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

Abstract

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®) type—below 40–50 μm 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 ∼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® conductor and two APC conductors (reacted at 700 °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®. 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® conductor.