Performance of a new coaxial higher-order-mode damper

A new type of rf damper, designed to suppress higher-order modes which may excite beam instabilities, was built and tested on a ferrite-tuned coaxial cavity proposed for use in the KAON Factory at TRIUMF The damper consisted of a series of rod and disc structures arranged at the open end of the cavity around the beam axis to form the capacitances and inductances of a multi-element Chebyshev high-pass filter. The damper was tested on the cavity and found to significantly damp all higher-order TEM modes with minimal effect at the fundamental frequency. A second version of the damper was also built and attached to a simple coaxial lambda/4 cavity on which cavity perturbation bead-pull measurements were made. These showed that the damper significantly reduced the shunt impedance of TEM higher-order-modes while leaving the shunt impedance of the system at the fundamental frequency intact.