In the process that leads a flawless Nb 3 Sn round strand to become part of a Rutherford cable first, and of a coil next, the same cabling process affects strands of different kinds in different ways, from filament shearing to subelement merging to composite decoupling. Due to plastic deformation, after cabling the filament size distributions in a strand usually change. The average filament size typically increases, as does the width of the distribution. This is consistent with the low field transport current of strands in cables being typically lower and less reproducible than for round strands [1]. To better understand the role of filament size in instabilities and to simulate cabling deformations, strands to be used in cables can be tested by rolling them down to decreasing sizes to cover an ample range of relative deformations. A procedure is herein proposed that uses both microscopic analysis and macroscopic measurements of material properties to study the effects of deformation.Index Terms-Critical current density, magnetic instability, Nb 3 Sn, Rutherford cable.

A major thrust in Fermilab's accelerator magnet R&D program is the development of wires which meet target requirements for high field magnets, such as high critical current density, low effective filament size, and the capability to withstand the cabling process. The performance of a number of strands with 150/169 restack design produced by Oxford Superconducting Technology using the Restacked-Rod Process was studied for round and deformed wires. To optimize the maximum plastic strain, finite element modeling was also used as an aid in the design. Results of mechanical, transport and metallographic analyses are presented for round and deformed wires.

An important part of superconducting accelerator magnet work is the conductor. To produce magnetic fields larger than 10 T, brittle A15 conductors are typically used. The original round wire, in the form of a composite of Copper (Cu), Niobium (Nb) and Tin (Sn), is assembled into a so-called Rutherford-type cable, which is used to wind the magnet. The magnet is then subjected to a high temperature heat treatment to produce the chemical reactions that make the material superconducting. At this stage the superconductor is brittle and its superconducting properties sensitive to strain. This work is based on the development of a 2D finite element model, which simulates the mechanical behavior of Nb-Sn composite wires under deformation before heat treatment. First the composite was modeled in detail and its behavior analyzed under flat rolling using Finite Element Analysis (FEM). To identify a critical criterion, the strain results of the model were compared with those measured experimentally on cross sections of the deformed composite. Then the model was applied to a number of different wire architectures.Index Terms-Finite element model analysis, Nb 3 Sn wires, plastic work, principal strain, restacked-rod process.

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