Improvements in curing coupled bunch instabilities at ELETTRA by mode shifting after the installation of the adjustable higher order mode frequency shifter (HOMFS)

Abstract

Mode shifting is the technique used at the third generation synchrotron light source ELETTRA to cure Coupled Bunch instabilities (CBI). Temperature tuning of the RF cavities, based on an analytical prediction algorithm, allows longitudinal stable operating conditions to be obtained. However, once longitudinal stability has been achieved transverse instabilities can be observed, particularly during the energy ramp from 1.0 to 2.0 GeV. There has therefore been the need to extend the flexibility of the mode shifting technique. This has been achieved by the installation of adjustable HOMFS on the storage ring RF cavities. The HOMFS is effective on those longitudinal and transverse cavity Higher Order Modes which are unstable over a wide temperature range, by shifting them out of the operating temperature interval of the cavity. The installation of the HOMFS has improved the reliability and reproducibility of machine operation, also in view of the beam current upgrade.