A Tabletop Persistent-Mode, Liquid Helium-Free 1.5-T MgB2 "Finger" MRI Magnet: Construction and Operation of a Prototype Magnet.

This paper presents results of construction and operation of a persistent-mode, liquid-helium-free, small-scale prototype magnet for the development of a tabletop 1.5-T "finger" MRI system for osteoporosis screening. The prototype magnet, composed of 2 MgB2 coils, one superconducting joint, and a persistent-current switch (PCS) built from a portion of one coil, was wound with a one continuous ~80-m long unreacted and monofilament MgB2 wire and then reacted. The test magnet was charged successfully and generated the estimated target field of 1.75 T at 5 K with the proposed PCS operation. During initial persistent-mode, the field was slightly decayed due to the index dissipation of the joint; thereafter it sustained the persistent field of 1.7 T for 35 h. The test results validated the joint resistance of < 1.2 × 10-11 as well as the proposed approach involving the PCS coil circuit model.

High Speed Spin Testing of Reinforced 2212 Coils for High Field NMR Magnets.

Bi-2212 superconductors have very good performance in field, and recent developments by Solid Materials Solutions (SMS) of Chelmsford, MA to mechanically reinforce this material will help realize the potential of this material for these highfield (> 1 GHz-class) NMR magnets. While the strength of these materials can be tested using a conventional tensile test, it is difficult-to-impossible to test coils in the high-field environment required to impose the large Lorentz stresses on the superconductor, as the available warm bore for high-field magnets is usually too small to test typical NMR insert coils, which typically have either a 60 or 80-mm winding diameter. Since it is important to test the coils-and not just wire-in the high-stress environment, as such factors as differential thermal contraction (between mandrel, wire, insulation and epoxy) and stress-concentrations (due to layer-to-layer crossover, for example) only can be tested in coil form, the objective of this study is to simulate the high-field magnet environment by spinning these coils at very high speed (up to 100,000 rpm) using the spin test facilities of Barbour-Stockwell (BSI) in Woburn, MA. By spinning coils wound on a 60-mm diameter mandrel at a speed of 100,000 rpm, the hoop stress is ~700 MPa, which is sufficient to exceed the yield strength of the reinforced Bi-2212 conductor. This paper summarizes the early stage status of this 3-year, NIH-funded project.

Progress in High-Speed Spin Testing of Superconducting Wire and Tapes for High-Field NMR Magnet Qualification.

This paper summarizes the status of a 3-year, NIH-funded research project to study the strength of high temperature superconductors under high circumferential hoop stress, in order to qualify these materials for high-field (> 1 GHz-class NMR magnets. The unique approach presented here is to spin test coils at high rotational speeds, approaching 100,000 rpm, in order to induce the necessary hoop stress. Thermal strain compatibility between the Bi-2212 wire and Inconel wire has been qualified, including thermal cycling. Assembly and testing of the first low-speed (< 30,000 rpm) rotor is now in process, and the design of second, higher speed (> 60,000 rpm) rotor, is also underway.