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Observation, modelling and cure of transverse instabilities at the ESRF

Multibunch operation at the ESRF is affected by transverse resistive-wall instabilities in horizontal and vertical planes. Chromaticities are shifted to positive values and a partial filling is used to raise the instability thresholds. Systematic measurements have been carried out to characterise the instabilities as a function of the filling pattern, chromaticity and RF voltage. A beam-based modelling of the impedance has been made, using combined BBR and resistive wall impedance. For single bunch, the mechanism that drives the vertical instabilities is presented, while for horizontal ones recent observations are discussed. Incoherent betatron tune shifts with the average beam current, observed in both plane but with opposite polarities, have been investigated for single and multibunch. First results from preliminary tests on a new hybrid mode are discussed. Besides resistive-wall instabilities, ion trapping as well as fast ion-beam blow up have been observed. The effectiveness of a mode-by-mode transverse feedback to damp the instabilities is also presented.