A persistent-mode MgB₂ 0.5-T/240-mm solid-nitrogen-cooled magnet for MRI

Abstract

Magnetic resonance imaging (MRI) has become one of the major noninvasive diagnostic imaging tools today. The annual $5-billion market drives magnet engineers to develop advanced and innovative, high-quality, low-cost, easy-to-operate MRI magnets. Because superconductors carry more than 100 times higher current density than copper, while generating no Joule heat, they are the only practical choice for diagnostic MRI magnets with field strengths above 1 T. A low-temperature superconductor of niobium titanium (NbTi) with a critical temperature of 9.8 K stands out among other superconductors because of its excellent mechanical properties, adequate electromagnetic properties and low manufacturing cost. Since the MRI magnet became available in the 1970s, most commercial superconducting MRI magnets have been of NbTi wire and operated in liquid helium bath at 4.2 K. Nowadays, the sharply increasing price of helium has driven magnet designers to consider other superconductors with higher critical temperature for liquid-helium-free MRI magnets. Discovered in 2001, a new high-temperature superconductor of magnesium diboride (MgB₂) with a critical temperature of 39 K has spurred intensive R&D effort. A combination of high critical temperature, low manufacturing cost, and good in-field performance makes it viable competitor to NbTi and the basis for this thesis study. This dissertation, a result of the 0.5-T/240-mm MgB₂ magnet project performed in the Magnet Technology Division of the Francis Bitter Magnet Laboratory, includes design, construction and operation details of the magnet. Each key component of the magnet, i.e., superconducting joint and persistent-current switch (PCS), was designed and tested to evaluate its performance. Each PCS was designed also to absorb energy when protecting the magnet; its protection performance, as well as switching function, was evaluated before deployed. Each finished coil module was tested separately before assembled to complete the magnet. The magnet was operated in persistent-mode in the temperature range 10-15 K. During operation, this liquid-helium-free magnet was immersed in a volume of solid nitrogen that provided a needed thermal mass to the magnet. Temporal and spatial field homogeneity are critical parameters of MRI magnets. Thus actual fields were measured and compared with the designed values to demonstrate its acceptability to MRI application. Operation also examined and validated a protection technique deployed for this magnet, as protection is one of the vital requirements of the superconducting magnet.