Abstract

During the beam commissioning of the Large Hadron Collider (LHC) [LHC Design Report No. CERN-2004-003-V-1, 2004 [http://cds.cern.ch/record/782076?ln=en]; O. Bruning, H. Burkhardt, and S. Myers, Prog. Part. Nucl. Phys. 67, 705 (2012)] with 150, 75, 50, and 25-ns bunch spacing, important electron-cloud effects, like pressure rise, cryogenic heat load, beam instabilities, or emittance growth, were observed. Methods have been developed to infer different key beam-pipe surface parameters by benchmarking simulations and pressure rise as well as heat-load observations. These methods allow us to monitor the scrubbing process, i.e., the reduction of the secondary emission yield as a function of time, in order to decide on the most appropriate strategies for machine operation. To better understand the influence of electron clouds on the beam dynamics, simulations have been carried out to examine both the coherent and the incoherent effects on the beam. In this paper we present the methodology and first results for the scrubbing monitoring process at the LHC. We also review simulated instability thresholds and tune footprints for beams of different emittance, interacting with an electron cloud in field-free or dipole regions.

Highlights

  • For almost 15 years photoemission and secondary emission have been predicted to build up an electron cloud inside the Large Hadron Collider (LHC) beam pipe [1], similar to the photoelectron instability in positron storage rings [2,3,4]

  • Various types of electron-cloud effects have been observed in the LHC for bunch spacings of 150 ns or below

  • The effects get much stronger for closer distances between bunches, but fast surface conditioning in 2010 and 2011 has made it possible to operate, in 2012, at a bunch spacing as small as 50 ns without any significant perturbation from electron cloud

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Summary

INTRODUCTION

For almost 15 years photoemission and secondary emission have been predicted to build up an electron cloud inside the LHC beam pipe [1], similar to the photoelectron instability in positron storage rings [2,3,4]. Since dedicated in situ measurements of the LHC electron cloud density and the LHC vacuum-chamber surface properties are not available, we are developing a method to determine the actual surface properties of the vacuum chamber related to secondary emission and to the electron-cloud buildup (max, "max, and R [16]; see Fig. 1 for a graphical definition of these three quantities), and their evolution in time, based on benchmarking computer simulations of the electron flux on the chamber surface using the ECLOUD code against pressure measurements for different beam characteristics (e.g. for varying spacing between bunch trains). The simulations were performed with the code HEADTAIL [21,22]

PRESSURE BENCHMARKING FOR THE LHC WARM SECTIONS
Methodology
Results
HEAT-LOAD BENCHMARKING FOR THE COLD ARCS
EMITTANCE GROWTH
Numerical model
Coherent emittance growth
Incoherent effects
LHC CONDITIONING STATUS AND GOALS
CONCLUSIONS

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