Microwave instability driven by terahertz-scale resistive-wall impedance in electron synchrotrons

Vacuum vessels of many existing and planned synchrotron light sources feature metallic coatings. In some combinations, they might cause a resonator-like peak in the longitudinal impedance. In this simulation study, we demonstrate how different parameters of non-evaporable getter (NEG) can increase momentum-spread growth and, hence, reduce microwave-instability thresholds. The first key insight of this study: The spread of the coating-layer thickness amongst vacuum vessels results in significantly reduced momentum-spread growth. Therefore, we recommend including randomised coating when designing vacuum components and modelling the impedance for a synchrotron light source. The second key insight of this study: Insertion devices featuring rectangular geometry and NEG coating might cause a multi-peak structure of the longitudinal impedance which can drive additional momentum-spread growth.