EXCITATION OF INTRA-BUNCH VERTICAL MOTION IN THE SPS- IMPLICATIONS FOR FEEDBACK CONTROL OF ECLOUD AND TMCI INSTABILITIES∗

Electron cloud (ecloud) and transverse mode coupled-bunch instabilities (TMCI) limit the bunch intensity in the CERN SPS. This paper presents experimental measure-ments in the SPS of single-bunch motion driven by a GHz bandwidth vertical excitation system [1]. The final goal is to quantify the change in internal bunch dynamics as in-stability thresholds are approached, and quantify the fre-quencies of internal modes as ecloud effects become sig-nificant. Initially, we have been able to drive the beam and view its motion. We show the excitation of barycentric, head-tail and higher vertical modes at different bunch in-tensities. The beam motion is analyzed in the time domain, via animated presentations of the sampled vertical signals, and in the frequency domain, via spectrograms showing the modal frequencies vs. time. The demonstration of the exci-tation of selected internal modes is a significant step in the development of the feedback control techniques