  The increasing demand for improving the high-field (16-22 T) performance of
Nb$_3$Sn conductors requires a better understanding of the properties of modern
wires much closer to irreversibility field, H$_{Irr}$. In this study we
investigated the impact of Ta, Ti and Hf doping on the high-field pinning
properties, the upper critical field, H$_{c2}$, and H$_{Irr}$. We found that
the pinning force curves of commercial Ti and Ta doped wires at different
temperatures do not scale and that the Kramer extrapolation, typically used by
magnet designers to estimate high-field critical current density and magnet
operational margins from lower field data, is not reliable and significantly
overestimates the actual H$_{Irr}$. In contrast, new laboratory scale
conductors made with Nb-Ta-Hf alloy have improved high-field J$_c$ performance
and, despite contributions by both grain boundary and point defect pinning
mechanisms, have more predictable high-field behavior. Using Extended X-ray
Absorption Fine Structure spectroscopy, EXAFS, we found that for the commercial
Ta and Ti doped conductors, the Ta site occupancy in the A15 structure
gradually changes with the heat treatment temperature whereas Ti is always
located on the Nb site with clear consequences for H$_{c2}$. This work reveals
the still limited understanding of what determines H$_{c2}$, H$_{Irr}$ and the
high-field J$_c$ performance of Nb$_3$Sn and the complexity of optimizing these
conductors so that they can reach their full potential for high-field
applications.
