Observations of a longitudinal coupled bunch instability in CESR

During the last several years of operating with trains of bunches, a longitudinal dipole coupled bunch instability has been observed at the Cornell Electron Storage Ring (CESR). Without the use of multibunch longitudinal feedback this instability would limit the total stored current in the two beams during High Energy Physics operation. This paper gives an updated report on observations of this instability, its strength, its dependence on bunch spacing within in the trains, its relationship to changes made in the RF accelerating system and results of simulations using the observed mode spectra of the RF cavities