Investigation of instability in high Jc Nb3Sn strands

Magnetization measurements show that modern high current-density Nb3Sn strands made for HEP programs exhibit flux-jump instabilities at low fields, due to their having large effective filament diameters. Such instabilities might be problematic because they can initiate a quench in low-field regions of magnets. We explored magnetization and transport measurements of the most recent high J(c) Nb3Sn strands and cables to probe the instability behavior. In the regime where flux jumps are seen by magnetization measurements, transport current measurements show a threshold for stability. This threshold is significantly lower than the critical current at higher fields, and above this threshold, quenching in the strand could be initiated by ramping the magnetic field. The threshold current depends on the wire size and internal filament design, and is consistent with stability criteria. In cables, quench currents were nearly independent of field after training, and were far below the expected critical currents. Details of these measurements and their implications for testing and use in magnets are discussed.