INVESTIGATIONS OF CAVITY INDUCED LONGITUDINAL COUPLED BUNCH MODE INSTABILITY BEHAVIOUR AND MECHANISMS∗

The narrowband impedances of RF-resonators in a circu-lar accelerator can drive coupled bunch mode instabilities (CBI), which might spoil the overall beam quality. Often the instability does not lead to beam loss, but to a severe degradation of the source brilliance, as e.g. in synchrotron light sources. Investigations of longitudinal CBIs have been performed at the DELTA storage ring [1] with a single DORIS-type cavity for future comparision with the behaviour of a HOM-damped cavity to be tested at DELTA. This resonator has been developed and built within an EU-collaboration [2].The beam was deliberately driven into instability us-ing the beam current, as well as the cavity temperature as individual parameters. At low energy (542 MeV) the longitudinal beam behaviour is completely dominated by instabilities, even for very low currents. At high energy (1.5 GeV) threshold currents between 30 and 75 mA have been detected for single higher order modes (HOM). The synchrotron sideband analysis shows a sattelite structure around the resonant mode. Numerical estimations includ-ing an FFT algorithm show that the fractional filling of the storage ring is responsible for this sideband structure