Investigation on the Effect of Ta Additions on $J_{c}$ and $n$ of $({\rm Nb},{\rm Ti})_{3}{\rm Sn}$ Bronze Processed Multifilamentary Wires at High Magnetic Fields

We have analyzed the influence of Ta on the superconducting properties of Ti doped bronze processed $({\rm Nb},{\rm Ta},{\rm Ti})_{3}{\rm Sn}$ multifilamentary wires. The study was carried out on wires with identical configuration, consisting of 14,641 filaments with 4.5 $\mu{\rm m}$ diameter embedded in a Ti bearing Osprey bronze and externally stabilized by copper. Several wires were made using either Nb7.5Ta or pure Nb as a filament material, some of them containing a core of NbTi as a supplementary Ti source. All wires were manufactured by three hot extrusion steps, cold drawing and several intermediate anneals. Wire samples with round and rectangular cross sections have been prepared and reacted with various heat treatments between 550 and 730$^{\circ}{\rm C}$. The residual Sn content in the Cu-Sn-Ti bronze after reaction and the degree of reaction of the filaments were determined by means of EDS and SEM analysis, respectively. AC susceptibility has been used to determine the effect of prestrain and doping on the superconducting transition temperature $T_{c}$. At 21 T and 4.2 K, similar non-Cu $J_{c}$ values up to 90 ${\rm A/mm}^{2}$ (overall $J_{c}$: 63 ${\rm A/mm}^{2}$) and $n$ values up to 25 have been observed on both compositions $({\rm Nb},{\rm Ta},{\rm Ti})_{3}{\rm Sn}$ and $({\rm Nb},{\rm Ti})_{3}{\rm Sn}$. These results suggest that Ta as a quaternary dopant does not lead to further improvements of $J_{c}$ and $n$ of ternary $({\rm Nb},{\rm Ti})_{3}{\rm Sn}$.