Feasibility of persistent-mode operation of HTS magnet with flux pump compensation for MRI

Abstract We study the feasibility of persistent-mode operation of a high temperature superconducting (HTS) magnet with a flux pump for magnetic resonance imaging (MRI). For the experiment, a non-insulated REBCO pancake coil magnet was constructed, featuring an inner diameter of 30 mm, an outer diameter of 112 mm, and an inductance of 12 mH. The current loop of the magnet, the switch and the joints had a resistance of 2.67 × 10 −9 Ω at 77 K caused by the soldered joints between REBCO tapes. A flux pump was coupled with a section of REBCO superconducting wire in the loop to compensate for the energy dissipation in the loop assembly due to its resistance. In the persistent-mode test, the magnet was immersed in liquid nitrogen and charged to a central magnetic field of 0.35 T. The decay rate of the magnetic field was 2.8 × 10 −4 T hr −1 without flux pump compensation and 3 × 10 −10 T hr −1 with flux pump compensation. This result shows that flux pump compensation can improve the long-term stability to meet MRI scanning requirements. The flux pump supplies energy through electromagnetic induction to sustain the persistent current in the magnet coil. Meanwhile, the pulsed flux generated by the flux pump will also cause fluctuations in the central magnetic field. The present experiments reveal that the magnitudes of these fluctuations are as high as a few μ T and can generate phase artifacts in MRI. However, we found that these magnetic field fluctuations exhibit precisely the same periodic behavior as the flux pump pulses and their influences on MRI could be eliminated in two ways: 1. Halting the flux pump during MRI scanning; 2. Identifying and excluding the noise sections in the data processing by leveraging the known periodicity of the flux pump-induced fluctuations. This study thus demonstrates that the persistent-mode operation of an HTS magnet with a flux pump is practical approach for MRI.