Quench Induced Eddy Current and Mechanical Stresses in the Bobbin and Thermal Shield of the 1.5 T MRI Magnet System

An actively shielded 1.5 T superconducting MRI magnet is designed for a whole-body clinical scanner. During a quench, its fast-decaying field induces a strong eddy current in all the conductive parts of the cryostat. The eddy current together with the decaying magnetic field generates Lorentz force, which generates stress and deformation in the bobbin structure, thermal shield, and other conductive parts of the cryostat. In this article, we report the analysis of the eddy current distribution induced in the bobbin structure and thermal shield made of various alloys of the MRI magnet system using a finite element analysis program. The net forces on each part of the bobbin structure and the thermal shield and the associated stress distribution, the deformation are analyzed in detail using OPERA-3D software. The eddy-current-induced effects on a few metallic alloys are compared for generating design inputs for the bobbin structure and the thermal shield of the MRI magnet system. We also report the transient behavior of the eddy current in correlation with the decaying current during a quench.