New Method for Quick Decay of Shielding Current in REBCO Superconducting Coils
We are developing superconducting magnets for MRI with Re1Ba2Cu3O7-δ (Re: Rare Earth, Y, Gd, Eu, REBCO) superconducting tapes. We intend to operate them in subcooled liquid nitrogen at 65 K. Superconducting magnets for MRI need uniformity of magnetic field in time and space. The disturbance should be mainly caused by the shielding current induced in the wound tapes. To investigate this, we made small test coils with a laser-scribed four-filament EuBCO tape and a nonscribed GdBCO tape. After cooling down to 65 K with subcooled liquid nitrogen and applying a dc bias magnetic field, transport current was applied step by step to the test coil and the magnetic field variation at the coil center was observed. The actual magnetic field followed the variation of transport current with a delay. We derived the fitting expression for the observed field variations. The magnitude and the decay time constant of the shielding current increased with transport current. However, when further increasing transport current close to the critical current, the shielding current decayed quickly. Using this tremendous knowledge, we proposed a new operating method so that the magnetic field reached to the target one in a short time. In addition, the comparison between both coils demonstrated that not only the magnitude of shielding current but also the decay time constant was drastically reduced by scribing.