Higher-order modes of storage ring RF cavities and their interaction with the beam at the Advanced Photon Source (APS)

The higher-order modes (HOMs) of APS storage ring (SR) RF cavities and waveguides were measured under various operating conditions. The HOMs of the 352-MHz RF cavity can be one of the major contributors to the coupled bunch instability. The distribution of HOMs under various conditions of beam current, cavity temperature, cavity tuning, single-bunch and multi-bunch operation, and fill patterns, are presented. The HOMs' shunt impedance of the loaded cavities were also measured. The effect of stagger tuning of the 16 cavities and their waveguide system is compared, and the HOM dampers are examined.