Higher-Harmonic RF System for Landau Damping in the CERN PS

Longitudinal coupled-bunch instabilities after transition crossing and at the flat-top limit the intensity of LHC-type beams in the CERN Proton Synchrotron (PS). A dedicated coupled-bunch feedback for dipole oscillation modes, using a Finemet cavity as wide-band longitudinal kicker, suppresses the instabilities up to an intensity of about 2·10¹¹ particles per bunch at extraction. However, dipole and quadrupole coupled-bunch oscillations are observed beyond this intensity. At the flat-top they were damped with a 40 MHz RF cavity operated as a higher-harmonic RF system to increase Landau damping, in addition to the principal RF system at 10 MHz. The existing 40 MHz RF system, designed for RF manipulations at fixed frequency, does not cover the frequency range required during acceleration. It is therefore proposed to install a tunable RF system with a 5% relative frequency swing. This paper summarizes the observations of instability damping at the flat-top and presents preliminary parameters for the higher-harmonic RF system.