OBSERVATION AT CESRTA OF THE REDUCTION OF THE VERTICAL BEAM SIZE OF THE LEAD BUNCH IN A TRAIN DUE TO THE PRESENCE OF A PRECURSOR BUNCH

Electron cloud-induced beam dynamics is being studied at CESRTA under various conditions. These measurements make use of instrumentation for the detection of the coherent self-excited spectrum for each bunch within the train and bunch-by-bunch vertical beam size. In the position spectrum coherent betatron dipole and head-tail motion is detectable for each individual bunch within the train. These techniques are utilized to study the electron cloud-related interactions, which cause the growth of coherent motion and beam size along the train. We report on the observations of the vertical enlargement of the first bunch(es) in 30 bunch-long trains. We also report that the addition of a precursor bunch following the train of bunches and before the start of the next train can counteract the vertical enlargement of the first bunch(es) in the train. Results from these observations will be presented. PREVIOUS OBSERVATIONS During the study of EC buildup in the storage ring CESR as part of the CESRTA project an unusual phenomenon has been observed. The studies utilized trains, containing 30 or 45 bunches, with the total number of positrons or electrons in the train of 3.6x10 11 particles. Initially the bunches were spaced 4 or 14 ns, but in subsequent studies with positrons the spacing was varied in multiples of 4 or 14 nec to a much as a 56 ns spacing. As expected, the buildup of the EC along positron trains caused the growth of coherent oscillations and the enlargement of the vertical beam size in the later bunches in these trains. The phenomenon that was unexpected, which was observed for positrons and not for electrons, was the enlargement of the vertical beam size and the presence of the (m=-1) coherent head-tail mode for the