Design and Optimization of the High Order Modes Damper for a 1.5 GHz Superconducting Harmonic Cavity

The installation of a superconducting radio-frequency (SRF) cavity can improve the beam lifetime via increasing the electron bunch length. A prototype 1.5-GHz SRF passive harmonic cavity working at 4.5 K was designed for The Taiwan Photon Source (TPS). As the beam might become unstable because of the coupled-bunch instability resulting from Higher Order Modes (HOMs) of the SRF harmonic cavity, HOMs damping of this cavity is an important issue. Ferrite absorbers placed in the beam tubes are designed to damp the HOMs. A theoretical calculation for the instability threshold for the HOMs is given. A tracking method is used to simulate the instabilities for operation with this HOMs-damped SRF harmonic cavity. The design and optimization of the HOMs damper, as well as the results of instability studies for this SRF harmonics cavity, are presented here.