Significant reduction in the low-field magnetization of Nb₃Sn superconducting strands using the internal oxidation APC approach

Nb3Sn superconductors are promising for building accelerator magnets for future energy-frontier circular colliders. A critical factor for this application is the low-field persistent-current magnetization because it leads to several critical issues: e.g. low-field instability (including flux jumps), hysteresis loss, and field errors in magnet bores. Suppression of low-field magnetization requires reduction of low-field critical current density (J (c)) or effective subelement size (d (eff)). However, reduction of d (eff) of state-of-the-art Nb3Sn conductors-the restacked-rod-process (RRP & REG;) type-below 40-50 & mu;m without a pronounced decrease in high-field J (c) is difficult. On the other hand, the internal oxidation method which forms artificial pinning centers (APC) in Nb3Sn offers an alternative approach to reducing the low-field magnetization. Compared with a conventional Nb3Sn conductor whose flux pinning force versus field (F (p)-B) curve peaks at & SIM;20% of its irreversibility field (B (irr)), the F (p)-B curve peaks of APC conductors shift to higher fields due to the point pinning effect, leading to flattening of the J (c)-B curves. The goal of this paper is to quantitatively study how much the APC approach can reduce the low-field magnetization. We measured the J (c)-B curves of an RRP & REG; conductor and two APC conductors (reacted at 700 & DEG;C) from zero field to B (irr) using a high-field vibrating sample magnetometer. The results showed that the APC conductors have higher non-Cu J (c) at high fields (e.g. 32%-41% higher at 16 T) and simultaneously lower non-Cu J (c) at low fields (e.g. 28%-34% lower at 1 T) compared with the RRP & REG;. This effect is due to a competition between their Nb3Sn layer fraction ratios and layer F (p) ratios. Suppose they reach the same 16 T non-Cu J (c), then the 1 T non-Cu J (c) and magnetization of the APC conductors are only half or even less compared with the RRP & REG; conductor.