Preliminary Design of the Recombination Dipole for Future Circular Collider

The Future Circular Collider (FCC) project aims at producing a conceptual design of a post-Large Hadron Collider (LHC) particle collider, able to reach higher beam energy with higher luminosity both for proton-proton and electron-electron configurations. In the proton-proton configuration, the FCC must be able to circulate 50 GeV protons in a 100 km tunnel. A key role in this project is played by the R&D on superconducting magnets, which must be able to produce magnetic fields significantly higher than in the LHC. Beside the bending dipoles, which must produce a 16 T field, also the multiplets in the interaction regions represent a technological challenge: as an example, the recombination dipoles are expected to produce a field of 10 T and therefore Nb3Sn must be used. In this contribution, the preliminary design of a double aperture recombination dipole for the FCC will be presented in details. It features a two layers, cosine-theta design, in order to produce the required field of 10 T in the bore. The magnetic field has the same direction in both apertures, generating a not negligible crosstalk which is minimized using asymmetric coils. The magnetic design will be described, focusing on the main features of the Nb3Sn conductor. The winding configuration and the iron yoke shape have been optimized to achieve a suitable field quality. A solution for the mechanical design will also be presented: the necessary prestress will be given by the so-called "bladder and keys" concept, which avoids the collaring and allows to obtain, thanks to differential thermal contraction of the components, most of the prestress after the cool down, when the Nb 3 Sn is less brittle. The proposed solution fulfills all the standard requirements both from the magnetic point of view, i.e., field quality and current margin, and from the mechanical one, with a viable construction schema and reasonably low stress on conductor and support structures, during all the magnet operations.