Impact of short-range wakefields from radio-frequency cavity resonant modes on bunch lengthening

Abstract: The high-performance operation of fourth-generation synchrotron light sources critically depends on harmonic cavities (HCs) to alleviate statistical collective effects through bunch lengthening. Active HCs are preferred over passive ones for achieving theoretically optimum bunch lengthening in Timing-mode operation, which is characterized by low average current and high bunch charge. This advantage stems from their ability to control cavity voltages via generator current. However, this study reveals a previously overlooked limitation: the detrimental impact of short-range wakefields from RF cavity resonant modes on bunch lengthening at high bunch charge. Using Hefei Advanced Light Facility parameters, we demonstrate that these wakefields can significantly degrade bunch lengthening. Further analysis with PETRA-IV parameters reveals that fine-tuning the HC voltage can optimize bunch lengthening. Notably, under specific HC settings, there exists two equilibrium bunch distributions. Our findings highlight the critical influence of RF cavity short-range wakefields on beam dynamics at high bunch charge, emphasizing their essential inclusion in the evaluation and optimization of HC performance for new-generation synchrotron light sources.