Modeling of the emittance growth due to decoherence in collision at the Large Hadron Collider

X Buffat,D Valuch,W Herr,T Pieloni

doi:10.1103/physrevaccelbeams.23.021002

Abstract

The transverse emittance growth rate of colliding hadron beams driven by external sources of noise is investigated based on existing analytical model as well as on macro-particle simulations and comparison to experimental data at the Large Hadron Collider (LHC). It is shown that an analytical description of the emittance growth rate neglecting the existence coherent beam-beam mode can nevertheless provide accurate estimate for operational conditions, featuring notably a high chromaticity. The model is used to investigate the level of noise experienced by the LHC beams. The results indicate that a significant reduction of the noise floor of the transverse feedback's beam position monitor is required for operation with a large beam-beam tune shift, as the one anticipated for the High Luminosity LHC (HL-LHC).

Highlights

In existing high energy hadron colliders, such as the Large Hadron Collider (LHC) at CERN, the preservation of the transverse emittances is a major concern due to the absence of significant damping mechanisms
The results indicate that a significant reduction of the noise floor of the transverse feedback’s beam position monitor is required for operation with a large beam-beam tune shift, as the one anticipated for the High Luminosity LHC (HL-LHC)
Considering beams colliding with twice the beam-beam parameter, corresponding to the HL-LHC design [9], we find that the vertical emittance growth rate is significantly reduced with respect to the expectations from the weakstrong formula yet remaining above the estimation from the first order strong-strong formula (Fig. 8)

Summary

INTRODUCTION

In existing high energy hadron colliders, such as the Large Hadron Collider (LHC) at CERN, the preservation of the transverse emittances is a major concern due to the absence of significant damping mechanisms. For more realistic configurations of the LHC and of the High Luminosity LHC (HL-LHC) involving complex bunch train structures, multiple IPs with asymmetric phase advances between them, as well as chromaticity, there exists a variety of coherent beam-beam modes which frequencies may lay inside or outside of the incoherent spectrum [8]. In these conditions the formalism developed in [5] usually leads to integrals that require numerical solution. These information allow for extrapolation to the configuration of the HL-LHC [9], highlighting the need for technological improvements of the transverse feedback BPM

NUMERICAL MODEL

COMPARISON TO ANALYTICAL MODELS

EXPERIMENTAL STUDIES

EXTRAPOLATION TO THE HL-LHC

Findings

CONCLUSION