Analytic modeling of instabilities driven by higher-order modes in HLS II RF system with a higher-harmonic cavity

TL;DR

This paper presents an analytical model demonstrating that a passive fourth-harmonic cavity effectively suppresses certain beam instabilities in the HLS II electron storage ring, supporting its use for improved performance.

Contribution

The paper develops an analytical model for higher-order mode driven instabilities in a fourth-harmonic cavity, confirming its stabilizing effects and aligning with previous simulation results.

Findings

Passive fourth-harmonic cavity suppresses parasitic coupled-bunch instability.

Cavity effectively mitigates microwave instability.

Model supports successful application at HLS II.

Abstract

The utility of a passive fourth-harmonic cavity plays key role in suppressing longitudinal beam instabilities in the electron storage ring and lengthens the bunch by a factor of 2.6 for the phase II project of Hefei Light Source(HLS II). Meanwhile, instabilities driven by higher-order modes(HOM) may limit the performance of the higher-harmonic cavity. In this paper, the parasitic coupled-bunch instability which is driven by narrow band parasitic modes and the microwave instability which is driven by broadband HOM are both modeled analytically. The analytic modeling results are in good agreement with that of our previous simulation study and indicate that the passive fourth-harmonic cavity suppresses parasitic coupled-bunch instabilities and the microwave instability. The modeling suggests that a fourth-harmonic cavity may be successfully used at HLS II.