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 of Chelmsford, MA, USA to mechanically reinforce this material will help realize the potential of this material for these high-field (>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 in Woburn, MA, USA. 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.