Further Studies on Betatron Sidebands Due to Electron Clouds

We have observed vertical betatron sidebands in the transverse beam spectra of positron bunches at the KEKB LER which are associated with the presence of electron clouds in single-beam studies[1], and which are also associated with a loss of luminosity when the KEKB beams are in collision[2]. The sidebands appear to be signals of a fast head-tail instability due to short-range wakes within the electron cloud, providing a diagnostic for exploring the mechanism for transverse beam blow-up due to electron clouds. We report here on further studies on the behavior of the sidebands under varying beam conditions, including varying chromaticity, emittance and synchrotron tune. These results strengthen the interpretation of the spectra as being due to head-tail oscillations. The sideband is also found to disappear in the presence of forced excitation.