Tracking Down a Fast Instability in the PEP-II LER

During Run 5, the beam in the PEP-II Low Energy Ring (LER) became affected by a predominantly vertical insta-bility with very fast growth rate of 10...60/ms and varying threshold. The coherent amplitude of the oscillation was limited to approx. 1 mm peak and would damp down over a few tens of turns, however, beam loss set in even as the amplitude signal damped, causing a beam abort. This led to the conclusion that the bunches were actually blowing up. The appearance of a 2νs line in the spectrum suggested a possible head-tail nature of the instability, although chro-maticity was not effective in changing the threshold. The crucial hints in tracking down the cause turned out to be vacuum activity near the rf cavities and observance of sig-nals on the cavity probes of certain rf cavities.