Ion Instabilities in Low Emittance Storage Rings

Ion trapping occurs when a negatively charged beam ionizes residual gas inside an accelerator vacuum chamber, and the resulting ions become trapped in the beam's potential. These ions can cause a variety of undesirable effects, including instability and emittance growth. Ion trapping is of particular concern for next-generation electron storage rings, such as the upcoming Advanced Photon Source Upgrade (APS-U), because of their challenging emittance and stability requirements. Ion instability has been studied experimentally in the present APS using a gas injection system. This system was designed to create a controlled pressure bump of Nitrogen gas, so that the resulting instability could be studied in detail. In particular, the growth of individual unstable modes has been measured using a bunch-by-bunch feedback system. In addition, the particle tracking code ELEGANT was enhanced to permit modeling both the coherent and incoherent aspects of the ion instability. This code has been used to better understand the results of the gas injection experiments, and to develop a plan for mitigating the ion instability in the APS-U.