Sextupole scheme optimization for HL–LHC

One essential upgrade of the High Luminosity LHC (HL–LHC) to reach the desired luminosity, is the large reduction of the β-function value at the interaction points (IP) hosting the high-luminosity experiments ATLAS and CMS. The achromatic telescopic squeeze (ATS [1]) approach has been adopted for HL–LHC, in order to overcome various limitations stemming from the ring quadrupoles and sextupoles when the β function at the IP is strongly reduced. The resulting large β waves at the sextupole locations excite strong non-linearities that are compensated in the baseline design by adding four sextupoles to the regular part of the magnetic lattice. This avoids life-time losses due to a reduction of the dynamic aperture (DA). In this study, alternative sextupole correction schemes are proposed and compared with the baseline. These schemes have the advantage of avoiding the costly and time-consuming installation of the additional sextupoles, while keeping the DA at the required level and the optics parameters within the constraints. The optimization process and the impact on the linear optics, aberrations, and dynamic aperture are summarized and discussed in this report.