Transverse and longitudinal multi-bunch feedback systems for PETRA

A severe intensity limitation for the PETRA and HERA e- rings is connected with the coupled bunch motion. The multi-bunch instabilities are driven by the parasitic modes of the accelerating cavities, thus limiting the total current. The threshold currents depend on the actual configuration of the parasitic modes and the Landau damping. During the last run, the Landau decay time in the transverse direction was measured at around 10 msec, the threshold current was just below 3 mA. In the longitudinal direction the decay time was 15 msec and the threshold current around 10 mA. In order to get rid of the brutal current limitations, a wide-band multi-bunch system was built and installed in PETRA for all three directions. The transverse system had been tested earlier. After the longitudinal system had been completed and after an essential improvement of the signal quality in the transverse direction, the damping time achieved was around 200 μsec in all three directions for all transverse and longitudinal modes for 70 bunches with a spacing of 96 nsec. The total current reached was 56 mA which is the current needed to fill HERA. The beam was completely stable at those currents and the intensity of the bunches distributed around the ring was constant within less than 5%. (orig.)