Achromatic telescopic squeezing scheme and by-products: From concept to validation

The achromatic telescopic squeezing (ATS) scheme has brought an essential conceptual foundation to the HL-LHC project, making possible a strong and clean (achromatic) reduction of β*, an important parameter with respect to which several HL-LHC sub-systems were dimensioned (e.g., the 150 mm aperture of the new inner triplet quadrupoles) or justified (crab-cavities for mitigating the geometric luminosity loss factor at low β* and subsequent large crossing-angle). The basic mechanics of the scheme is shortly reminded, highlighting as well some of its by-products (Landau damping, long-range beam-beam mitigation with octupoles), and a recent improvement which made possible an early, but still partial, implementation of ATS optics in the LHC for the 2017 and 2018 LHC physics runs. The main focus of the paper will be on the experimental validation of the scheme, via the development of dedicated machine configurations and high-intensity beam tests which took place in Run 2. The paper will conclude on the configuration presently in mind to operate the LHC during its third exploitation period (Run 3), while trying to ensure a smooth enough transition toward the HL-LHC, in terms of optics, beam parameters, and dedicated beam manipulations (β*-leveling).