Research investigates low activation and low cost  superconducting material for magnetic coils in next generation



fusion reactors



ansto.gov.au



/AboutANSTO/MediaCentre/News/ACS125707



ANSTO has participated in collaborative research investigating the microstructure and superconducting properties of



a material made with isotopically pure boron,



11



B,



for use in the magnetic coils of nuclear fusion reactors, such as



the International Thermonuclear Experimental Reactor (



ITER



).



Jacketed cable for ITER's toroidal field conducting, superconducting and non-conducting strands around a



central channel for helium



Image: ITER



An international group of researchers led by



Md Shahriar Hossain



, Senior Research Fellow from the



University of



Wollongon



g, who carried out the investigations at ANSTO,



published



in



Scientific Reports,



found that the



superconducting compound Mg



11



B



2



filament made with very low cost starting materials showed an optimal



electrical transport current performance.



For practical applications, maximum electrical transport current density, in which superconductivity is maintained



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without resistance, is considered the most important characteristic of any superconductor.



The authors describe the result as a significant breakthrough, which strongly supports the feasibility of replacing



NbTi wires in highly radioactive fusion reactors with high performance and low radioactivity



Mg



11



B



2



wires.



X-ray diffraction patterns of all Mg



11



B



2



wires sintered at different temperatures



Co-author, Instrument Scientist



Dr Vladimir Luzin



of ANSTO’s



Australian Centre for Neutron Scattering



(ACNS), who



has carried out previous collaborative research on superconducting cables for ITER, supervised X-ray diffraction



experiments and assisted with data analysis.



PhD student Hyunseock Jie, holds a joint position at the ACNS and the University of Wollongong and  the Head of



Superconducting Systems at ITER,



Arnaud Devred



are also both co-author on the publication.



Superconductors used in magnet technology carry extreme currents because of their ability to keep magnetic flux



motionless.



With the aim of further improving the critical electrical transport current density, the researchers probed the effects of



sintering temperatures used in the production of microfilament wires made from isotopically pure boron in the



compound Mg



11



B



2



and transport current capacity at high magnetic field in the study.



According to the authors, sintering at 750Â° C resulted in the highest magnetic properties and best electrical transport



density in the Mg



11



B



2,



producing better phase composition and crystallinity than other



temperatures (700Â°, 770Â° and



800Â° C) that were investigated.



Although X-ray diffraction confirmed that the phase composition and superconductivity of the Mg



11



B



2



was retained at



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all temperatures, a vanishing of transport current occurred at 800Â° C.



Scanning electron microscopy revealed that this was attributable to severe deterioration of inter grain connectivity,



the ability of current to flow one grain to an adjacent grain, caused by the formation of big microcracks, voids, and



Mg



11



B



2



clusters at 800Â° C.



The superconducting fragments become isolated from each other and eventually, very little current passes through



the wire.



The transport critical current measurements determined in magnetic fields up to 15 T were undertaken at the



Institute for Superconducting and Electronic Materials



at the University of Wollongong, which has world-class



facilities for the fabrication and advanced characterization of superconducting wires, coils and devices.



The superconducting magnets for the ITER experimental reactor are being fabricated using NbTi and Nb3Sn but



there is a major drawback.



These low temperature superconductor cannot operate without the liquid helium which is scarce and becoming very



expensive. “Because of its transition temperature at 39 K,



Mg



11



B



2







can operate without the need for costly



liquid



helium,” said Dr Hossain



Irradiating Nb based superconductors results in the production of the long lived nuclide,



94



Nb, which requires tens of



thousands of year to cool down.



In contrast, Mg



11



B



2



, which was known to have superconducting properties, is much more stable in an irradiation



environment with a significantly shorter decay time of about a year.



There are additional advantages. The fabrication costs of Mg



11



B



2



,wire are a third less than that of Nb



3



Sn.



Also, because the Mg



11



B



2



has



working temperature



of 20Â°K and can be cryo-cooled, it could cut operating costs by



50%; while Nb materials need liquid helium to cool them to 4Â° K for superconductivity.



The origin of the superconducting properties of magnesium diboride, MgB



2



, which has a transition temperature of 39



K, was first reported



by researchers from Japan in



Nature



in 2001 who attributed it to a two band electron structure.



Conventional materials superconduct when electrons are induced by an energy gap to form  “Cooper pairs” at a



critical temperature.



In MgB



2



the pairing arises from attractive interactions between the electrons that are mediated by atomic vibrations,



or phonons.



The electron pairs possess different binding energies, corresponding to two different energy gaps.



ANSTO signed a



cooperation agreement



with ITER in September 2016 to collaborate on research in diagnostics,



materials, superconducting technology, and fusion plasma theory and modelling for the reactor.



The superconducting materials group at University of Wollongong led by Dr Hossain is also part of this consortium.







doi:10.1038/srep36660











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Published: 21/02/2017



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