The Design of Superconducting Separation Dipoles D2 for the High Luminosity Upgrade of LHC

The luminosity upgrade of the Large Hadron Collider involves large modifications of the magnets close to the interaction regions. In particular, the double-aperture separation/recombination dipoles D2 shall deliver a field integral of 35
$\text{T}\cdot\text{m}$
. Given the physical and geometrical constraints limiting the length to less than 10 m, these magnets must be superconducting and shall generate a magnetic field higher than 3.5 T. Since the magnetic field direction is identical in both apertures (105 mm each one), the magnetic field between the two apertures sums up to high values, locally saturating the shielding iron yoke surrounding both coils. In order to decouple the magnetic field in the two apertures, a strategy has been developed based on three pillars: 1) no iron is placed in between the coils (thus limiting saturation effects); 2) each coil is asymmetric in a way to cancel the magnetic crosstalk to each other; 3) the yoke is suitably profiled for minimizing the variation in the harmonic components due to iron saturation. Based on these concepts, a 2-D magnetic optimization was carried out, leading to an acceptable field quality with a limited variation of the multipoles as the magnet field is raised from the injection value to the maximum value (4.5 T). A 2-D mechanical design was also performed with the aim at simplifying the construction issues and keeping under control the parameters relevant to beam optics (beam separation and magnetic multipoles). Finally, the coil ends were designed, too, and a complete model 1.5 m long drafted. This paper discusses in detail all these aspects, with some considerations about the future developments preliminary to the mass production.