Dynamic stability threshold in high-performance internal-tin Nb3Sn superconductors for high field magnets

Modern Nb3Sn strands can now exceed 3000 A mm-2 critical current density Jc at 4.2 K and 12 T within the non-copper area. However, the aggressive reaction used to achieve this performance causes the Nb3Sn filaments to coalesce into a single large, continuous ring of superconductor, and also allows tin to penetrate through diffusion barriers and alloy with the copper stabilizer. This results in a lack of adiabatic stability, due to the combination of high Jc and large superconductor diameter, and a strong reduction of dynamic stability, due to the reduction of the copper's thermal conductivity. Under these circumstances, flux jumps at low fields are inevitable, and the associated heat release could propagate along the conductor in a quench. In magnets, this means that quenches could be initiated in low-field regions at currents well below the designed operating current. We show that by limiting the final reaction duration, it is possible to keep the quench current density above Jc, thus ensuring flux-jump recovery along the entire magnet load line. For the example studied, keeping the residual resistivity ratio above ∼ 20 ensures safe operation. This was achieved for final reactions of 40 h or less, instead of the typical 72-200 h. Surprisingly, the performance penalty was small: a 24 h final reaction reached >90% of the highest Jc obtained. Energy-dispersive spectroscopy in a SEM did not reveal any detectable tin in the copper for stable strands, but in unstable strands as much as 4% Sn was found in the copper between sub-elements, suggesting that the contamination is rather local. The thermal conductivity of the stabilizer should then vary strongly with distance from the sub-element pack to the strand perimeter, complicating stability analyses. (rapid communication)