Photoelectron trapping mechanism for transverse coupled bunch mode growth in CESR

An anomalous damping or growth of transverse coupled bunch modes has long been observed in CESR. The growth rates and tune shifts of these modes are a highly nonlinear function of current. The effect is associated with the operation of the distributed ion pumps, as it disappears when the pumps are not powered. We show that this effect can be explained by the presence of electrons trapped in the CESR chamber by the field of the dipole magnets and the electrostatic leakage field of the distributed ion pumps. Photoelectrons are introduced into the chamber by synchrotron radiation and can be ejected from the chamber by the passage of an e⁺ or e^- bunch. The transverse position of the beam thus modulates the trapped photoelectron charge density, which in turn deflects the beam, creating growth or damping and a tune shift for each coupled bunch mode. Predictions of the dependence of growth rate and tune shift on bunch current, bunch pattern, and mode frequency by a numerical model of this process are in approximate agreement with observations