Maps for electron cloud density in Large Hadron Collider dipoles

T Demma,F Zimmermann,G Rumolo,F Ruggiero,S Petracca

doi:10.1103/physrevstab.10.114401

Abstract

The generation of a quasistationary electron cloud inside the beam pipe through beam-induced multipacting processes has become an area of intensive study. The analyses performed so far have been based on heavy computer simulations taking into account photoelectron production, secondary emission, electron dynamics, and space charge effects, providing a detailed description of the electroncloud evolution. Iriso and Peggs [U. Iriso and S. Peggs, Phys. Rev. ST Accel. Beams 8, 024403 (2005)] have shown that, for the typical parameters of RHIC, the bunch-to-bunch evolution of the average electron-cloud density at a point can be represented by a cubic map. Simulations based on this map formalism are orders of magnitude faster compared to those based on standard particle tracking codes. In this communication we show that the map formalism is also applicable to the case of the Large Hadron Collider (LHC), and that, in particular, it reproduces the average electron-cloud densities computed using a reference code to within 15% for general LHC bunch filling patterns. We also illustrate the dependence of the polynomial map coefficients on the physical parameters affecting the electron cloud (secondary emission yield, bunch charge, bunch spacing, etc.).

Highlights

Photoemission and/or ionization of the residual gas in the beam pipe produces electrons, which move under the action of the beam field and their own space charge
In this paper we show that a simple cubic map can be effective in modeling the bunch-to-bunch evolution of the electron-cloud density in the Large Hadron Collider (LHC) arc dipoles, and for identifying the bunch filling patterns for which the e-cloud saturation density does not exceed some critical threshold
As an illustration of this important property, we use the map coefficients corresponding to the reference filling pattern of LHC (72 charged bunches) to predict the e-cloud density evolution for different filling patterns, and compare the result to those obtained from ECLOUD

Summary

INTRODUCTION

Photoemission and/or ionization of the residual gas in the beam pipe produces electrons, which move under the action of the beam field and their own space charge. Averaged over the time interval between successive bunch passages, could be accurately described by a simple (cubic) map. Such an approach is useful because the map is computable, and allows a quick scan of some key design parameters, such as the bunch filling pattern, while effectively boiling down the detailed description of the underlying physics, including the secondary emission yield ( SEY), the quantum efficiency, etc., into a few effective parameters.

THE CUBIC MAPS

FILLING PATTERNS

Findings

CONCLUSIONS AND OUTLOOK