STUDIES ON MAXIMUM RF VOLTAGES IN FERRITE-TUNED ACCELERATING CAVITIES

The GSI SIS100 project requires very high accelerating voltages. With ferrite-tuned cavities the gap voltage is often strongly limited by the Q-Loss Effect at medium dc bias fields, whereas with lower bias fields, much higher voltages can be reached. 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. With increasing bias, the usable amplitudes decrease continuously to the lowest Q-Loss level. In this fall-off region there is still a tuning factor up to 2 available, with rf flux densities by at least a factor 1.5 above the Q-Loss level. Measurements on small samples of the ferrite material used in the GSI cavities could be verified very well in the GSI SIS and ESR full-size cavities. The tests were mainly limited by the available anode voltage. It seems possible to generalize the main results for other ferrite materials, also. Based on the results, an improved scenario for the SIS100 rf system is discussed.