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 (Jc) or effective subelement size (deff). However, reduction of deff of state-of-the-art Nb3Sn conductors – the restacked-rod-process (RRP®) type – below 40-50 μm without a pronounced decrease in high-field Jc 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 (Fp-B) curve peaks at ~20% of its irreversibility field (Birr), the Fp-B curve peaks of APC conductors shift to higher fields due to the point pinning effect, leading to flattening of the Jc-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 Jc-B curves of an RRP® conductor and two APC conductors (reacted at 700°C) from zero field to Birr using a high-field vibrating sample magnetometer (VSM). The results showed that the APC conductors have higher non-Cu Jc at high fields (e.g., 32-41% higher at 16 T) and simultaneously lower non-Cu Jc 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 Fp ratios. Suppose they reach the same 16 T non-Cu Jc, then the 1 T non-Cu Jc and magnetization of the APC conductors are only half or even less compared with the RRP® conductor.

Characteristic Length for Pinning Force Density in Nb3Sn

“…In this report, we reanalyzed experimental data on the dependence of the maximum pinning force density, , from the average grain size, d , in practical low- T c multifilamentary Nb 3 Sn conductors [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 55 , 56 , 58 ] fabricated by bronze and power-in-tube technologies.…”

“…Extensive (over nearly five decades) R&D studies of Nb 3 Sn-based conductors have shown that the key factors affecting the in-field critical current in these wires are the local composition, structure, and morphology of the superconducting A-15 phase [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ].…”

Characteristic Length for Pinning Force Density in Nb3Sn

“…Extensive (nearly five decades) R&D studies of Nb3Sn-based conductors have shown that the key factors affecting the in-field critical current in these wires are the local composition, structure, and morphology of the superconducting A-15 phase [9][10][11][12][13][14][15][16][17].…”