Evaluation of residual stress and texture in isotope based (MgB2)-B-11 superconductor using neutron diffraction

Magnesium diboride (MgB2) superconducting wires have demonstrated commercial potential to replace niobium-titanium (NbTi) in terms of comparable critical current density. Its higher critical temperature makes MgB2 wire suitable for liquid-helium-free operation. We recently reported boron-11 isotope-based low-activation Mg11B2 superconducting wire with decent critical current density appropriate for low-cost superconducting fusion magnets. In this study, we have mainly focused on the neutron diffraction technique to measure the residual stress in Mg11B2 superconducting wire for the first time. The residual stress state was given qualitative and quantitative interpretation in terms of micro- and macrostress generation mechanisms based on the isotropic model confirmed by neutron texture measurements. The relationship between the stress/strain state in the wire and the transport critical current density is also discussed. This investigation could pave the way to further enhancement of the critical current density of low-activation Mg11B2 superconducting wires suitable for next-generation fusion grade magnets

AC loss and contact resistance of different CICC cable patterns: Experiments and numerical modeling

© 2020 The Author(s) For upcoming nuclear fusion energy reactors, like the China Fusion Engineering Test Reactor (CFETR) and EU-DEMO, the superconducting Cable-In-Conduit Conductors (CICC) are in the design phase, and the operating conditions like electromagnetic forces can be higher than in previous devices like ITER. The prototype conductors for the Central Solenoid (CS) coils in the CFETR, for example, are designed to produce a peak field of 19.9 T and are expected to be made of high current density Nb3Sn strands. Investigations are also ongoing on the application of bismuth strontium calcium copper oxide (BSCCO) and MgB2 strands for CICCs in fusion reactors. The latter material, MgB2, could be applied for superconductors subjected to lower magnetic fields, such as Poloidal Field coils, Correction Coils, and Feeders. The performance of all these strands is sensitive to strain, and the mechanical strength of the brittle filaments is relatively weak. This requires a thorough analysis of the cable pattern in terms of the mechanical support of the strands along their length in combination with the minimization of the interstrand coupling currents and strand indentation. As an initial step to finding the most appropriate cable pattern for CICCs, three prototype CICCs made of ITER type Nb3Sn strands with significantly different cable twist patterns are tested experimentally for AC coupling loss, interstrand contact resistance, and strand indentation. The three cabling patterns referred to as the Twente, CWS (copper wound superconducting strand), and CFETR-CSMC (CFETR Central Solenoid Model Coil) design. The numerical code JackPot ACDC developed at the University of Twente is used to analyze the interstrand coupling loss and contact resistance. The new ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences) triplet modified CWS design is aimed at reducing strand pinching during cabling, which causes degradation of transport properties during compaction and cyclic loading. The Twente design has the same objective but also aims at reducing the coupling loss while maximizing the mechanical lateral support for the strands by making the twist pitch ratio of the sequential cabling stages close to one. The CFETR-CSMC, taken as a reference for comparison, has cable a pattern mostly similar to the ITER CS cable design

Evaluation of residual stress and texture in isotope based Mg11B2 superconductor using neutron diffraction

Magnesium diboride (MgB2) superconducting wires have demonstrated commercial potential to replace niobium-titanium (NbTi) in terms of comparable critical current density. Its higher critical temperature makes MgB2 wire suitable for liquid-helium-free operation. We recently reported boron-11 isotope-based low-activation (MgB2)-B-11 superconducting wire with decent critical current density appropriate for low-cost superconducting fusion magnets. In this study, we have mainly focused on the neutron diffraction technique to measure the residual stress in (MgB2)-B-11 superconducting wire for the first time. The residual stress state was given qualitative and quantitative interpretation in terms of micro- and macrostress generation mechanisms based on the isotropic model confirmed by neutron texture measurements. The relationship between the stress/strain state in the wire and the transport critical current density is also discussed. This investigation could pave the way to further enhancement of the critical current density of low-activation (MgB2)-B-11 superconducting wires suitable for next-generation fusion grade magnets

New design of cable-in-conduit conductor for application in future fusion reactors

The China Fusion Engineering Test Reactor (CFETR) is a new tokamak device whose magnet system includes toroidal field, central solenoid (CS) and poloidal field coils. The main goal is to build a fusion engineering tokamak reactor with about 1 GW fusion power and self-sufficiency by blanket. In order to reach this high performance, the magnet field target is 15 T. However, the huge electromagnetic load caused by high field and current is a threat for conductor degradation under cycling. The conductor with a short-twist-pitch (STP) design has large stiffness, which enables a significant performance improvement in view of load and thermal cycling. But the conductor with STP design has a remarkable disadvantage: it can easily cause severe strand indentation during cabling. The indentation can reduce the strand performance, especially under high load cycling. In order to overcome this disadvantage, a new design is proposed. The main characteristic of this new design is an updated layout in the triplet. The triplet is made of two Nb3Sn strands and one soft copper strand. The twist pitch of the two Nb3Sn strands is large and cabled first. The copper strand is then wound around the two superconducting strands (CWS) with a shorter twist pitch. The following cable stages layout and twist pitches are similar to the ITER CS conductor with STP design. One short conductor sample with a similar scale to the ITER CS was manufactured and tested with the Twente Cable Press to investigate the mechanical properties, AC loss and internal inspection by destructive examination. The results are compared to the STP conductor (ITER CS and CFETR CSMC) tests. The results show that the new conductor design has similar stiffness, but much lower strand indentation than the STP design. The new design shows potential for application in future fusion reactors

Evaluation of isotopic boron (\u3csup\u3e11\u3c/sup\u3eB) for the fabrication of low activation Mg\u3csup\u3e11\u3c/sup\u3eB2 superconductor for next generation fusion magnets

© 2020 The American Ceramic Society In this study, we analyze the properties of boron isotope (11B)-rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk Mg11B2 superconductors and evaluate their superconducting properties. While 11B-rich powder is an essential precursor to fabricate Mg11B2 superconductors for fusion magnet applications, the properties of the 11B 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 11B to better than 99 at % quality. In addition, Pavezyum\u27s 11B 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. Mg11B2 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 11B shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge 11B and hence, Pavezyum 11B boron has the potential for manufacturing fusion grade Mg11B2 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 Mg11B2 samples fabricated from the powder. Thus, the low-neutron-activation Mg11B2 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

Evaluation of isotopic boron (11B) for the fabrication of low activation Mg11B2 superconductor for next generation fusion magnets

In this study, we analyze the properties of boron isotope (11B)-rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk Mg11B2 superconductors and evaluate their superconducting properties. While 11B-rich powder is an essential precursor to fabricate Mg11B2 superconductors for fusion magnet applications, the properties of the 11B 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 11B to better than 99 at % quality. In addition, Pavezyum's 11B 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. Mg11B2 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 11B shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge 11B and hence, Pavezyum 11B boron has the potential for manufacturing fusion grade Mg11B2 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 Mg11B2 samples fabricated from the powder. Thus, the low-neutron-activation Mg11B2 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

Evaluation of isotopic boron (11B) for the fabrication of low activation Mg11B2 superconductor for next generation fusion magnets

In this study, we analyze the properties of boron isotope (B)-rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk MgB superconductors and evaluate their superconducting properties. While B-rich powder is an essential precursor to fabricate MgB superconductors for fusion magnet applications, the properties of the B 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 to better than 99 at % quality. In addition, Pavezyum's B 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. MgB 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 shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge B and hence, Pavezyum B boron has the potential for manufacturing fusion grade MgB 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 MgB samples fabricated from the powder. Thus, the low-neutron-activation MgB 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