Various techniques for overcoming coupled bunch instabilities were discussed including higher order mode (HOM) shifting, HOM free cavities and feedback systems as well as Landau cavities for increased tune spread either within the bunch or between bunches.

Mode Shifting and Mode Free Cavities

Coupled bunch instabilities are successfully cured at ELETTRA by means of mode shifting, using a combination of temperature control and HOM frequency shifters. The first direct evidence for a HOM driven instability has been seen at the SRS; the effect on the ID spectra as a function of HOM excitation was presented, as well as the cure of the instability through a combined change of cavity temperature and tuner position. To avoid the problems due to HOMs, SOLEIL is studying the use of superconducting cavities with HOM dampers.

Feedback Systems and Other Techniques

New possibilities for multibunch feedback systems have recently been opened up by the use of digital signal processing (DSP) systems. ELETTRA is developing a transverse feedback system in collaboration with DAf

MAX II will install a Landau cavity for the simultaneous increase in Touschek lifetime and to combat multibunch instabilities via the increased longitudinal tune spread within the bunch. The cavity will be passive because of the difficulty in finding klystrons at the frequency of 1.5 GHz. The resulting beam current range could however be quite small, because of the need to overcome the Robinson instability. BESSY I also has a double r.f system for bunch length control.

prepared by C.J. Bocchetta and R.P. Walker