MULTIPHYSICS MODELLING OF OPERATIONAL STRESSES IN A 1.5 T SUPERCONDUCTING MAGNET FOR WHOLE-BODY CLINICAL MRI SCANNER

Abstract

Ministry of Electronics and Information Technology (MeitY), Government of India, has initiated a project to develop a 1.5 T superconducting MRI scanner in India. Inter- University Accelerator Centre (IUAC), New Delhi, is primarily responsible for the development of a 1.5 T superconducting magnet and an ever-cooled cryostat for the MRI scanner. An actively shielded 1.5 T superconducting MRI magnet has been designed for a whole-body clinical scanner. The MRI magnet generates a 1.5 T magnetic field in a 45 cm Diametrical Spherical Volume (DSV) with ±5.5 ppm homogeneity. The magnet has been wound by using braided polyethylene terephthalate (PET) insulated wire in channel (WIC) Niobium Titanium (Nb-Ti) (low temperature superconductor) wire on the bobbin. MRI magnet operates at 4.2 K (liquid helium) temperature, and its operating current is 420 A. Through this thesis, the operational stress and its effects on the magnetic field quality of the 1.5 T MRI magnet are studied. This includes the effects of Lorentz forces, cooldown to a cryogenic temperature of 4.2 K, and the winding tension experienced during the magnet winding process. Initially, the composite material properties of the magnet are calculated to simplify the calculation. This allows the assumption that the magnet coils are a single homogeneous material instead of a composite made up of multiple materials. This is achieved by calculating the speared properties of a Representative Volume Element (RVE). Further, an analytical model is created to calculate the magnetic field, and based on the magnetic field values, the Lorentz forces are applied to the magnet. This information, along with the thermal boundary conditions and winding tension, is used to create a computational model to calculate the operational stresses in the magnet. In order to simulate the winding tension, a code is developed in APDL that utilizes initial stress states and the Element Death and Birth method. x Using the models, the design of the magnet is optimised for the required state of stress. The winding tension, number of layers of over-bind, and over-bind tension are taken as variable parameters and are used to compensate for the effect of Lorentz forces and thermal stresses. Through simulation, the final state of stress is identified, and the deformations of the coil due to various forces are calculated. This information is then used to calculate out the final field quality and variation in field homogeneity from the design. By utilising a bond graph, a multi-physics model is created to study the operation of the superconducting magnet during an emergency shutdown/quench. The model constitutes two domains, i.e. thermal and electrical. The thermal domain looks at the thermal propagation of heat generated during the process and the associated quench propagation. In contrast, the electrical domain looks at the current flow in the circuit and the effect of various quench protection systems. The electrical domain is modelled using a pure bond graph, while the thermal domain is modelled as a pseudo bond graph. The model is then used to simulate the quench in a 6 T solenoid magnet, and the results are compared with the commercial software OPERA and experimental results.