Improvement of the critical current of multifilamentary Nb3Sn conductors under tensile stress

In multifilamentary Nb
3
Sn conductors produced by solid state diffusion process, the Nb
3
Sn is subjected to a tensile stress, because the bronze contracts stronger than the Nb
3
Sn during cooling from the diffusion temperature to 4 K. The measurements carried out demonstrate that under externally applied tensile stress at 4 K, the critical current I
c
of these conductors increases and goes through a maximum I
cm
at a strain
\varepsilon_{m}
. This is caused by the decrease in pressure on the Nb
3
Sn through the external tensile stress; a further increase in the external stress leads to internal tensile stress. The compressive stress and the resulting shrinkage of the Nb
3
Sn increase with decreasing layer thickness, thereby I
cm
/I
c0
increases (I
c0
: initial value without strain). I
cm
/I
c0
is solely dependent on
\varepsilon_{m}
and reaches (at 4.4 T) values above 1.4 at
\varepsilon_{m} = 0.9
% for very thin Nb
3
Sn layers.
\varepsilon_{m}
increases as the share of Nb
3
Sn in the conductor cross-section decreases and accordingly behaves qualitatively as the thermally affected shrinkage of the Nb
3
Sn.