Identification of impedance sources responsible for longitudinal beam instabilities in the CERN PS

Abstract

Longitudinal instabilities in the CERN PS are an important limitation to obtain the expected beam intensity and longitudinal emittance at the PS extraction in the framework of the LHC Injector Upgrade (LIU) project. The observed coupled-bunch instabilities lead to dipolar and quadrupolar oscillations, as well as to an uncontrolled longitudinal emittance blow-up for proton beams. A microwave instability observed with ion beams develops quickly at transition crossing. To identify the potential impedance sources of these instabilities, two strategies were adopted. Firstly, beam measurements were performed for dierent impedance configurations, i.e. by partially detuning the main RF cavities. Secondly, a thorough survey of the machine elements together with the RF studies allowed to refine the PS impedance model in order to find potentially missing contributions. Measurements were compared with particle simulations using the updated impedance model of the PS. Although the source of the dipolar coupled-bunch instability was already identified in the past, this study led to an identification of the impedance sources driving other types of longitudinal instabilities.