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Beam instability induced by the rf cavity accelerating modes and its suppression in the Super Tau-Charm Facility

Wenshu Liang1, Tianlong He1,*, Weiwei Li1,†, Yelong Wei1,‡, Ye Zou2, Qing Luo2, and Jingyu Tang1,2

  • *Contact author: htlong@ustc.edu.cn
  • †Contact author: liwe@ustc.edu.cn
  • ‡Contact author: wylong@ustc.edu.cn
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Phys. Rev. Accel. Beams 29, 074902 – Published 30 July, 2026

DOI: https://doi.org/10.1103/v8mq-6nh6

Abstract

The Super Tau-Charm Facility (STCF), a proposed high-luminosity electron-positron collider in China, requires a beam current of 2 A to achieve its design luminosity. Such intense current induces severe coupled-bunch instability driven by the rf cavity accelerating modes. This paper presents a systematic analysis of these instabilities across the STCF operational energy range of 1–3.5 GeV. Without active control, the low-order coupled-bunch modes, particularly the mode −1, exhibit growth rates significantly exceeding the radiation-damping rate. We demonstrate that the instability can be effectively suppressed using only the proportional-integral feedback loop of the baseline low-level rf system. The key lies in setting a unit proportional gain and a loop delay of approximately half the revolution period. This configuration creates a notch-filter response that attenuates the most dangerous mode −1. By combining analytical calculations with particle-tracking simulations, we show that this simple and robust approach maintains beam stability for the baseline design parameters across all design energies, thereby enabling reliable high-current operation without requiring additional dedicated feedback measures, such as direct or one-turn-delay feedback. A stability assessment based on the Nyquist criterion confirms the closed-loop stability of the system and indicates sufficient gain margins. These results provide a reliable foundation for the STCF rf and feedback system design, enabling stable high-current operation under realistic constraints. The proposed approach and the associated analysis framework also offer a practical reference for other high-current storage rings facing similar instability challenges.

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Phys. Rev. Accel. Beams 29, 074902– Published 30 July, 2026
  • Received 21 February 2026
  • Accepted 15 June 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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