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Preprint
Report number CERN-ACC-2023-0005
Title Long-range beam-beam mitigation for round optics
Author(s) Fartoukh, Stephane (CERN) ; Kostoglou, Sofia (CERN)
Publication 2023
Imprint 01 Sep 2023
Number of pages 22
Subject category Accelerators and Storage Rings
Accelerator/Facility, Experiment CERN LHC
Processes Beam Optics
Keywords Long-range beam-beam mitigation ; Octupoles ; wires
Abstract When colliding high intensity bunch trains at low beta*, a sufficiently large crossing angle needs to be imposed at the interaction point (IP) in order to separate the two beams before and after the collision, and mitigate the so-called long-range beam-beam (BBLR) effect, which otherwise can be very detrimental to the beam lifetime. This, in turn, can impact severely the performance of the collider, unless crab-cavities for restoring the geometric overlap of the bunches at the collision point are part of the design (which is the strategic choice made by the HL-LHC project), and/or active beam-beam compensation schemes are put in place. The grail of such a scheme is based on DC current wires (one per beam per IP side), which should be ideally inserted at a rather small normalized transverse distance from the beam, which scales with the targeted normalized crossing angle, typically 8 beam sigma (for one of the two wires) when the targeted normalized crossing angle is 9.5 sigma. This ideal distance is however, deeper or much deeper, than the LHC or HL-LHC settings planned for the tertiary collimators (TCT). Since its early proposal in [1], the wire option was therefore known to be very challenging, if not incompatible, with the machine protection and collimation hierarchy. Purely numerical studies [2], however, recently showed that very good compromises could be found with quite relaxed wire settings, at least for round optics (i.e. with same beta* values in both transverse plane), and provided a net increase of the wire current. In this framework, this paper will elaborate a systematic and semi-analytical analysis of the pros and cons of this compromise, together with its limits and sensitivity to other optics parameters such as the beta-function aspect ratio at the wire location. One output of this study is the maximum allowed normalised gap for the TCTs (for round optics), such that the wires are not exposed to the tertiary halo while warranting their functionality in aggressive beam and optics conditions (preservation of the dynamic aperture at the level of 5.5 sigma up to a bunch population of 2.2e11 p/b with a normalised crossing angle as low as 9.5 sigma). The alternative of using pure octupole magnets, which would be installed near Q4, at sufficiently large beta-function and at an optimal beta-function aspect ratio, will also be discussed. While this option would strictly decouple the long-range beam-beam mitigation scheme from the collimation and machine protection sub-systems, it is shown to be less performing than the wire option for preserving the dynamic aperture with aggressive beam and optics parameters.
Submitted by [email protected]

 


 Record created 2025-01-14, last modified 2025-01-29


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