Gilardi, Antonio (2021) Measurements of wakefields and bunch length with beam in linear electron accelerators: a case study at the CLEAR facility. [Tesi di dottorato]

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Abstract

The thesis focuses on the study of two phenomena, both connected to transverse electromagnetic forces, affecting the particles inside an accelerator. The first study regards the undesired transverse kick given to the particles by the so-called wakefields, electromagnetic fields generated by the interaction of the particles themselves with the surrounding environment (e.g., the vacuum beam pipe of an accelerator, or the inner copper surface of an accelerating structure). In the thesis, a novel method for assessing the transverse kick induced by wakefields is proposed. This method is used to study the wakefields excited by the beam in the latest prototype of the high gradient accelerating structure developed for future compact accelerators, in the framework of the CLIC study at CERN. The experimental results are compared with simulations, and the comparison shows a good agreement. The results clearly indicate that the dominant effect in this case is given by the short-range part of the wakefield. The investigation explored also the often neglected dependence of the wakefield kick on the length of the particle bunch. The second study also regards the interaction of a particle bunch with a transverse electromagnetic field, but in this case the field is intentionally applied in order to perform a measurement of the bunch length. Firstly, an overview of the standard measurement technique is presented. In this layout two novel measurement methods that allows to obtain additional information on the bunch properties are presented. Such extension allows in particular to estimate the energy chirp and energy spread and to obtain information about the correlations between particle position, angular divergences, and energy. Later, an improved layout for performing the measurement is presented. Such layout does not alter the conventional measurement properties and, at the same time, gives the flexibility to directly measure the correlations between particle position, divergences, and energy. Besides, using the improved layout it is possible to enhance the method's metrological performance improving the resolution and the uncertainty. A detailed study of these two topics is presented, by comparing the result obtained with the two layouts. All derived theoretical models were benchmarked with simulation and experimental measures performed on the CLEAR linear electron accelerator at CERN. A satisfying agreement was found in all cases.

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