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The GSI SIS100 project requires very high accelerating voltages. With ferrite-tuned synchrotron cavities the gap voltage is often strongly limited by the Q-loss effect appearing at medium dc bias fields. At low bias fields, considerably higher voltages can be reached, however. The maximum usable amplitudes over the bias region have been studied. At zero bias, the ferrites could be driven to more than a factor 3 above the Q-loss limit. Except overheating, no other problems appeared. With increasing bias, the maximum amplitudes decrease continuously to the Q-loss level. In this fall-off region there is still a tuning factor up to 2.5 available, with rf flux densities by at least a factor 2 above the Q-loss level. Measurements on small samples of the ferrite material used in the GSI cavities could be verified very well in a full-size cavity, for the most part. The tests were mainly limited by the available anode voltage and the fear of damaging the cavity. It seems possible, to generalize the main results for other ferrite materials, also. Based on the results, a possible scenario for the SIS100 rf system is given. Additionally, the results lead to an alternative cavity design for higher voltages, which is described as well.
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