| Popis: |
The use of magnesium diboride (MgB2) superconducting wires inside a conduction cooled MRI main magnet could reduce the amount of liquid helium (LHe) from 2000 to only a few liters. The quench protection of MgB2 superconducting magnets remains a primary challenge because the higher enthalpy margin and slower normal zone propagation velocity (NZPV) of MgB2 wire compared to conventional niobium titanium (NbTi) wire leads to a higher temperature rise, which could damage the magnet, necessitating the use of an active protection system. Both 0.5 T and 1.5 T whole-body conduction cooled MRI main magnet designs with MgB2 wire have been presented here with dimensions comparable to current scanners. The quench propagation throughout the magnet has been numerically modelled using custom code in MATLAB, and the procedure consists of the interaction between thermal, magnetic, and circuit models. The governing heat equations were solved using the implicit Douglas-Gunn and Peaceman-Rachford methods, and the governing circuit equations were solved using Heun’s method. From these simulations, it was found that the temperature rise inside a quenched MRI coil could be reduced at the time of quench detection by increasing the thermal and/or electrical conductivity of the wire composite. A quench protection system using Coupling Loss Induced Quench (CLIQ) has been investigated for the two magnet designs where an external charged capacitor introduces an oscillating current into the coils, which generates heat inside the coils due to inter-filament coupling currents. The coil’s increased resistance reduces the current, leading to a lower hot-spot temperature. Various parameters were varied including the wire’s twist, number of CLIQ units, and the voltage and capacitance of each unit to determine their effect on the magnet’s protection. Finally, these quench simulations were performed on a single MgB2 test coil to determine the quench propagation inside the coil and the effect of a dump resistor and/or external protection heater on the protection of the coil. Therefore, this work has numerically investigated the quench propagation of MgB2 MRI coils and demonstrated both ways to reduce the hot-spot temperature and a potential method of quench protection. |
