Transverse ´monopole´ instability driven by an electron cloud?

We simulate the long-term emittance growth of a proton beam due to an electron cloud of moderate density. This emittance growth is sometimes characterized by a rapid blow up of the bunch tail, and it appears to be different from the strong head-tail instability, which is observed at higher electron densities. We study whether this instability can occur in the absence of transverse dipole motion along the bunch, and its sensitivity to various simulation parameters, such as the number of beam-electron interaction points (IPs) and the phase advances between them. Using a frozen-potential model, we compute tune footprints, which reveal the resonances contributing to the incoherent part of the emittance growth.