Dong-Keun Park

Monofilament

This paper presents recent results from our continued development of a 0.5 T whole-body MRI magnet at the Francis Bitter Magnet Laboratory. HyperTech Research Corp. (Columbus, OH) manufactures the MgB2 conductor for this project. During the past year, we have found that our technique, originally developed successfully to splice unreacted multifilament MgB2 wires, works much better, i.e., of higher reliability, with unreacted monofilament MgB2 wires. This has led us to wind the entire coil components in our persistent-mode MRI magnet with unreacted monofilament MgB2 wire, having a MgB2 core of 0.4 mm in diameter, an overall diameter of 0.8 mm bare, 1 mm S-glass insulated. To verify that these coils would not suffer from flux jumping, as they would if wound with monofilament NbTi wire, magnetization studies were performed on monofilament wires of MgB2 and NbTi (as a reference) at 4.2 K. For the monofilament MgB2 wire, the results were affirmative. To further ensure the absence of flux jumping that may quench these current-carrying coils, two test coils were wound with unreacted monofilament MgB2 wire. One MgB2 coil was operated in driven mode, while the other MgB2 coil, equipped with a persistent current switch and terminated with a superconducting joint, was operated in persistent mode. The operating temperature range was 4.2-15 K for these MgB2 coils. The driven mode coil was operated in self-field. The persistent mode coil achieved a persistent current of 100 A, corresponding to a self-field of ~ 1 T in the winding, for 1 hour with no measurable decay. Both test coils were operated quench free.

\hbox{MgB}_{2}

In this paper, we present two design options for a tabletop liquid-helium-free, persistent-mode 1.5-T/90-mm MgB 2 “finger” MRI magnet for osteoporosis screening. Both designs, one with and the other without an iron yoke, satisfy the following criteria: 1) 1.5-T center field with a 90-mm room-temperature bore for a finger to be placed at the magnet center; 2) spatial field homogeneity of <5 ppm over a 20-mm diameter of spherical volume (DSV); 3) persistent-mode operation with temporal stability of <0.1 ppm/h; 4) liquid-helium-free operation; 5) 5-gauss fringe field radius of <50 cm from the magnet center; and 6) small and light enough for placement on an exam table. Although the magnet is designed to operate nominally at 10 K, maintained by a cryocooler, it has a 5-K temperature margin to keep its 1.5-T persistent field up to 15 K. The magnet will be immersed in a volume of solid nitrogen (SN2) that provides additional thermal mass when the cryocooler is switched off to provide a vibration-free measurement environment. The SN2 enables the magnet to maintain its persistent field over a period of time sufficient for quiescent measurement, while still limiting the magnet operating temperature to ≤15 K. We discuss first pros and cons of each design, and then further studies of our proposed MgB2 finger MRI magnet .

Wire for a Whole-Body MRI Magnet: Superconducting Joints and Test Coils

The rotary flux-pump using HTS tape has been studied for superconducting rotating machinery application. The charging speed and saturation current of the rotary HTS flux-pump is closely related to magnetic flux linkage passing through the HTS tape. To analyze charging parameters that effect pumping rate and saturation current of the flux-pump, methods of changing the rotating speed, shape of permanent magnet, width of HTS tape, and magnetic flux intensity have been investigated in previous studies [1]– [3]. In this paper, we have tried to test three cases to investigate the pumping rate and saturation current: 1) two different background materials, iron and Bakelite, were used to compare the magnetic flux linkage reinforcement; 2) two HTS tapes were overlapped to extend the magnetic flux linkage area, and each HTS tape was connected to an HTS coil; and 3) the parallel joint was conducted between the flux-pump and the HTS coil to compose a closed loop for persistent current mode. In order to measure the charging speed and pumping rate, a Hall sensor was installed at the center of the HTS coil.