Head-tail instability caused by electron clouds in positron storage rings

In positron or proton storage rings with many closely spaced bunches,an ele ctron cloud can build up in the vacuum chamber due to photoemission or seco ndary emission. We discuss the possibility of a single-bunch two-stream ins tability driven by this electron cloud. Depending on the strength of the be am-electron interaction, the chromaticity and the synchrotron oscillation f requency, this instability either resembles a linac beam breakup or a head- tail instability. We present computer simulations of the instabilities, and compare the simulation results with analytical estimates.