Superconducting joints using multifilament MgB2 wires for MRI application

MgB2 Superconducting Joint Architecture with the Functionality to Screen External Magnetic Fields for MRI Magnet Applications

“…The fabrication steps to join unreacted C-doped multifilament MgB 2 wires with the functionality to screen external magnetic fields are shown in Figure a. First, Monel and copper (Cu) from the end of the two wires were etched using nitric acid (HNO 3 , 60% w/w, 0.5 h) to eliminate the possibility of their reaction with magnesium (Mg) during heat treatment, and then, the ends were cut at the 20° angle to increase the cross-section of the exposed MgB 2 filaments’ core . For the cutting, a diamond cutting wheel was used with 3000 rpm rotation and 1 mm min –1 cutting speed without any protective fluid in a cutting machine.…”

“…Unexpectedly, after removing the background field, joint 1 showed only an I c of 7.7 A, which was much lower than the earlier measured I c of 39.3 A at 20 K in a self-field. It was expected that the MgB 2 bulk inside the joint would have trapped some magnetic field when a high magnetic field was applied to it . This trapped magnetic field would not have completely vanished when the background field was reduced to zero.…”

“…The C-doped multifilament MgB 2 wire manufactured using the in situ process was utilized for fabricating the joints. ,, The field-dependent I c (s) of the MgB 2 wire at 20 K are shown in Figure with a cross-sectional optical image of the wire. The specifications of the MgB 2 wire are listed in Table S1 in the Supporting Information.…”

“…Superconducting joints for both monofilament ,,− and multifilament MgB 2 conductors ,,,− have been reported. The joints fabricated for multifilament MgB 2 conductors showed inferior performance compared with the joints fabricated for monofilament MgB 2 conductors owing to their finer filament size. ,,, Carbon (C)-doping is usually used in the MgB 2 material to improve its in-field performance. − However, the performance of the joints fabricated using C-doped MgB 2 conductors was found to be poorer compared with that of undoped MgB 2 conductors .…”

“…A cryogen-free MRI scanner using an MgB 2 conductor has already been developed by ASG Superconductors. , This MRI scanner, however, is operated in the driven mode unlike most of the mainstream MRI scanners, which are operated in the persistent mode for cost-effective operation and the necessity to generate a steady magnetic field (<0.1 ppm h –1 ) to produce high-resolution scans. ,, The persistent mode in a superconducting magnet is realized by short-circuiting ends of the magnet with a persistent current switch (PCS, a small noninductively wound superconducting coil) using “ superconducting joints ” following initial current charging as shown in Figure a. Because in MRI (Figure b) more than one coil is used to construct the magnet, the coils are connected in series using superconducting joints and the ends are short-circuited using a PCS and superconducting joints. , Unavailability of reliable superconducting joining technology is one of the prime reasons for the driven-mode operation in ASG Superconductors’ MRI magnets and its limited deployment in hospitals .…”