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

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 750° 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 (700°, 770° and

800° 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 800° 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 800° 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 20°K and can be cryo-cooled, it could cut operating costs by

50%; while Nb materials need liquid helium to cool them to 4° 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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