Measurements of bunch motion due to the longitudinal dipole-coupled bunch instability at the Cornell Electron-Positron Storage Ring

In the past, a longitudinal dipole-coupled bunch instability had limited high current operation at the Cornell Electron-Positron Storage Ring (CESR) and resulted in a degradation of luminosity performance. A longitudinal feedback system successfully damps this instability, and the exchange of superconducting rf cavities for normal conducting rf cavities in CESR has further reduced the instability's strength. A description of the longitudinal dynamics with the instability present are described in this paper along with detailed measurements of the instability using a dual-axis synchroscan streak camera. The measurements were made on single trains of bunches, multiple trains, and colliding beams. These measurements give a characterization of the instability's degradation of luminosity, modes of oscillation, and bunch distribution changes.