Development of high homogeneity and high stability 1.5T superconducting magnet for whole body MRI scanner

A multi-coil actively shielded 1.5T superconducting magnet has been designed for a whole-body MRI clinical scanner. The magnet has six primary coils and two active shield coils that have a reverse polarity of current with respect to the primary coils. The spatial field homogeneity achieved in 45cm of the field of view is ±5 ppm at its iso-center. The peak field on the coils is 4.3T. The MRI magnet would be operated in a persistent mode to achieve the temporal field stability of 0.1ppm/hr. The wire-in-channel (WIC) NbTi conductor with copper to superconductor ratio of 10 will be used for all the eight coils of the 1.5T magnet. To achieve the desired field stability in the persistent mode of operation, superconducting joint technology has been developed and the joints have been tested at 4.2K using an indigenously developed field decay test probe. In zero field, the minimum joint resistance achieved is 1.3×10⁻¹³ Ω. Since the inter-coil joints will be placed in a 0.5T field region we have measured the joint resistance in 0.5 T field as well. In the 0.5T background field, the joint resistance is found to be 7×10-¹² Ω, low enough to provide required field stability.