DESIGN, INSTALLATION, AND COMMISSIONING RESULTS FOR SUPPRESSING ROGUE TE MODES IMPACTING VERTICAL BPM READINGS OF THE APS STORAGE RING*

The Advanced Photon Source (APS) storage ring beam position monitor (BPM) system presently is impacted by transverse electric (TE) longitudinal resonances trapped in the beam vacuum chamber. These TE-like modes are excited in the large-aperture sections and become trapped between the bellows end flanges. Ideally the chamber would be in cut-off for frequencies well above the BPM operating frequency of 352 MHz, but due to the geometry of the chamber the cut-off frequency is nominally 330 MHz. The modes are vertically oriented and are superimposed on the beam position signals resulting in erroneous step changes in beam position measurements and systematic intensity dependence in the vertical plane. This problem places a fundamental limitation on vertical beam position monitor performance. In this paper we will discuss the mode suppression design, simulation, installation, and commissioning results of the first three sectors installed in the APS storage ring.