Difference of Irreversible Strain Limit in Technical RHQT Nb3Al Superconductors

Strain limit for critical current degradation in various technical rapid-heating, quenching, and transformation-processed Nb3 Al 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 Jc 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 restackingmethod. In normal Nb and Ta matrix strands with a filament diameter of more than 30 μm, 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 appearsmore 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 μm. The Ag layer between the Nb3 Al 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 Ic degradation in a higher strain range.