Horizontal Instability and Feedback Performance in DAFNE e+ Ring

In DA\NE, after the 2003 shutdown for the installation of FINUDA, a strong horizontal multibunch instability was found to limit the positron beam for currents>450mA. The author has performed transverse grow-damp measurements in order to estimate the instability growth rates as well as the feedback damping rates for each bunch at different beam currents and to evaluate the tune shift along the bunch train. In particular, a strong dependence of oscillation amplitudes on the position of the bunch in the train has been observed. In this paper, the author describes the set-up for multibunch oscillation amplitude recording, summarizing some observations on the transverse instability and discussing the feedback performance. The feedback raises the threshold by about a factor of two, depending on the machine configuration