Abstract
A novel optics concept, the achromatic telescopic squeezing (ATS) scheme has been invented in the context of the Large Hadron Collider (LHC) upgrade studies, and chosen as the baseline scheme for the optics and layout of the recently approved high luminosity LHC project (HL-LHC). This scheme offers an extremely powerful and flexible machinery in order to strongly reduce ${\ensuremath{\beta}}^{*}$ in a symmetric or asymmetric way (i.e. without necessarily imposing the same ${\ensuremath{\beta}}^{*}$ in both planes), while perfectly controlling the chromatic aberrations induced, namely the linear and nonlinear chromaticities, the off-momentum $\ensuremath{\beta}$ beating, and the spurious dispersion from the large crossing angle which is required at small ${\ensuremath{\beta}}^{*}$ in the particular case of the (HL)-LHC. The initial motivations of the scheme will be reviewed, followed by a detailed description of its fundamental theoretical foundations. An effective construction of ATS optics will be given and its main features illustrated in the case of the LHC and HL-LHC.
Highlights
A strong reduction of à requires in principle longer final focus systems (FFS) in order to enable a smooth matching of the optics between the interaction point (IP) and the regular lattice of the machine, which are the arcs in the case of a circular machine, or the two facing linacs in the case of a linear collider
Appropriate solutions, or even acceptable compromises, might be very hard if not impossible to find, when strongly relying on this approach for upgrading the performance of a machine which is already built. Another prerequisite to reduce à might be a reduction of LÃ, which is the distance between the last final focus quadrupole and the interaction point. This intervention aims at mitigating the inevitable increase of the chromatic aberrations due to the larger functions in the final focus quadrupoles induced at smaller Ã: the momentum à acceptance in the specific case of a linear collider, or, in the case of a circular machine, the linear and nonlinear chromaticities, the off-momentum beating, and the spurious dispersion induced by an eventual nonzero crossing
The achromatic telescopic squeezing scheme represents a real evolution with respect to the standard concept of a lowinsertion, where the reduction of à is generally achieved by using a certain number of matching quadrupoles, confined in a rather small fraction of the ring on either side of the IP
Summary
Reducing the beam spot size at the interaction point (IP), which is acting on à at constant transverse emittances, is a key ingredient to boost the performance of any collider. Appropriate solutions, or even acceptable compromises, might be very hard if not impossible to find, when strongly relying on this approach for upgrading the performance of a machine which is already built. Another prerequisite to reduce à might be a reduction of LÃ, which is the distance between the last final focus quadrupole and the interaction point. The price to pay might be again in this case a substantial lengthening of the final focus systems of the collider or, for circular machines, a delicate surgery into the heart of the lattice by replacing the existing chromaticity sextupoles by new ones, much stronger, in all the arcs of the ring. III in the framework of the luminosity upgrade program of the LHC (HL-LHC [2])
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