Index number (n) below "critical" current in Nb-Ti superconductors

For multifilamentary superconductors such as Nb-Ti and Nb/sub 3/Sn, a phenomenological theory ("sausage model") predicts a monotonic increase in the index number n as operating current is decreased below a "critical" current. An experiment was conducted to test the prediction; preliminary experimental results appear to confirm the prediction. For multifilamentary Nb-Ti with a measured n value of 26 at 3.4 T and 4.2 K - deduced from the shape of a V versus I plot at the conductor's nominal critical current of 542 A (defined at E/sub c/=0.1, /spl mu/V/cm) - the experiment reported here finds n to increase to 40 at I/sub op//I/sub c/=0.8; this increase in n agrees quite well with that predicted by the theory. One important implication of this theory is that it is permissible to specify n at I/sub c/ to be smaller than the required value if the conductor's normalized operating current (I/sub op//I/sub c/) is to be smaller than 1. The theory also can quantify n at I/sub c/ for required operating values of n and I/sub op//I/sub c/.<
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