• R.M. Lill, G. Decker, J.E. Hoyt, X. Sun
  ANL, Argonne

The Advanced Photon Source (APS) storage ring rf beam position monitors (BPMs) are impacted by the presence of beam-excited transverse electric (TE) modes. These modes are excited in large-aperture vacuum chambers and become trapped between the bellow end flanges. The TE modes are vertically oriented and are superimposed on the TEM beam position signals, corrupting the BPM measurements. Erroneous step changes in beam position measurements and systematic intensity dependence in the vertical plane have been traced to these modes, placing a fundamental limitation on vertical beam position stabilization. Experiments were conducted suppressing these modes on a test vacuum chamber. These experiments were simulated with MAFIA and Microwave Studio, confirming experimental results. We will describe the measurements, simulations, and prototype test results.