Terahertz scale microbunching instability driven by high resistivity nonevaporable getter coating resistive-wall impedance

Abstract

Nonevaporable getter (NEG) coating is widely required in the new generation of light sources and circular $e^{+}{e}^{-}$ colliders for small vacuum pipes to improve the vacuum level, which, however, also enhances the high-frequency resistive-wall impedance and often generates a resonator-like peak in the terahertz frequency region. In this paper, we will use the parameters of the planned Hefei Advanced Light Facility storage ring to study the impact of NEG-coating resistive-wall impedance on the longitudinal microwave instability via particle tracking simulation. Using different NEG-coating parameters (resistivity and thickness) as examples, we find that the impedance with a narrow and strong peak in the terahertz frequency region can cause terahertz scale microbunching instability, which has a low instability threshold current and contributes to a large energy spread widening above the threshold. In order to obtain a convergent simulation of the beam dynamics, one must properly resolve such a peak. The coating with a lower resistivity has a less sharp peak in its impedance spectrum, and there is a regime that it is helpful to suppress the terahertz scale microbunching instability and in return contributes to a higher instability threshold current.