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Commissioning Experimence from PEP-II HER Longitudinal Feedback


The DSP-based bunch-by-bunch feedback system installed in the PEP-II HER has been used to damp HOM-induced instabilities at beam currents up to 605 mA during commissioning. Beam pseudospectra calculated from feedback system data indicate the presence of coupled bunch modes that coincide with the 0-M-2 cavity HOM. Bunch current and synchronous phase measurements are also extracted from the data. These measurements reveal the impedance seen by the beam at revolution harmonics. The impedance peak at 3*frev indicates incorrect parking of the idle cavities, and explains the observed instability of mode 3. Bunch synchrotron tunes are calculated from lorentzian fits to the data. Bunch-to-bunch tune variation due to the cavity transient is shown to be large enough to result in Landau damping of coupled bunch modes.

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