  The recently discovered iron-based superconductors with very high upper
critical field and small anisotropy have been regarded as a potential candidate
material for high field applications. However, enhancements of superconducting
properties are still needed to boost the successful use of iron-based
superconductors in such applications. Here, we propose a new sheath
architecture of stainless steel (SS)/Ag double sheath and investigate its
influence on the microstructures and Jc-H property. We found that the transport
Jc-H curves for rolled and pressed tapes both show extremely small magnetic
field dependence and exceed 3*10^4A/cm^2 under 28 T, which are much higher than
those of low-temperature superconductors. More interestingly, 12 cm long rolled
tape shows very high homogeneity and sustains Jc as high as 7.7*10^4 A/cm^2 at
10 T. These are the highest values reported so far for iron-based
superconducting wires fabricated by scalable rolling process. The
microstructure investigations indicate that such high Jc was achieved by higher
density of the core and uniform deformation resulting better texturing. These
results indicate that our process is very promising for fabricating long Ba122
wires for high field magnet, i.e. above 20 T.
