Elettra | Machine Coherent synchrotron radiation instability in low-emittance electron storage rings Modern low-emittance electron storage rings aim to produce extremely bright x-ray beams for advanced scientific research. However, as beam emittances shrink and bunch lengths shorten, previously negligible effects become crucial. One of the most significant among them is coherent synchrotron radiation (CSR), which can cause beam instabilities and limit the achievable performance of these next-generation light sources. CSR is emitted when relativistic electrons travel through curved paths in bending magnets. While synchrotron radiation is a well-known and desirable phenomenon in light sources, its coherent component, emitted collectively by electrons in short bunches, can disturb the beam’s stability. In low-emittance rings, where the electron bunches are both compact and intense, CSR becomes a dominant factor in defining the beam’s behavior. Our study, grounded in theory and simulation, investigates CSR-driven microwave instability (MWI) in two representative accelerators: Elettra 2.0, a forthcoming fourth-generation light source, and SuperKEKB, an existing high-luminosity collider. We analyzed how CSR interacts with other highfrequency effects, such as resistive wall (RW) impedance, and developed scaling laws that link the onset of instability to machine parameters like bunch length, energy spread, and vacuum chamber geometry. At Elettra 2.0, we found that in the absence of a third harmonic cavity (3HC)—used to lengthen the bunch and flatten the charge profile—the CSR instability threshold is very low, around 0.7 mA. This means that even modest beam currents could trigger unwanted effects. With the 3HC included, the threshold increases significantly, confirming its essential role in suppressing CSR-driven instabilities. Our approach combines instability theory, numerical simulations, and detailed impedance modeling using codes like CSRZ, which calculates CSR effects in realistic vacuum chamber geometries. We also introduced practical tools for predicting when and how CSR will affect a given machine, including a new scaling law showing that the instability threshold depends primarily on the ratio between bunch density and vacuum chamber height. In parallel, we examined the effect of RW and broadband resonator (BBR) impedances, finding that CSR dominates at high frequencies, while RW effects contribute more at lower currents and longer wavelengths. This understanding helps separate different instability sources and design mitigation strategies accordingly. The findings are not only theoretical. Our simulations closely match previous experimental results from machines like the Advanced Light Source and predict observable CSR effects at SuperKEKB, reinforcing the model’s predictive power. Beyond individual machines, our results are relevant to any future light source or collider operating with ultra-low emittance beams. As these facilities push the limits of beam brightness and stability, understanding and managing CSR becomes a critical design and operational challenge. This work serves as a practical guide to identifying and avoiding CSR-induced limitations, offering both a theoretical framework and computational tools. For designers and operators of nextgeneration storage rings, it highlights the importance of balancing brightness with beam stability—and ensures that the brilliance of the light source doesn’t come at the cost of its reliability. Elettra Highlights 2024–2025 140
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