Abstract
At the natural bunchlength the single bunch currents in PETRA are limited by satellite resonances and a new vertical instability, which causes a vertical blow-up of a oarticle bunch if the single bunch current reaches the threshold value of that instability. The "PETRA instability" was explained in terms of head-tail mode coupling and according to this model the vertical blow-up was cured by bunchlengthening changing the longitudinal damping partition. The disadvantage of this kind of bunchlengthening is the increase of energy spread leading to a reduction of quantum life time during energy ramminq. The satellite resonances are avoided by a careful steering of the longitudinal and transverse tunes. A more effective method of bunch lengthening is the use of a higher harmonic rf system. Such a system was installed in PETRA. Wiith the higher harmonic rf system the threshold current of the vertical "PETRA instability" could be increased from 3 mA (natural bunchlength) to 15 mA (with higher harmonic rf system). Besides bunchlengthening the higher harmonic system leads to a considerable reduiction of satellite effects in the range of normal PETRA tunes.
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