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Threshold studies of coherent synchrotron radiation induced microwave instability beyond adiabatic approximation

J. H. Bian1, X. J. Deng2,*, W. H. Huang1,†, and C. X. Tang1

  • *Contact author: dengxiujie@mail.tsinghua.edu.cn
  • †Contact author: huangwh@mail.tsinghua.edu.cn
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Phys. Rev. Accel. Beams 28, 074401 – Published 15 July, 2025

DOI: https://doi.org/10.1103/jp4b-1mhy

Abstract

In typical studies of collective effects in a storage ring, an important assumption is that the longitudinal bunch length remains constant over one turn, which is referred to as the adiabatic approximation. This approximation is reasonable for conventional storage rings. However, for extremely short bunch length storage rings, such as steady-state microbunching storage ring, the global phase slippage is very small, and the local or partial phase slippage can significantly influence the longitudinal dynamics of the bunch. Consequently, the bunch length can vary greatly around the ring, and the adiabatic approximation no longer holds. In this work, we investigate the microwave instability caused by the coherent synchrotron radiation, which represents one of the dominant collective effects in short bunch storage rings, beyond the adiabatic approximation. We present a general formula for the instability threshold considering the variation of longitudinal optics around the ring and offer valuable guidance for the design of extremely short bunch storage rings. Our results show that depending on the distribution of the partial phase slippage, the instability threshold of a real lattice can be increased by a factor of 2 or even more compared to the classical prediction.

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Phys. Rev. Accel. Beams 28, 074401– Published 15 July, 2025
  • Received 20 January 2025
  • Accepted 20 June 2025
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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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