Critical-Current Measurements on an ITER Nb$_{3}$Sn Strand: Effect of Axial Tensile Strain

The dependence of transport critical current I
_c
on axial tensile strain was measured for a developmental Nb
₃
Sn multifilamentary strand as a function of magnetic field B between 12 T and 16 T, at the temperature of 4 K. This conductor was from the first stage of strand pre-production for the central solenoid of the International Thermonuclear Experimental Reactor (ITER) project. Straight samples were measured with a stress-free-cooling strain apparatus. The compressive pre-strain and the irreversible strain limit epsiv
_irr
were 0.19% and 0.8%, respectively; and the ultimate strain where the wire physically broke was about 0.95%. The pinning force F
_p
( = I
_c
x B ) was proportional (B*
_c2
)
^s
b
^p
(1 - b )
^q
to , where b = B / B*
_c2
is the reduced magnetic field, and the scaling constants had values p = 0.58, q = 1.86, and s = 0.7. The strain dependence of the effective upper critical field (the field at which F
_p
extrapolates to zero) was well described within the measured strain range by B*
_c2max
[1 - alpha |epsiv - epsiv
_max
|
^u

_],
where B*
_c2
is the maximum value of B*
_c2
as a function of strain, u = 1.7, and alpha was about 1230 for the compressive strains and 1670 for the tensile strains. Ekin's strain scaling law was applied to calculate the strain sensitivity of I
_c
at various intrinsic strains between -0.5% and 0.5%, and magnetic fields from 12 T to 16 T.