Trapped modes in the PEP-II B-factory interaction region

The design of the PEP-II B-factory interaction region is based primarily on beam-stay-clear requirements and on synchrotron radiation background considerations (masks are required to shield the detector beam pipe from all sources of synchrotron radiation). A complicated 3-dimensional structure results from these requirements. A high intensity beam traversing this structure will generate wake fields that lead to energy deposition on the beam pipe, as well as to decelerating and deflecting forces acting back on the beam. Computation of wake fields and impedances in frequency-domain and time-domain using 2-D and 3-D electromagnetic codes revealed the existence of trapped modes in the interaction region, which if not controlled could enhance the higher order mode heating of the beam pipe. We will present the simulation results and the design strategy to avoid resonant conditions between these trapped modes and the bunch train frequency.