Beam Stability Analysis of the PLS-II Storage Ring for Taper Shape Change of the Superconducting RF Cavities

The higher-order-mode (HOM)-induced power in a RF cavity could cause beam instabilities in the longitudinal and the transverse directions if it cannot be damped sufficiently. In order to damp the HOM-induced power, the superconducting RF cavities of the light source storage rings employ two coaxial ferrite HOM dampers, which are located on both ends of the cavity connecting to the warm tapers. A taper shape change can lead to variations in the HOM power absorbability on ferrite HOM absorbers and cause the HOM damping effects to change. Two commercial cryomodules will be installed in a long-straight section of the Pohang Light Source (PLS)-II storage ring. The available length of the long-straight section is not enough to install two commercial cryomodules. The most effective way to install the two cryomodules in the section is to change the taper shapes to reduce the total length of the cryomodules. The main concern for this change is the effects on the beam instability. In this paper, the HOM damping effect and the loss factor changes for different taper shapes have been analyzed. Based on the analysis results, we present the optimized modification choice. KCI Citation Count: 1