Abstract
In view of the High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) at CERN, different materials were investigated for the upgrade of the LHC collimation system. A key objective was ...to determine how the jaws of the new collimators could be manufactured to meet the demanding requirements of HL-LHC, such as thermo-mechanical robustness and stability, beam coupling impedance, Ultra-High Vacuum (UHV), etc. During the Long-Shutdown 2 (LS2), five primary and ten secondary low-impedance collimators were already produced using novel materials. For LS3, in addition to more secondary collimators, the production and installation of other types of devices, including tertiaries and physics-debris collimators, is planned. This paper details the final mate-rial choices and rationale for each collimator family.
The High Luminosity (HL) upgrade of the Large Hadron Collider (LHC) will increase the peak luminosity at the experiments by more than a factor of 5 with respect to the LHC design value. To achieve ...this goal, among the upgrade of several beam and machine parameters, the beam intensity will nearly double with respect to the operational LHC value, and the transverse beam emittance will decrease by 50% compared to the LHC design value. Past operational experience showed that coherent beam instabilities may occur for low, positive values of chromaticity, and a higher tune spread than predicted from simulations is required from the dedicated octupole magnets to provide enough Landau damping. With the HL-LHC brighter beams, stability margins will become tighter, and coherent instabilities become stronger if no dedicated mitigation measures are taken. An impedance reduction plan is therefore taking place targeting the collimation system, and the main contributor to the transverse beam coupling impedance at the flattop energy. New collimators with lower resistivity materials will replace the current LHC ones. In this work, we assess the benefits of this impedance reduction with respect to the transverse mode coupling instability threshold. This study quantifies the discrepancy between measured and predicted beam stability thresholds at low chromaticity. It also probes the expected gain of the impedance reduction plan of HL-LHC.
Beam Instabilities in Hadron Synchrotrons Metral, E.; Argyropoulos, T.; Bartosik, H. ...
IEEE transactions on nuclear science,
04/2016, Letnik:
63, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Beam instabilities cover a wide range of effects in particle accelerators and they have been the subjects of intense research for several decades. As the machines performance was pushed new ...mechanisms were revealed and nowadays the challenge consists in studying the interplays between all these intricate phenomena, as it is very often not possible to treat the different effects separately. The aim of this paper is to review the main mechanisms, discussing in particular the recent developments of beam instability theories and simulations.
Most hadron synchrotrons rely on lattice nonlinearities for Landau damping of impedance driven coherent modes of oscillation. However, in a collider, the presence of beam-beam interactions strongly ...modifies the transverse amplitude detuning and therefore the resulting stability diagram. A numerical tool to evaluate the effect of beam-beam on the stability diagram has been developed and is used to discuss observations during different phases of the operational cycle of the Large Hadron Collider during the 2012 proton run. In particular, we show the evolution of the stability diagram when the strength of long range beam-beam interactions is increased, during the betatron squeeze. Also, we investigate the stability of beams colliding with a small transverse offset and compare to observations of instabilities when bringing the beams into collision and while leveling the luminosity.
In high brightness circular colliders, coherent and incoherent beam dynamics are dominated by beam-beam interactions. It is generally assumed that the incoherent tune spread introduced by the ...beam-beam interactions is sufficiently large to cure any instabilities originating from impedance. However, as the two counterrotating beams interact they can give rise to coherent dipole modes and therefore modify the coherent beam dynamics and stability conditions. In this case, coherent beam-beam effects and impedance cannot be treated independently and their interplay should be taken into account in any realistic attempt to study the beam stability of colliding beams. Due to the complexity of these physics processes, numerical simulations become an important tool for the analysis of this system. Two approaches are proposed in this paper: a fully self-consistent multiparticle tracking including particle-in-cell Poisson solver for the beam-beam interactions and a linearized model taking into account finite bunch length effects. To ensure the validity of the results a detailed benchmarking of these models was performed. It will be shown that under certain conditions coherent beam-beam dipole modes can couple with higher order headtail modes and lead to strong instabilities with characteristics similar to the classical transverse mode coupling instability originating from impedance alone. Possible cures for this instability are explored both for single bunch and multibunch interactions. Simulation results and experimental evidences of the existence of this instability at the LHC will be presented for the specific case of offset collisions.
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