Stress effects on multifilamentary Nb3Sn wire

Critical current I
c
measurements were obtained on highly stabilized mf Nb
3
Sn wires as a function of heat treatment, stress, temperature, and applied magnetic field. The ratio of the area of the copper to bronze core-niobium tube is about 8, and the filaments are concentrated in the inner 30% of the wire's cross section. Values of I
c
and T
c
were determined for samples subjected to a wide range of heat treatments. Diffusion reaction times and temperatures in the ranges 16 to 128 h and 700 to 750°C provided a number of mf Nb
3
Sn wires having similar I
c
characteristics. To some extent the residual compressive loading on the Nb
3
Sn wires varied with the particular heat treatment. This loading arises primarily from the differential contraction of the remaining bronze and the Nb
3
Sn layer when cooled from the reaction temperature to the operating temperature. Like other investigators, we find that, by controlled bending or stretching of the wires, whereby some of the strain in the Nb
3
Sn is relieved, the I
c
at 14 K is increased by as much as 30% and the critical temperature is increased by up to 1 K. The pinning force in strained and relieved wires was determined from I
c
measurements in applied fields up to 10 T. We analyzed changes in the position and height of the pinning force peak as a function of strain relief using Kramer's theory of flux pinning. With the Kramer model, by varying the effective strength and number of pinning sites, we were able to describe qualitatively differences in the pinning force curves that were obtained in the as-reacted (strained) and bent (strain-relieved) states.