Identification of the A15 Contours in Multifilamentary Nb3Sn Wires by Means of a Magnetic Method

A model for evaluating thecontours of the A15 layer in Nb3Sn wires is proposed. These borders, crucial for the correct assessment of the layer current density J(c) and the volume layer pinning force F-p (B) are in fact non trivial to assess in some cases from scanning electron microscopy (SEM) polished surface images, eventually leading to non-negligible discrepancies between independent measurements reporting the same m - H or I-c values. A self-reliant method from magnetometry based (Bean) models is proposed, allowing the outer (r(o)) and inner (r(i)) contours of the A15 phase to be assessed. The magnetic moment of' short samples of' a prototype powder in tube (PIT) wire with artificial pinning centres (APC) was measured between 6.5 T (H-0) and 6.95 T at 4.2 K in a superconducting quantum interference device (SQUID) magnetometer, allowing the calculation of r(o) from the initial slope of dm/dH. Then, the derived function for the remanent magnetic moment m(rem) is fit to the experimental data in order to derive r(i), under the condition that the saturation moment exactly matches the experimental value.