Characterization of impedance and wakefields of accelerator devices in the short bunch limit and beam dynamics effects for the CLIC Damping Rings

The Damping Rings (DRs) complex of the Compact LInear Collider (CLIC) has to provide ultra low emittances to the linear accelerating sections in order to reach a luminosity of $2\times10^{34}$ cm$^2$ s$^{-1}$ at the centre of mass energy $3$ TeV. Due to the high intensity, short bunches and low transverse emittances, collective effects are expected to seriously affect the performance of the DRs, limiting the electron and positron beam intensity at extraction, or causing an intolerable degradation of the beam quality. The design of the machine has to be optimized already at a very early stage in order to ensure that all the instability thresholds stay above the operational target intensities with sufficient safety margin. To this end, the impedance model of the DRs is developed and used for studying instability thresholds with the PyHEADTAIL code. Starting from a simplified model of the whole machine, the code allows to take into account the radiation damping and quantum excitation effects due to the synchrotron radiation emission and also the resistive wall contribution. The knowledge of the surface impedance up to hundreds of GHz, to which the bunch spectrum extends, is essential for the correct resistive wall impedance modeling of the coatings deposited on the vacuum chambers of the machine. Specifically, Non Evaporable Getter (NEG) is a commonly used coating to allow a distributed and continuous pumping in vacuum chambers and the amorphous Carbon (a-C) is deposited in order to avoid the onset of the Electron Cloud (EC) in the accelerator vacuum chamber. The core of this thesis consists in the development of a reliable, handy, and inexpensive measurement system for the Electromagnetic Characterization (EMC), in the sub-THz frequency range, of coating materials. The method is based on time domain measurements of an electromagnetic wave passing through a Device Under Test (DUT) made of a waveguide with a thin central slab, where the coating material is deposited on both sides. This device has two main advantages: the deposition homogeneity with predictable thickness and the possibility to reuse the system for further measurements on different coating materials. The assessment of the signal attenuation through the DUT is analytically evaluated and confirmed with numerical simulations. This novel technique is tested on slabs coated with NEG and allows the electromagnetic characterization and the surface impedance evaluation up to hundreds of GHz.