Development of a 0.5-T/240-mm MgB₂ MRI Magnet: Assembly Design and Module Coils

This paper presents the latest results from our continued development of a 0.5-T/240-mm MgB₂ MRI magnet at the MIT Francis Bitter Magnet Laboratory. Because we have successfully developed our superconducting joint technique with a monofilament MgB₂ wire, manufactured by Hyper Tech Research, Inc. (Columbus, OH), we have decided to use a monofilament wire to wind our MgB₂ MRI magnet. The magnet, comprising eight module coils, has a winding inner diameter of 276 mm, an outer diameter of 290 mm, and a total height of 460 mm. Each coil has its own persistent-current switch (PCS) and a superconducting joint. In order to guard against a few bad coils forcing the entire magnet to be inoperative, each coil will be heat-treated and tested individually. After eight coils are successfully operated, they will be assembled into an MRI magnet and series-connected with soldering joints between adjacent coils. The PCS in each coil is designed in such way that it will also serve as a detect-and-heat protection absorber when the magnet quenches over a small "localized" region: The conductor volume in the eight switches is designed to absorb the entire magnet energy while still remaining below 200 K. This paper reports 1) the design of the whole magnet and 2) the fabrication and test results of the two real-size test coils, with their PCSs and superconducting joints. The tests were conducted in gas helium in the temperature range of 10-15 K and in the self-field of the coils.