The relationship between the n-value and applied strain
The most popular fit for describing the volt-ampere characteristic of a superconducting material is a power law with exponent n. Usually for a Nb3Sn wire the n-index is parametrized as a function of the sole critical current (I-c). An experiment to determine whether the n-index also shows an explicit dependence on the strain (epsilon) has been performed for an ITER (International Thermonuclear Experimental Reactor) wire. From the same wire, three pairs of samples were prepared. The three pairs were mounted on two Ti-ITER barrels, on two 316L stainless steel barrels and on two INCONEL (TM) alloy C-276 barrels, and heat treated. The critical current was then measured in the field range of 9-15 T. From the I-c measurements it was possible to derive three n versus I-c curves. Each curve corresponds to a different applied strain, as the three barrel materials have different thermal contraction coefficients. By analysing the three curves, the explicit dependence of the n-index on the strain is evaluated. In the strain range covered by the measurements (up to approximate to-0.65% intrinsic strain), the n-value scales with the sole critical current I-c.