Stress-Induced Magnetic Field Inhomogeneity in a 1.5 T Superconducting MRI Magnet

The heart of the present day MRI scanners is a superconducting magnet producing the required central field. A 1.5 T multicoil magnet with ±5.5 ppm field homogeneity in 45 cm field of view (FOV) is designed. These coils will operate at an average current density of 130 A/mm
²
. The magnet bobbin will be made of 5083 aluminium alloy, while the winding pack of the magnet will be a composite structure which will consist of Nb-Ti, copper, epoxy and insulations. The winding pretension, thermal strains, due to cooling, and Lorentz forces contribute to the overall stress, which during operation can cause deformation of the magnet. To design a structurally sound and stable magnet system, it is critical to evaluate the stresses that are generated in the magnet during winding, cool down and operation. This paper discusses the stresses generated on the magnet and deformations produced due to the forces acting on the coils. The effect of deformation on the field homogeneity and the influence of winding tension on the overall operational stress are also studied. The von-Mises stress during the operation is found to be 40 MPa along the median line. The deformations of the magnet due to operational stresses increased the field homogeneity to ±85 ppm in the FOV.