Simulation of Multipacting with Space Charge Effect

Gennady Romanov

doi:10.11648/j.ajpa.20170506.15

Abstract

The electron multiplication on surfaces exposed to an oscillating electromagnetic field causes the phenomenon of multipacting, which can degrade significantly the performance of vacuum RF devices, especially accelerating cavities. It is a serious obstacle to be avoided for normal operation of particle accelerator and their RF components. Many types of room temperature and superconducting accelerating cavities are designed and produced at Fermilab for different projects. The extensive simulations of multipacting in the cavities with updated material properties and comparison of the simulation results with experimental data are routinely performed during electromagnetic design of the cavities. The new advanced computing capabilities made it possible to take the space charge effect into account in the multipacting simulations. The basic new features of multipacting process that appear due to the space charge effect are shown for the classic case of the parallel plates and discussed. As the first practical application of the multipacting simulations with space charge effect the study of multipacting in the low-beta and high-beta 650 MHz elliptical superconducting cavities is also presented.

Highlights

Multipacting can affect practically all accelerating RF cavities and their components in the entire range energies and frequencies
Control over the multipacting phenomena is important for this project, and as a part of overall RF design we routinely perform the extensive simulations of multipacting (MP) in each SC and room temperature (RT) cavity and other RF components under development
Such MP dynamics is representative for the initial stages of multipacting development, and the multipacting thresholds predicted by the models without space charge effect usually are in a reasonably good agreement with the experiments

Summary

Introduction

Multipacting can affect practically all accelerating RF cavities and their components in the entire range energies and frequencies. The new advanced GPU acceleration for Particle-In-Cell calculations made it possible to take space charge effect into account in this study It is shown in [4, 5, 6, 7] that the space charge effect plays a prominent part in the secondary electron resonance discharge, i.e. multipacting. In the elementary theory of multipacting and in the most MP simulation codes the space charge effect is neglected, which results in infinite growth of electron number in the calculations or in the simulations (a growth is typically exponential, but not always) Such MP dynamics is representative for the initial stages of multipacting development, and the multipacting thresholds predicted by the models without space charge effect usually are in a reasonably good agreement with the experiments. As a practical application of the multipacting simulations with space charge effect the study of multipacting in the PIP-II low-beta (LB, β=0.6) and high-beta (HB, β=0.9) 650 MHz elliptical superconducting cavities were performed

Multipacting Between Parallel Plates

Multipacting of First Order

Multipacting in PIP-II 650 MHz Cavities

Conclusion