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.
Highlights
The APS-Upgrade is a 4th-generation light source currently under development at Argonne National Laboratory [1]
IV, we propose using high charge “guard bunches” before and after the train gaps, to minimize rf transients and provide a stronger kick for clearing out ions
Ion trapping is expected for 324 bunch mode in the APSUpgrade storage ring, and is likely to result in a vertical instability
Summary
The APS-Upgrade is a 4th-generation light source currently under development at Argonne National Laboratory [1]. The hybrid MBA cell is symmetric and features two dispersion bumps, within each of which are placed three sextupoles. As indicated, these are referred to as the “multiplet” sections, since they contain quadrupoles, sextupoles, skew quadrupoles, and octupoles. These are referred to as the “multiplet” sections, since they contain quadrupoles, sextupoles, skew quadrupoles, and octupoles These sections have larger-than-average beta functions, which has implications for ion trapping, as will be seen below. For the APS-U storage ring, the simulations indicate that train gaps are required to prevent the instability. IV, we propose using high charge “guard bunches” before and after the train gaps, to minimize rf transients and provide a stronger kick for clearing out ions
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