The Electron - Cloud Instability in the Arcs of the PEP - II Positron Ring ∗

We have applied our simulation code “POSINST” to evaluate, in linear approximation, the contribution to the growth rate of the electron-cloud instability (ECI) from the pumping sections and the dipole bending magnets in the arcs of the PEP-II positron ring. A key ingredient in our model is a detailed description of the secondary emission process off the TiN-coated chambers. Another important element is the analytic computation of the electric field produced by the beam, including the effects from surface charges. Space-charge forces of the electron cloud upon itself are also included, although these forces are negligible under nominal conditions. Bunch-length effects are optionally included by slicing the bunch into several kicks. We conclude that the growth rate is dominated by the pumping sections and scales linearly with the photoelectric yield Y ′. For Y ′ = 1, our present estimate is in the range ∼ 1000− 1300 s−1, depending upon the value of the photon reflectivity R. This is in the range controllable by the transverse feedback system. The contributions to the growth rate from other magnets and from other sections of the ring remain to be evaluated.