Voltage Versus Magnetic Field Measurements on Nb3Sn Wires

We measured voltage versus magnetic field (V‐H) on commercial Nb3Sn wires. Typically voltage‐current (V‐I) at constant field is measured to determine the critical current (Ic). Recently, V‐H at constant or ramping current is being measured to assess the relative stability of Nb3Sn wires. Our low‐noise measurements were made with the same equipment, apparatus, and sample mount as used in Ic measurements. High‐performance Nb3Sn wires exhibit flux‐jump instabilities at low magnetic fields, and low‐noise V‐H curves on these wires show indications of flux jumps. Two nominal 0.8 mm diameter Nb3Sn wires were measured: a high‐performance wire with an Ic of 750 A at 12 T and a stable wire with an Ic of 150 A at 12 T. V‐H measurements also reveal that less stable wires will quench (abrupt and irreversible transition to the normal state) at currents much smaller than Ic at the lower magnetic fields. The measured V‐H curves and quench currents of these two wires are contrasted.