Dynamic stability threshold in high-performance internal-tinNb3Sn superconductors for high field magnets

Abstract

Modern Nb3Sn strands cannow exceed 3000 A mm−2 criticalcurrent density Jc at 4.2 K and 12 T within the non-copper area. However, theaggressive reaction used to achieve this performance causes theNb3Sn filaments to coalesce into a single large, continuous ring of superconductor, andalso allows tin to penetrate through diffusion barriers and alloy with the copperstabilizer. This results in a lack of adiabatic stability, due to the combination of highJc and large superconductor diameter, and a strong reduction of dynamic stability, due to thereduction of the copper’s thermal conductivity. Under these circumstances, flux jumps atlow fields are inevitable, and the associated heat release could propagate along theconductor in a quench. In magnets, this means that quenches could be initiated in low-fieldregions at currents well below the designed operating current. We show that by limiting thefinal reaction duration, it is possible to keep the quench current density aboveJc, thus ensuring flux-jump recovery along the entire magnet load line. For the examplestudied, keeping the residual resistivity ratio above ensures safe operation. This was achieved for final reactions of 40 h or less, instead of thetypical 72–200 h. Surprisingly, the performance penalty was small: a 24 h final reaction reached>90% of thehighest Jc obtained. Energy-dispersive spectroscopy in a SEM did not reveal any detectable tin in thecopper for stable strands, but in unstable strands as much as 4% Sn was found inthe copper between sub-elements, suggesting that the contamination is ratherlocal. The thermal conductivity of the stabilizer should then vary strongly withdistance from the sub-element pack to the strand perimeter, complicating stabilityanalyses.