameter=1 m, length=1.86 m). The stability state magnetic-field decay was 7*10^-8/h. The initial magnetic-field decay was 3*10^-6/h. Overshoot currents (101 and 105% of the rated current) were applied to the magnet and the current reduced to the rated current to improve the initial decay. The energizing and de-energizing rate of the field was 1.8 G/s. No initial decay was observed when 105% current pattern was applied to the magnet.