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Description
For the second phase of the LHC collimation system upgrade, with beam operation at nominal and ultimate intensities, Fermilab and CERN have started the development of 11 T Nb3Sn dipoles 11 m long to replace a few longer Nb-Ti dipoles and free space for cold collimators in the LHC dispersion suppression areas. An important step in the design of these magnets is the development of the high aspect ratio Nb3Sn cable to achieve 11 T at the nominal LHC operating current of 11.85 kA with 20% margin. The desired goal is of producing nominal cable both at FNAL and at CERN.
Using FNAL two-step cable fabrication technology, a number of cable design studies were performed to finalize cable specifications. This included the effect of the number of strands on the cable parameters, the sensitivity of electrical properties to cable compaction, measurements of cable expansion during reaction, and the effect of intermediate annealing between the two cable fabrication steps. The cable optimization process was aimed at achieving both mechanical stability and minimal damage to the internal architecture of the Restacked-Rod Process (RRP®) Nb3Sn strands to be used in the magnet short models. Both the 108/127 RRP wire, which is presently a baseline conductor for Nb3Sn magnet R&D in the US, and a new 150/169 RRP wire were used for the cable optimization studies. In particular, the 150/169 RRP wire was used to develop a 40-strand cored cable technology, and in fabrication of 300 m of cable with stainless steel core for an 11 T dipole short model. Strand and cable studies included electrical characterization of transport properties at high fields and of flux jumps stability at low fields, magnetization measurements, metallographical analyses of internal damage and ANSYS finite element modeling.
