Author :
Seeman, J. ; Adams, K. ; Billing, M. ; Blum, E. ; Gareyte, J. ; Kirchgessner, J. ; Littauer, R. ; McDaniel, B. ; Meller ; Peck, R.S. ; Peggs, S. ; Rice, D. ; Rouse, G. ; Siemann, R. ; Sundelin, R. ; Talman, R. ; Tigner, M. ; vonBorstel, E.
Abstract :
A transverse coupled bunch instability has been observed in the Cornell Electron Storage Ring CESR for both positrons and electrons. This instability shows strong horizontal or vertical coherent signals but very weak longitudinal signals. Positron injection, which requires sixty-one uniformly spaced bunches in CESR, was originally restricted by the enlarged effective horizontal beam size resulting from this instability. Although several cures have been discovered, only external octupoles provide a sufficient increase in the threshold and are compatible with injection. The theory for coupled bunch motion correctly predicts the instability threshold assuming that the driving mechanism is a 1140 MHz parasitic resonance in the RF cavity. Changes in the threshold with tune and octupole field strength are also correctly predicted.
