Study on the Instability of TPS With Superconducting Harmonic Cavity Under High Beam Current Operation

The Taiwan Photon Source (TPS) is a third-generation, 3 GeV low-emittance light source situated at the NSRRC in Taiwan. The TPS has a bunch length of approximately 4.5 mm at routine operation with a beam current of 500 mA. This short bunch length leads to a brief Touschek lifetime and high parasitic loss in in-vacuum insertion devices. To address this issue, a 1.5 GHz two-cell superconducting RF (SRF) passive harmonic cavity (HC) operating at 4.5 K has been designed for the TPS. The anticipated bunch lengthening factors, accounting for phase transient effects, range from 1.5 to 2.3 for different bunches. Beyond performance enhancement, the study of the instability of the electron beam is also an important task. This work investigates the instability during high beam current operation with the SRF HC. This paper delves into the details of theoretical calculations, the methodology of multi-bunch multi-particle tracking simulation, and presents the results of the instability study.