Fault Current Limiter for Temporal Stabilization Process of Very High Fields in NMR or MRI

Abstract

Applications in MRI & NMR need higher fields to improve imaging performances. For such high field magnets, it is sometimes necessary to operate them in driven mode. However, the NMR & MRI requires a field drift of less than the best power supply can make (typically, MRI need less than 0.05 ppm/hr and NMR is more demanding, in comparison best stabilized power supply have drift of about 1 ppm/hr). A solution had been found in literature: a semi persistent mode with a resistance (R1) connected in parallel with the magnet and to the power supply, filters the power supply drift. But it is not sufficient for application in large magnet. In fact, in case of quench, energy contained in the magnet will destroy the resistance in parallel to the magnet because its value is too small. A discharge resistance in parallel to the magnet and to the resistance R1 is necessary and a switch in series with the resistance R1 is opened when a quench is detected. During the quench, the voltage on the switch could be large. An alternative of a switch is a fault current limiter which is usually used for the protection of an electrical network (it limits fault current) and to increase its quality. A study has been done to verify the feasibility of this object and an experimental assessment has been done on a very small magnet operating at 7 T using a prototype of fault current limiter made of 100 meters of superconducting wire.