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SummaryOne of the most important questions during the study and the construction period of PETRA was connec ted with the high particle intensity effects. There fore the first high current experiments at PETRA were performed to study average current limitations, higher order mode losses, bunchlengthening, single bunch in stabilities and single bunch current limitations.

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SummaryIn the electron-and positron rings of DORIS strong transverse and longitudinal, single beam insta bilities were observed, which caused current limitations. The severest transverse and longitudinal instabilities were induced by excitation of higher modes in the rfcavities. When these instabilities were cured by damping the higher cavity modes, the thresholds for this type of instabilities were greatly increased. The origin of the remaining transverse and longitudinal instabilities is not yet understood. The thresholds of these instabilities were increased by Landau-damping and changing the betacron-and synchrotron frequencies from bunch to bunch. The head-tail effect was observed at high peak currents and was cured by compensating the chromaticity.In the electron ring a transverse instability is present which is induced by the ions. Ion clearing in combination with some vacuum improvements is sufficient to avoid this instability. 1, Cavity induced instabilitiesThe severest single beam instabilities observed in DORIS were induced by the excitation of higher parasitic modes in the rf-cavities. Except for a very short filling of the ring (10 to 20 bunches) the current limitation was nearly independent of the circumferential distribution of the bunches, so that mainly the mean currents were limited. The thresholds for the transverse and longitudinal instabilities appeared in the range of 1 to 5 mA. However, the thresholds and the maximum currents, that could be stored were strongly correlated to the tune of the 8 cavities in each ring, so that by varying the tune of the cavities at random 200 mA could be stored or even more.It turned out, that the transverse instabilities were induced by higher deflecting modes of the rfcavities, while the longi tudinal instabilities were induced by higher accele.rating modes.After improvement of the detecting equipment rfsignals from the beam picked up from wide-band elec trodes in the rings and from loops in the cavities, were spectrum analysed.In the beam and in the cavities we found higher-modefrequency signals, coincident with the excitation of instabilities.In order to identify the higher cavity modes causing the instabilities a special program of mode spectroscopy was started, based on the following properties of the beam: Thus the observable frequencies are multiples of die revolution frequency shifted by 6Q-f (6Q a frac tional part of the Q-value) in the transverse case and by Q s *f0 (synchrotron frequency) in the longitudinal case.