The Study of a 3 T Conduction-Cooled Superconducting Magnet for Animal Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is one of the most advanced imaging techniques and is widely used in clinical diagnosis and basic life science research, with advantages including noninvasiveness, high/multiple soft tissue contrast, parametric imaging, and the richness of information. Animal MRI which takes animals as the research object is widely used in life science research, medical research, and drug mechanism research. The superconducting magnet system which is usually adopted the cryogenic technology of bath-cooling with 4.2 K liquid helium is one of the core components of the MRI system. Since liquid helium (LHe) is a costly nonrenewable resource with a diminishing reserve and the prices have kept steadily increasing, a 3 T superconducting magnet adopting conduction-cooled technology that does not require LHe was developed for small-size and low-cost animal MRI systems. An active-shielded magnet structure composed of six main coils and two shielding coils arranged coaxially was designed by using both linear programming and nonlinear programming algorithm. The magnet produces a 3 T central magnetic field in a φ330 mm warm bore with theoretical homogeneity of 12×10−4 % in a φ180mm DSV. In order to adapt to the condition that the coil’s temperature may be higher than 4.2 K in a conduction-cooled system, sufficient current-sharing temperature margins are reserved in the magnet design. The minimum current-sharing temperature margin is 2.05 K in coils 1 and 6. A finite element analysis on the stress in the magnet was carried out by using averaged model in which mechanical characteristics of the elements are averaged on different material components. We can see that the maximum hoop stress in the magnet is about 40.9 MPa which is located at coils 3 and 4, indicating that the magnet design is safe. A passive protection method based on coil subdivision and quench propagation acceleration is a study to protect the magnet from damage caused by a quench. The superconducting coils are divided into three subdivisions to restrict the peak voltage. A heater can deliberately trigger the coil to quench to prevent the harmful concentration of heat in the confined normal zone. The outside interfaces of all coils are equipped with three heaters respectively. All heaters are made of nonmagnetic stainless-steel foils designed to go from one end of the coil to the other end and turn back for two turns. Each heater is made with the following characteristics width and thickness are 10 mm, and 0.2 mm, respectively. Each subdivision is in parallel with a pair of back-to-back diodes in series with eight heaters. Each heater is driven by the loop current in the subdivision. The cryostat uses a two-stage GM cryocooler to cool the superconducting magnet from room temperature to operating temperature and no LHe is needed. The cryostat mainly consists of a vacuum vessel, thermal shields, a GM cryocooler, pull rods, thermal connections, and a thermal switch. The GM cryocooler is mounted on the vacuum vessel. The first stage is directly connected to the thermal shields. The second stage cools the magnet via a thermal connection braid made of high-purity copper. A pair of hybrid current leads each of which consists of a copper lead and a Bi2223 HTS lead is employed to power the magnet. The cryostat and the superconducting coils were tested successively after the magnet system had been assembled. The final measured temperature at the first stage is 50 K. The final measured temperature at the second stage is 4.0 K. The maximum temperature of the magnet is 5.0 K in the shielded coil. The magnet was magnetized to 3.001 3 T after undergoing five training quenches and the measured peak-to-peak homogeneity in 180 mm DSV was about 1.33×10−2 %. © 2023 Chinese Machine Press. All rights reserved.