Instability studies and double rf-system operation at BESSY

Longitudinal multi-bunch oscillations are observed at the BESSY storage ring even with beam currents of 1-2 mA, although stored currents up to 970 mA can be achieved. With a double rf-system the threshold currents can be increased when operating the system in the tune splitting or in the 'Landau'-cavity mode. A longitudinal impedance of exclamation Z/n exclamationapprox. = 12 Ω is obtained from bunchlength measurements. No transverse multi-bunch instabilities have been observed. This may be explained by Landau damping induced by the non-linear fields of the ions trapped in the beam