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. CURING CBI BY HOM SHIFTINGThe technique of curing CBI by shifting the frequency of the Higher Order Modes (HOM) of the RF cavities can be an interesting solution, particularly for high energy and compact storage rings. At ELETTRA the distribution of the Coupled Bunch Modes (CBM) and their growth rates have been calculated as a function of cavity temperature [1], [2]. This allows a quite flexible operation of the storage ring, with the possibility of a lifetime increase by controlled use of CBI which lengthen the bunch [3]. Characterisation of the excitation of Low Frequency Oscillations for different levels of CBI has also been possible [4]. Secondary effects on the RF cavities like cross-talk between cavities, cavity temperature oscillation and spurious vacuum interlocks could be observed and understood [5]. Mode shifting by temperature tuningCavity temperature intervals where the growth rates of all CBM are below the radiation damping level, which is a conservative condition to achieve stability, are more easily available if :• the radiation damping time is short; • the width of the temperature interval where a cavity HOM doesn't drive any unstable CBM is large; • the number of HOMs one has to deal with is small; • the temperature tuning range of the cavity is wide The first two conditions are more easily met by small, high energy rings. The RF cavities should not be provided with any HOM damping system, so that the HOM resonances are still sharp, high Q resonances. The experience at ELETTRA shows that the number of modes one has to deal with does not exceed about ten for the longitudinal case and ten for the transverse case (considering the double polarisation for the dipole modes). A cavity temperature tuning range of about 30 °C, as used in ELETTRA, is usually large enough to provide stable temperature windows. However, even a small bandwidth HOM can be unstable over a wide temperature range, if its temperature coefficient τ is low. Then the temperature tuning method can fail. This was the case for the first longitudinal mode, L1, of the E...

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