Considerations on a Partial Energy Upgrade of the LHC

In the frame of the HL-LHC project, a few accelerator dipole and quadrupole magnets of higher critical field and/or larger aperture are being produced. The new inner triplet quadrupoles and dispersion-suppressor dipoles are made from Nb3Sn superconductor, which supports a higher field than the classical Nb-Ti magnets used for the LHC. For the longer term future, it has been proposed to replace a fraction of the Nb-Ti arc magnets in the LHC arcs with Nb3Sn magnets of higher field (e.g. 11 T), in order to boost the beam energy. Here we examine several options: the replacement of every third dipole by a stronger one, the substitution of the present Nb-Ti quadrupole by Nb3Sn combined-function magnets, the excitation of the horizontal orbit correctors, and pushing all the dipole magnets to their ultimate field. We discuss challenges and constraints, including issues related to mechanical aperture, powering, or other hardware limitations, and we estimate the potential energy reach for each of the options