RF Feedback Simulation Results for PEP-II*

A model of the RF feedback system for PEP-II has been developed to provide time-domain simulation and frequency-domain analysis of the complete system. The model includes the longitudinal beam dynamics, cavity fundamental resonance, feedback loops, and the nonlinear klystron operating near saturation. Transients from an ion clearing gap and a reference phase modulation from the longitudinal feedback system are also studied. Growth rates are predicted and overall system stability examined. I. Model Description The development of a model of the PEP-II RF feedback system and longitudinal dynamics has been described previously[1]. The model has been in use predicting the overall stability of the High Energy Ring (HER) and the Low Energy Ring (LER) RF systems under heavy beam loading[2]. Feedback loops are included, as are the klystron, cavity, and longitudinal dynamics of 36 rigid macro bunches. Three feedback loops are required to stabilize the coupled-bunch oscillations that arise from the detuned cavity fundamental. A direct RF feedback loop, comb filter feedback, and a connection from the bunch-by-bunch longitudinal feedback are required. Signal from the longitudinal feedback is fed to the reference phase shifter to provide additional damping of the low order coupled-bunch modes[3]. Digital components of the feedback system are simulated using the actual system sample rates. Signal propagation delays in the system are also included. Figure 1 is a block diagram of the model. Adaptive Feedforward Reference Longitudinal Feedback Phase Shifter Cavity Equalizer + One-turn Delay PEP-II RF Feedback System Comb Filter Feedback