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UBC Theses and Dissertations

Transient beam loading studies in radiofrequency cavities Thoeng, Edward


RF cavities are the driving technology behind most of modern particle accelerators. The diverse use of RF cavities in linear accelerators for are discussed and experiments have been performed to illustrate these principles. Experiments were conducted at two Rare-Isotope Beam facilities: GSI in Germany, and TRIUMF in Canada. More specifically, linear accelerators investigated are GSI UNILAC and 0.5 MW Superconducting electron LINAC, currently constructed and commissioned at TRIUMF as part of the ARIEL Project. Measurements at GSI UNILAC were performed parasitically while at TRIUMF, access to different beamline sections is available and dedicated measurements have been undertaken at both the low and medium energy sections of the LINAC. A main focus is to study beam-cavity interactions, in particular the effect of beam-loading. This effect acts back on the beam via induced resonance of the cavity, and can have negative effects on the beam, such as the increase of beam energy dispersion. This interaction can be explained using an equivalent circuit model. In Superconducting RF cavities, cavity voltage is controlled via a feedback loop. In pulsed beam operation, however, due to the high quality factor of the SRF cavity, feedback control alone is not adequate to compensate for the beam-loading effect and an additional feedforward control signal needs to be implemented. Combining experiments at GSI and TRIUMF, this effect is discussed in detail via RF cavity response to beam-driven, generator-driven, and with closed-loop control feedback and feed-forward, using RF signal analysis, beam diagnostics measurements, and dipole magnets. On top of beam-loading effects, the bunch length output of the ARIEL electron gun has also been studied with a deflecting cavity. Additional measurements with a buncher cavity have also been performed as a follow- up of the issues discovered during beam-loading measurements with ARIEL EINJ (injector cryomodule). The results obtained from both measurements indicate the necessity to improve the timing architecture of the ARIEL e-LINAC. A portion of the Appendix is dedicated to illustrate RF superconductivity and RF eigenmode simulation with commercial RF codes in order to explain the different modes of RF cavity operations applied throughout this thesis.

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