Measurements of electron cloud density in the CERN Super Proton Synchrotron with the microwave transmission method

S Federmann,F Caspers,E Mahner

doi:10.1103/physrevstab.14.012802

Abstract

The electron cloud effect can pose severe performance limitations in high-energy particle accelerators as the CERN Super Proton Synchrotron (SPS). Mitigation techniques such as vacuum chamber thin film coatings with low secondary electron yields ($\mathrm{SEY}<1.3$) aim to reduce or even suppress this effect. The microwave transmission method, developed and first applied in 2003 at the SPS, measures the integrated electron cloud density over a long section of an accelerator. This paper summarizes the theory and measurement principle and describes the new SPS microwave transmission setup used to study the electron cloud mitigation of amorphous carbon coated SPS dipole vacuum chambers. Comparative results of carbon coated and bare stainless steel dipole vacuum chambers are given for the beam with nominal LHC 25 ns bunch-to-bunch spacing in the SPS and the electron cloud density is derived.

Highlights

Inside the vacuum chamber of high-energy particle accelerators a small number of electrons are always present and unavoidable
The presence of electrons can be caused, for example, by ionization of residual gas molecules, by desorption of lost beam particles, and by photoemission due to synchrotron radiation. These electrons, once accelerated by the beam potential, impact on the vacuum chamber wall and create secondary electrons, which can lead to an avalanche creating a so-called electron cloud (EC). This EC effect can be detrimental for the performance of a particle accelerator, leading to significantly fast and high pressure rises, beam instabilities such as transverse blowup and particle losses resulting in a limitation of the beam intensity, and a reduction of beam quality
This paper describes the application on the microwave transmission method to study the physics of electron clouds in the Super Proton Synchrotron (SPS), and the EC suppression potential of carbon coated main dipole chambers, tested in 2009

Summary

Introduction

Inside the vacuum chamber of high-energy particle accelerators a small number of electrons are always present and unavoidable. The presence of electrons can be caused, for example, by ionization of residual gas molecules, by desorption of lost beam particles, and by photoemission due to synchrotron radiation These electrons, once accelerated by the beam potential, impact on the vacuum chamber wall and create secondary electrons, which can lead to an avalanche creating a so-called electron cloud (EC). This EC effect can be detrimental for the performance of a particle accelerator, leading to significantly fast and high pressure rises, beam instabilities such as transverse blowup and particle losses resulting in a limitation of the beam intensity, and a reduction of beam quality. Possible mitigation techniques have been explored in the past, comprising grooved vacuum chambers, clearing electrodes, rough surfaces, and coatings such as amorphous carbon (a–C) and/or TiZrV

Methods

Results

Conclusion