Critical state instability in Nb-clad MgB2 superconducting wires

Magnetization hysteresis loops of Cu/MgB2, Nb/MgB2, Cu/Nb/MgB2 and Fe/Cu/MgB2 wires in parallel magnetic fields of up to 5 T were studied in the temperature range from 5 to 35 K. All Nb-clad samples exhibited a thermomagnetic instability (TMI) in the form of magnetization jumps. In a thick wire (about 2 mm in core diameter), the TMI persisted up to the unexpectedly high temperature of 32 K. Thin wires showed low TMI which vanished at T > 10 K. Cu/MgB2 wires which did not contain a Nb barrier, showed no signs of TMI. The TMI in thin wires exhibited good reproducibility and stability in the jump pattern (JP) (jump amplitudes and positions), while thick wires showed the worst time stability. We found that moderate flat rolling of the round unstable Cu/Nb/MgB2 wire resulted in negligible TMI at 5 K in the processed flat tape. The TMI amplitudes of studied samples correlated with the adiabatic stability parameter, β−1.