Measurements of wakefields andbunch length with beam in linearelectron accelerators: a case study atthe CLEAR facility

The thesis focuses on the study of two phenomena, both connected to transverseelectromagnetic forces, affecting the particles inside an accelerator.The first study regards the undesired transverse kick given to the particlesby the so-called wakefields, electromagnetic fields generated by the interactionof the particles themselves with the surrounding environment (e.g., the vacuumbeam pipe of an accelerator, or the inner copper surface of an accelerating struc-ture). In the thesis, a novel method for assessing the transverse kick induced bywakefields is proposed. This method is used to study the wakefields excited bythe beam in the latest prototype of the high gradient accelerating structure de-veloped for future compact accelerators, in the framework of the CLIC study atCERN. The experimental results are compared with simulations, and the com-parison shows a good agreement. The results clearly indicate that the dominanteffect in this case is given by the short-range part of the wakefield. The investi-gation explored also the often neglected dependence of the wakefield kick on thelength of the particle bunch.The second study also regards the interaction of a particle bunch with a trans-verse electromagnetic field, but in this case the field is intentionally applied inorder to perform a measurement of the bunch length. Firstly, an overview of thestandard measurement technique is presented. In this layout two novel measure-ment methods that allows to obtain additional information on the bunch proper-ties are presented. Such extension allows in particular to estimate the energychirp and energy spread and to obtain information about the correlations be-tween particle position, angular divergences, and energy. Later, an improved lay-out for performing the measurement is presented. Such layout does not alter theconventional measurement properties and, at the same time, gives the flexibilityto directly measure the correlations between particle position, divergences, andenergy. Besides, using the improved layout it is possible to enhance the method’smetrological performance improving the resolution and the uncertainty. A de-tailed study of these two topics is presented, by comparing the result obtainedwith the two layouts. All derived theoretical models were benchmarked withsimulation and experimental measures performed on the CLEAR linear electronaccelerator at CERN. A satisfying agreement was found in all cases.