The CESR horizontal separators impedance study

The longitudinal multi-bunch beam instability significantly complicates Cornell Electron Storage Ring (CESR) operation. The observed dependence of the horizontal separator temperature on the space between two bunches circulating in the same direction suggested the presence of long lasting parasitic modes in the separators. The S-parameter measurement confirmed the existence of the modes and gave their frequencies and quality factors (Qs). The program URMEL was used to calculate the modes coupled to the beam, i.e. parameters R/Q. Using these characteristics the longitudinal instability growth rate and instability threshold current were computed. The calculated threshold is only 35% higher than observed. The conclusion has been made that the parasitic modes in horizontal separators are among the major factors causing longitudinal multibunch beam instability at the CESR. The multibunch beam stability at the CESR will be significantly improved if these modes are damped.