SPECTRAL CHARACTERIZATION OF LONGITUDINAL COUPLED-BUNCH INSTABILITIES AT THE ADVANCED LIGHT-SOURCE

We present the results of an experimental study of longitudinal coupled-bunch (CB) motion observed in the newly commissioned Advanced Light Source (ALS). Our study identifies the frequencies of the monopole higher-order modes (HOMs) of the RF cavities driving the beam by correlating the frequencies of the beam oscillations for different symmetric beam filling patterns with the measured cavity HOM frequencies. Self-limiting of the largest coherent coupled-bunch oscillation occurs at similar to 10 degrees at the RF frequency with an individual bunch amplitude of no greater than 20-30 degrees, depending on the total beam current. A qualitative discussion of a limiting mechanism to CB growth is presented along with computer tracking studies.