Influence of collective effects on seeded FEL and mitigation strategies at FLASH

The Free Electron Laser (FEL) user facility FLASH at DESY is currently being upgraded within the framework of the FLASH2020+ project with the goal of converting one of the beamlines into an externally seeded beamline. In particular, the Echo-Enabled Harmonic Generation (EEHG) method will be used to generate fullycoherent FEL pulses in XUV and soft X-ray wavelength regions down to 4 nm. As one of the most challenging of the planned operation modes, 4 nm EEHG can be especially sensitive to the parasitic beam dynamics, known as collective effects. In this work, we employ analytical models to investigate the influence of Incoherent Synchrotron Radiation and Intra-Beam Scattering on the efficiency of the EEHG setup. We perform numerical simulations to study the effect of Coherent Synchrotron Radiation (CSR) in the seeding section of the beamline on the spectral properties of EEHG bunching. We compare different models of CSR and highlight the importance of such aspects of the models as chamber shielding and transient effects in the parameter space of 4 nm EEHG set-up at FLASH. Furthermore, for CSR, which poses noticeable challenges, we explore a mitigation strategy using short seed laser pulses to optimize the spectral bandwidth. Finally, we demonstrate a promising solution to the Micro-Bunching Instability (MBI) using the recently installed Laser Heater. This thesis is a self-contained detailed investigation of the electron beam dynamics of EEHG in the seeding section in the presence of collective effects and an integral part of ongoing start-to-end simulation efforts for realistic characterization of the performance of the future seeded FEL beamline at FLASH2020+