Electromechanical properties of superconductors for DOE/OFE applications. Final report Page: 4 of 15

Executive Summary

Our first publication of research results under this interagency agreement,
Characterization of Multifilamentary Nb3Sn Superconducting Wires for Use in the 45-T
Hybrid Magnet, was a collaborative effort with researchers at the National High Magnetic
Field Laboratory (NHMFL) and Teledyne Wah Chang. The paper describes the results of
characterization studies of candidate Nb3Sn superconductors for a 45 T hybrid magnet
being constructed at NHMFL. When completed, this research magnet will generate the
world's highest steady state magnetic field and as a by product, extremely high
electromagnetic forces within the coil. Our measurements of the effect of axial tensile strain
on the critical current of the candidate conductor at 4 K and 12 T provided the basic
empirical data necessary to model the magnet's performance.
A second publication, High Compressive Axial Strain Effect on the Critical Current
and Field of Nb3Sn Superconductor Wire, was motivated by the need to model the
electromechanical properties of superconductors used in large stainless-steel reinforced
magnets, ITER for example, where the relatively high thermal contraction of the steel can
place the superconductor under an axial compressive strain of about -1%. In this study, the
axial strain characteristic of a Nb3Sn superconductor with a high matrix-to-superconductor
ratio was tested. The large matrix volume of this wire places the superconductor under an
initial compressive prestrain of approximately 0.95% at 4 K. Measuring the effect of
applied tensile strain on the critical current of the precompressed wire provides data over a
wide strain range including high compression. A comparison of the resulting data with the
Strain Scaling Law developed at NIST showed that this empirical relation can accurately
model Nb3Sn strain effects at high levels of compressive strain.
In the final publication, Tensile Measurements of the Modulus of Elasticity of
Nb3Sn at Room Temperature and 4 K, the stress-strain characteristics of several Nb3Sn
composite conductors were measured to determine the elastic modulus, E, of Nb3Sn. In
modeling Nb3Sn superconductor systems, E is a key parameter; however, there are large
discrepancies in the available data and, prior to this study, there were no published tensile-
test measurements of E for Nb3Sn at low temperature. This study showed that E decreases
with decreasing temperature and that its value at 4 K is approximately 65 GPa, compared
with 4 K values more than twice this high that have commonly been used for Nb3Sn. We
subsequently used these results for comparative modeling studies of axial and transverse
stress effects in Nb3Sn.
The two incremental progress reports describe additional results of the program
including numerous strain characterizations of ITER and TPX candidate conductors;
consultations with DOE laboratories and wire manufacturers on strain effects; a
comparative study of axial/transverse stress effects in a Nb3Sn monofilament conductor
aimed at extending the Strain Scaling Law from one to three dimensions; and a
collaborative study with Intermagnetics General Corp. on the combined effects of reaction-
mandrel material and prereaction annealing of Nb3Sn on critical-current-versus-strain
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Ekin, J.W. & Bray, S.L. Electromechanical properties of superconductors for DOE/OFE applications. Final report, report, September 1, 1998; United States. (digital.library.unt.edu/ark:/67531/metadc707370/m1/4/ocr/: accessed November 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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