IMPEDANCE EFFECTS IN THE CLIC DAMPING RINGS

Due to the unprecedented brilliance of the beams, the performance of the Compact Linear Collider (CLIC) damping rings (DR) is affected by collective effects. Single bunch instability thresholds based on a broad-band resonator model and the associated coherent tune shifts have been evaluated with the HEADTAIL code. Simulations performed for positive and negative values of chromaticity showed that higher order bunch modes can be potentially dangerous for the beam stability. This study also includes the effects of high frequency resistive wall impedance due to different coatings applied on the chambers of the wigglers for e-cloud mitigation and/or ultra-low vacuum pressure. The impact of the resistive wall wake fields on the transverse impedance budget is finally discussed.

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Eirini Koukovini Platia, Nicolas Frank Mounet, Giovanni Rumolo, Benoît Salvant
2011
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https://infoscience.epfl.ch/record/170542?ln=fr

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Electromagnetic characterization of nonevaporable getter properties between 220-330 and 500-750 GHz for the Compact Linear Collider damping rings

Eirini Koukovini Platia, Giovanni Rumolo, Carlo Zannini

Due to its effective pumping ability, nonevaporable getter (NEG) coating is considered for the vacuum chambers of the Compact Linear Collider (CLIC) electron damping rings (EDR). The aim is to suppress fast beam ion instabilities. The electromagnetic (EM) characterization of the NEG properties up to ultra-high frequencies is required for the correct impedance modeling of the damping ring (DR) components. The properties are determined using rectangular waveguides which are coated with NEG. The method is based on a combination of complex transmission coefficient S-21 measurements with a vector network analyzer (VNA) and 3D simulations using CST Microwave Studio (R) (CST MWS). The frequency ranges discussed in this paper are 220-330 and 500-750 GHz.

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

Eirini Koukovini Platia

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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