A REVIEW OF DIFFICULTIES IN ACHIEVING SHORT BUNCHES IN STORAGE RINGS

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During the last few years, the potential of electron storage rings for the production of short bunches has been thoroughly investigated. The beam-wall interaction in general was reconsidered for 3 rd generation synchrotron sources. The mechanism of the so-called microwave instability, responsible for bunch lengthening and energy widening, was clarified in all details from a theoretical point of view. Predictions were confirmed with results from tracking code simulations and experimental data. A result of these studies is that harmonic cavities are ineffective for either shortening the bunch or substantially raising the instability threshold. Extensive investigations were performed on quasi-isochronous lattices. Many rings were operated with very low momentum compaction optics, above and below transition energy. Experimental results in full agreement with theoretical prediction confirm that unfortunately short bunches are very hard to produce. rd generation storage rings were commissioned, the quasi-isochronous tuning of rings appeared promising to produce short electron bunches and accordingly short and intense X-ray pulses. The quasi-isochronous regime has now been tested. Unfortunately, the bunch lengthens strongly with current and its length becomes independent of both the momentum compaction and the energy. The strong defocusing effect of the self-induced voltage can be reversed when the momentum compaction is negative. But, a negative mass instability then starts developing, blowing up the energy spread. For a positive momentum compaction, the energy spread will start widening above the “microwave” instability threshold. The “microwave” instability mechanism was thoroughly investigated again to determine if, implementing harmonic cavities or a passive cavity-like structure, would help to reach higher peak brilliance on storage rings. Unfortunately, none of these structures will allow to boost the peak brilliance to the level of performances announced for linac driven sources.