Relaxation of Shielding Current in Test Coils for MRI With REBCO Superconducting Scribed Tapes

Superconducting magnets for MRI need uniformity of produced magnetic field within ppm order in time and space. To develop the superconducting magnets for MRI operating in subcooled liquid nitrogen at 65 K, two pieces of small test coils were fabricated with Re 1 Ba 2 Cu 3 O 7-δ (REBCO, Re: Rare Earth such as Y, Gd, and Eu) superconducting tapes. One was wound with a nonscribed tape and the other was with a laser-scribed tape into a four-filament structure. Each test coil was inserted inside a NbTi superconducting magnet. After bath cooling, the test coil with subcooled liquid nitrogen and applying a dc bias magnetic field of 0.5-2 T, transport current was applied step by step, and the variation of magnetic field at the coil center was observed by a Hall probe with high accuracy. Although magnetic field followed the variation of transport current with a delay in the respective coils, the drastic difference between them was revealed. It was demonstrated that the influence of the induced shielding current on the produced magnetic field was drastically reduced by scribing. Not only the magnitude of shielding current but also the decay time constant was reduced by scribing. The produced magnetic field of the test coil with the scribed tape reached to a target one in a short time as programmed. Scribing REBCO tapes brought about faster relaxation of magnetization.