Abstract
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 ${\ensuremath{\beta}}^{*}$, 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 ${\ensuremath{\beta}}^{*}$ 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 (${\ensuremath{\beta}}^{*}$-leveling).
Highlights
AND MOTIVATIONSReducing the beam sizes at the interaction point, namely acting on βà at constant beam emittances, is a key ingredient to boost the performance of any collider
The HL-LHC baseline optics fully relies on the achromatic telescopic squeezing (ATS) scheme, firstly in terms of low-βà optics feasibility
The machine was filled with 48 þ 12 þ 1 1⁄4 61 nominal bunches, in particular a train of 48 bunches colliding at IP1 and IP5
Summary
Reducing the beam sizes at the interaction point, namely acting on βà at constant beam emittances, is a key ingredient to boost the performance of any collider. Besides the need of larger-aperture magnets in order to safely operate the machine at lower βÃ, the first challenge is to design the optics, that is to match it to the (nonupgraded) part of the ring, while preserving its flexibility within the matching quadrupole strength limits, and its chromatic properties in terms of off-momentum β-beating, linear and nonlinear chromaticities, and spurious dispersion induced by the crossing bumps (see [1] for more detail). The achromatic telescopic squeezing (ATS) scheme [2,3,4] brought a definite and cost-efficient solution to this problem
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