Critical current and n-value modifications from superconducting strands to Rutherford cables

We propose a method for relating the critical current measured on superconducting wires with the one measured on a Rutherford cable made by cabling the same wires. The problem is mainly related to the definition of the critical magnetic field of strands or cables exposed to a non-uniform magnetic field due to the self-field. The field generated by the conductor may be a not negligible fraction of the total applied field, so to play a central role in determining both the measured value of the critical current and the shape of the superconducting transition as observed through voltage-current measurements. In this frame it is important to understand the effect of the inter-strand currents due to the fact that strands in saturated regions can exchange current with not saturated strands. We show how the V-I characteristic would be modified by this effect and what seems to happen in real conductors, by measurements performed on high-current multi-strand cables. (C) 2003 Elsevier B.V. All rights reserved.