Powder-in-tube (PIT) Nb3Sn wires are competing with Restacked-Rod-Process (RRP®) for the realization of the high luminosity upgrade of the Large Hadron Collider (LHC) at CERN. These two conductors have different properties and microstructures that are in both cases averages of an inhomogeneous A15 microstructure. PIT has in general a smaller fraction of A15 in the non-Cu cross-section than RRP® and a lower non-Cu Jc (12 T, 4.2 K) (2500–2700 A mm−2 versus 2900–3000 A mm−2) but it can be made in smaller filament diameters, which is an important property for LHC magnets. Another characteristic of PIT A15 is that ∼25% is made up of ∼1–2 μm sized grains (typically ∼10 times the small grain (SG) diameter) and their contribution to transport is uncertain. Here we studied a 192 filament Ta-doped, 1 mm diameter PIT wire and combined multiple characterization techniques in order to distinguish the different wire components, to determine their individual properties and to identify which components are current-carriers. We found multiple evidence that the large A15 grains, which are also the highest-Tc grains, do not contribute to transport at high field and that the only current-carrying A15 is the SG with Tc <17.7 K. However, because of the high density of grain boundaries in the SG A15 layer, PIT has an exceptionally high SG-layer Jc and high specific grain boundary pinning force, QGB. These findings clearly show that it is essential to increase the ratio of small to large and disconnected grains in order to improve PIT performance.

High critical current density (J c ) Nb 3 Sn A15 multifilamentary wires require a large volume fraction of small grain, superconducting A15 phase, as well as Cu stabilizer with high Residual Resistance Ratio (RRR) to provide electromagnetic stabilization and protection. In Powder-in-Tube (PIT) wires the unreacted Nb7.5wt.%Ta outer layer of the tubular filaments acts as a diffusion barrier and protects the interfilamentary Cu stabilizer from Sn contamination. A high RRR requirement generally imposes a restricted A15 reaction heat treatment (HT) to prevent localized full reaction of the filament that could allow Sn to reach the Cu. In this study we investigate recent high quality PIT wires that achieve a J c (12 T, 4.2 K) up to ~2500 A/mm 2 and find that the minimum diffusion barrier thickness decreases as the filament aspect ratio increases from ~1 in the inner rings of filaments to 1.3 in the outer filament rings. We found that just 2-3 diffusion barrier breaches can degrade RRR from 300 to 150 or less. Using progressive etching of the Cu we also found that the RRR degradation is localized near the external filaments where deformation is highest. Consequently minimizing filament distortion during strand fabrication is important for reducing RRR degradation. The additional challenge of developing the highest possible J c must be addressed by forming the maximum fraction of high J c small-grain (SG) A15 and minimizing low J c large-grain (LG) A15 morphologies. In one wire we found that 15% of the filaments had a significantly enhanced SG/LG A15 ratio and no residual A15 in the core, a feature that opens a path to substantial J c improvement.

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