Analysis of the longitudinal low-order mode beam dynamics in PEP-II rings at high current beams

PEP-II plans to achieve the final goal in luminosity will require an increase of the beam currents to 4 A for LER and 2.2 A for HER. These magnitudes are challenging in part because they will push the longitudinal low-order mode (LOM) beam stability and the station stability to the limit. To analyze the behavior of both rings at high currents and to understand the limits in the longitudinal feedback systems, a simulation tool has been developed at SLAC. This tool is based on a reduced model of the longitudinal LOM dynamics of the beam interacting with the effective impedance presented by the RF stations. Simulations and measurements of the longitudinal beam behavior in both rings have been performed to understand the ultimate limits of the systems. These studies have defined the impact of control loop parameters in the longitudinal beam dynamics, identified the limiting behavior of RF devices affecting the optimal performance of the RF stations and quantified the behavior of the longitudinal LOM beam dynamics. Results of sensitivity to parameter variations in the beam dynamics and limits in the maximum current that LER/HER can achieve based on the longitudinal beam stability are reported in this paper.