Difference of Irreversible Strain Limit in Technical RHQT Nb3Al Superconductors

Abstract

Strain limit for critical current degradation in various technical rapid-heating, quenching, and transformation-processed Nb3Al strands has been studied with respect to the matrix material and filament diameter. This property is one important factor as well as the strain sensitivity to $J_{{\rm{c}}}$ characteristics, especially when we consider to apply the so-called react and wind technique to large magnets such as DEMO reactors. We compared five strands, including a standard Nb matrix, standard Ta matrix, Nb/Ag/Nb three-layered barrier structure, and other two fine filament diameter strands prepared by the restacking method. In normal Nb and Ta matrix strands with a filament diameter of more than 30 μ m, the irreversible strain limit is about 0.3%. Refinement of the filament diameter enables us to significantly improve the irreversible strain limit up to more than 0.7%. The Nb/Ag/Nb three-layered barrier structure appears more attractive from a practical point of view. The irreversible strain limit was improved up to nearly 0.6%, even if the filament diameter is more than 20 μ m. The Ag layer between the Nb3Al filaments seems to act as a cushion to mitigate the stress concentration in the wire. The Ag barrier also seems to be effective for current bypass in existence of cracks, resulting in suppressing $I_{{\rm{c}}}$ degradation in a higher strain range.