Electron Cloud Trapping In Recycler Combined Function Dipole Magnets

TL;DR

This paper investigates how combined function dipole magnets in the Fermilab Recycler trap electron clouds, leading to significant multi-turn accumulation that can cause beam instability, and proposes a clearing method to mitigate this.

Contribution

It provides analytical estimates and numerical simulations showing electron cloud trapping in combined function magnets and demonstrates a clearing bunch method to prevent build-up.

Findings

Electron clouds are trapped at the beam center in combined function magnets.

Up to 1% of particles can be trapped, causing exponential growth.

A clearing bunch can effectively stop the electron cloud build-up.

Abstract

Electron cloud can lead to a fast instability in intense proton and positron beams in circular accelerators. In the Fermilab Recycler the electron cloud is confined within its combined function magnets. We show that the field of combined function magnets traps the electron cloud, present the results of analytical estimates of trapping, and compare them to numerical simulations of electron cloud formation. The electron cloud is located at the beam center and up to 1% of the particles can be trapped by the magnetic field. Since the process of electron cloud build-up is exponential, once trapped this amount of electrons significantly increases the density of the cloud on the next revolution. In a Recycler combined function dipole this multi-turn accumulation allows the electron cloud reaching final intensities orders of magnitude greater than in a pure dipole. The multi-turn build-up can be stopped by injection of a clearing bunch of $10^{10}$ p at any position in the ring.