Horizontal Instability in the KEKB HER

After a brief introduction to the problem, I study several aspects related to the horizontal instability in the KEKB HER, namely resistive-wall instability, ion trapping and ion instability, ion losses and the associated modification of the fast beam-ion instability, measurements of coupledbunch oscillations and their simulation assuming they are driven by ions, resistive wall, or electron cloud, and, finally, the measured and simulated instability mode spectra. I show that the observed bunch oscillations, mode spectra, and rise times cannot be explained by the resistive wall nor by a short-range electron-cloud wake, but that they are consistent with an instability driven by carbonmonoxide ions in a region without magnetic field. In the appendix I discuss the relation between head-tail damping and coherent tune shift for several types of impedance.