High Frequency Effects of Impedances and Coatings in the CLIC Damping Rings

The Compact Linear Collider (CLIC) is a 3 TeV e+e- machine, currently under design at CERN, that targets to explore the terascale particle physics regime. The experiment requires a high luminosity of 2x10^34 cm^2 s^-1, which can be achieved with ultra low emittances delivered from the Damping Rings (DRs) complex. The high bunch brightness of the DRs gives rise to several collective effects that can limit the machine performance. Impedance studies during the design stage of the DR are of great importance to ensure safe operation under nominal parameters. As a first step, the transverse impedance model of the DR is built, accounting for the whole machine. Beam dynamics simulations are performed with HEADTAIL to investigate the effect on beam dynamics. For the correct impedance modeling of the machine elements, knowledge of the material properties is essential up to hundreds of GHz, where the bunch spectrum extends. Specifically, Non Evaporable Getter (NEG) is a commonly used coating for good vacuum but its properties up to high frequencies were still widely unexplored. A new method using rectangular waveguides is proposed, benchmarked and applied for the first time to characterize NEG up to hundreds of GHz. The numerical tools used for the DR studies are applied and benchmarked with measurements in other light sources. In particular, single bunch measurements were performed in the ALBA light source and compared to the model prediction using HEADTAIL. The impedance budget of ALBA was estimated before and after the installation of a pinger magnet. Furthermore, studies were also carried out for the Swiss Light Source (SLS) upgrade to investigate the machine performance in terms of single bunch instabilities for lattices with negative momentum compaction factor.

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