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

TL;DR

This paper investigates how non-evaporable getter coating resistive-wall impedance influences terahertz microbunching instability in light source storage rings, revealing that impedance peaks can trigger instability and that lower resistivity coatings mitigate this effect.

Contribution

It provides a detailed analysis of the impact of NEG coating parameters on microbunching instability using particle tracking simulations, highlighting the importance of impedance peak resolution.

Findings

Narrow, high peak impedance causes micro-bunching instability.

Lower resistivity coatings reduce impedance peaks and suppress instability.

Proper peak resolution is essential for accurate beam dynamics simulation.

Abstract

Non-evaporable getter (NEG) coating is widely required in the next 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 (HALF) 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 high frequency region can cause micro-bunching 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 much less sharp peak in its impedance spectrum, which is helpful to suppress the micro-bunching instability and in return contributes to a weaker microwave instability.