Abstract Body

Superconducting (SC) magnets for MRI systems require strong magnetic fields and high-quality uniformity. Therefore, the conventional magnets are wound with low temperature superconducting (LTS) multifilamentary wires, but it is necessary to take a high cost because of consuming a large amount of helium. If high temperature superconducting (HTS) wires could be applied to SC magnets for MRI systems, it would be possible to substantially reduce the cooling cost. However, screening currents are induced inside the windings for energizing the HTS magnets because the commercially available HTS wires are in the form of a flat tape. These currents generate so-called screening-current-induced fields (SCFs) that significantly reduce field uniformity in the center of the magnet. To reduce the SCFs, we have proposed useful methods [1] based on the abnormal transverse field effects [2,3]. It has been demonstrated that both coaxial and toroidal shaking coils wound with copper wires can reduce SCFs successfully [4,5]. In this study, we experimentally evaluate the influence of the leakage fields generated by the shaking coils on the magnetization of externally placed HTS coils, because the MRI systems are usually composed of multiple element coils. We fabricate small-size HTS coils, two types of shaking coils, and a jig to adjust the spacing between the HTS and shaking coils. The entire experimental setups are immersed in liquid nitrogen. After energizing shaking coils, we measure the central fields in the adjacent HTS coils with Hall sensors as shown in figure. As a result, the HTS coils are not magnetized by the leakage fields from the shaking coils even at the closest distance as small as possible.

References

[1] K. Kajikawa et al.: IEEE Trans. Appl. Supercond. 22 (2012) 4400404.
[2] K. Funaki et al.: Jpn. J. Appl. Phys. 21 (1982) 299.
[3] E.H. Brandt et al.: Supercond. Sci. Technol. 17 (2004) S1.
[4] K. Kajikawa et al.: Supercond. Sci. Technol. 24 (2011) 125005.
[5] K. Kajikawa et al.: IEEE Trans. Appl. Supercond. 26 (2016) 4400504.