Evaluation of isotopic boron (B-11) for the fabrication of low activation (MgB2)-B-11 superconductor for next generation fusion magnets

Abstract

In this study, we analyze the properties of boron isotope (B-11)-rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk (MgB2)-B-11 superconductors and evaluate their superconducting properties. While B-11-rich powder is an essential precursor to fabricate (MgB2)-B-11 superconductors for fusion magnet applications, the properties of the B-11 powder turned out to be critical to determine the quality of the final superconducting product. Therefore, appropriate control of processing conditions is needed to comply with the requirements of the nuclear fusion application. Analysis of the B isotope ratio by accelerator mass spectroscopy and neutron transmission revealed that all three types of powder are enriched with B-11 to better than 99 at % quality. In addition, Pavezyum's B-11 shows the lowest crystallinity and smallest crystalline domain size as evidenced by the high-resolution X-ray diffractometer and scanning electron microscopy. The chemical states of the boron isotope investigated with near edge X-ray absorption fine structure spectroscopy and X-ray photoemission spectroscopy also reveals that Pavezyum boron has amorphous structure. (MgB2)-B-11 bulks and multi-filamentary (12-filament) wires have been manufactured, sintered at different temperatures and characterized via the transport critical current density. The wire with Pavezyum B-11 shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge B-11 and hence, Pavezyum B-11 boron has the potential for manufacturing fusion grade (MgB2)-B-11 based magnets. The results of this study demonstrated that Boron powders with higher purity, smaller grain size and lower crystallinity are critical for improving the superconducting and electronic properties of (MgB2)-B-11 samples fabricated from the powder. Thus, the low-neutron-activation (MgB2)-B-11 is possibly an affordable and technically viable candidate to replace NbTi superconductors in the low field poloidal field and correction coils for the next-generation fusion reactors.