Stability and flux jumping of internal-Sn, Nb3Sn conductors (and a model system MgB2)

The flux jump stability of Nb₃ Sn, internal-Sn strands was investigated using both experiment and calculation. An MgB₂ bulk sample was also used as a model system. M-H loops were measured in the temperature range of 4.2 K to 15 K for fields up to 1.7 T using a small vibrating sample magnetometer (VSM) system, while ±9 T loops were taken with a larger VSM. Measurement were made on samples up to 3 cm long in transverse fields, and in the form of spiral samples (in axial fields) containing up to 37 cm of sample length. Some strands displayed larger flux jumps at higher fields, and numerous small flux jumps at low fields. In one case, the outer Cu stabilizer was removed from the strand, and as a result the strand became less stable, with flux jumping occurring at higher fields (where the magnetization is lower). Additionally, the influence of ramp rate on flux jumping was investigated. This was done by direct experiment, the use of a model system (MgB₂), and the extension of existing theory. It was found that the maximum instability, which is related to the maxima in dJ/dT, could occur off-axis. Additionally, higher ramp rates led to decreased stability. This was seen to lead in some cases to a ramp rate dependence in the field position of the flux jump.