Modeling ion effects for the Argonne Advanced Photon Source upgrade

Abstract

Ions are produced in an accelerator when the beam ionizes residual gas inside the vacuum chamber. If the beam is negatively charged, ions can become trapped in the beams potential, and their density will increase over time. Trapped ions can cause a variety of undesirable effects, including instability and emittance growth. This paper will discuss the possibility of ion trapping and instability in the APS-Upgrade storage ring. The question of where ions will be trapped is addressed using simple analytical calculations, while the instability is modeled with computer simulations. A scheme is proposed for mitigating the ion instability, by using train gaps with a high charge bunch before and after the gap to minimize rf transients. The reduction in ion density due to nonevaporable getter coating is also discussed.

APS Science 2019 Volume 2: Research and Engineering Highlights from the Advanced Photon Source at Argonne National Laboratory

“…The electron beam can also ionize gas atoms, leading to a potentially unstable collective interaction with an ion cloud, particularly for 324-bunch operation. Modeling [10] has allowed developing countermeasures, namely, introducing a number of symmetric gaps into the 324-bunch train. These countermeasures were tested in experiments in the APS that involved deliberate injection of gas into the vacuum chamber.…”

Advanced Photon Source Upgrade Project Final Design Report

“…To reduce this effect, the "guard bunch" scheme [84] was introduced. In this scheme, the ion-clearing gaps are bracketed by a series higher-charge bunches that contain the charge removed from the gap.…”