Coherent synchrotron radiation instability in low-emittance electron storage rings

TL;DR

This paper investigates how coherent synchrotron radiation (CSR) and other high-frequency impedances affect the microwave stability of low-emittance electron storage rings, using simulations on Elettra 2.0 and SuperKEKB rings.

Contribution

It extends existing theories of CSR-driven microwave instability to include interactions with broadband impedances and provides detailed numerical analysis for specific modern light source and collider rings.

Findings

CSR significantly influences microwave instability thresholds.

High-frequency impedances interact with broadband impedances, affecting stability.

Numerical simulations confirm theoretical predictions.

Abstract

Longitudinal impedances at high frequencies, which extend far beyond the width of the beam spectrum, can pose a threat to the performance of modern low-emittance electron storage rings, as they can establish a relatively low threshold for microwave instability. In such rings, coherent synchrotron radiation (CSR) emerges as a prominent contributor to these high-frequency impedances. This paper undertakes a systematic investigation into the effects of CSR on electron rings, utilizing Elettra 2.0, a ring of fourth-generation light sources, and the SuperKEKB low-energy ring, a ring of $e^+e^-$ circular colliders, as illustrative examples. Our work revisits theories of microwave instability driven by CSR impedance, extending the analysis to encompass other high-frequency impedances such as resistive wall and coherent wiggler radiation. Through instability analysis and numerical simulations conducted on the two aforementioned rings, the study explored the impact of high-frequency impedances and their interactions with broadband impedances from discontinuities in vacuum chambers.