Critical current scaling and the pivot-point in Nb3Sn strands

Abstract

Detailed measurements are provided of the engineering critical current density (Jc) and the index of transition (n-value) of two different types of advanced ITER Nb3Sn superconducting strand for fusion applications. The samples consist of one internal-tin strand (OST) and two bronze-route strands (BEAS I and BEAS II—reacted using different heat treatments). Tests on different sections of these wires show that prior to applying strain, Jc is homogeneous to better than 2% along the length of each strand. Jc data have been characterized as a function of magnetic field (B ≤ 14.5 T), temperature (4.2 K ≤ T ≤ 12 K) and applied axial strain ( − 1% ≤ εA ≤ 0.8%). Strain-cycling tests demonstrate that the variable strain Jc data are reversible to better than 2% when the applied axial strain is in the range of − 1% ≤ εA ≤ 0.5%. The wires are damaged when the intrinsic strain (εI) is εI ≥ 0.55% and εI ≥ 0.23% for the OST and BEAS strands, respectively. The strain dependences of the normalized Jc for each type of strand are similar to those of prototype strands of similar design measured in 2005 and 2008 to about 2% which makes them candidate strands for a round-robin interlaboratory comparison. The Jc data are described by Durham, ITER and Josephson-junction parameterizations to an accuracy of about 4%. For all of these scaling laws, the percentage difference between the data and the parameterization is larger when Jc is small, caused by high B, T or |εI|. The n-values can be described by a modified power law of the form , where r and s are approximately constant and Ic is the critical current. It has long been known that pivot-points (or cross-overs) in Jc occur at high magnetic field and temperature. Changing the magnetic field or temperature from one side of the pivot-point to the other changes the highest Jc sample to the lowest Jc sample and vice versa. The pivot-point follows the B–T phase boundary associated with the upper critical field and is usually attributed to the different tin content profiles and pinning properties of internal-tin and bronze-route strands. We report that the strain dependence of the pivot-point in these strands is quite different from that of the upper critical field and suggest that its origin in optimized high tin content strands is the proximity of the tetragonal Nb3Sn phase, which has low superconducting critical parameters.