Temporal Characterization of SASE FEL pulses at FLASH

At Free-Electron Lasers (FELs) based on Self-Amplified Spontaneous Emission (SASE), the radiation properties such as pulse duration, arrival time, energy and spectrum fluctuate from shot to shot. To accurately interpret the experimental data on a single-shot basis, dedicated diagnostic tools for each parameter are essential. One such technique is terahertz (THz) streaking, which enables single-shot characterization of the temporal properties of SASE FEL pulses. This thesis presents the design, implementation, and experimental validation of the FL21 beamline, a dedicated photon diagnostics beamline at the Free-electron LASer in Hamburg (FLASH) for extreme ultraviolet (XUV) pulse characterization using THz streaking. The results confirm the feasibility of using this setup for systematic long-term studies of SASE FEL pulse properties with high temporal resolution. Another key focus of this work is the measurement and characterization of a possible frequency chirp of SASE XUV pulses, frequency chirp refers to a variation in frequency along the pulse. Using two independent methods, THz streaking, which directly measures the linear frequency chirp, and electron bunch energy observation through a transverse deflecting structure (PolariX TDS), a good agreement between both approaches was demonstrated across various FEL settings. By comparing XUV pulse profiles reconstructed from PolariX TDS and THz streaking, the strengths and limitations of each approach in the determination of the pulse shape were identified. The complementary nature of these techniques enables more accurate pulse characterization, particularly in complex phase space scenarios. A comprehensive investigation of the fundamental and third harmonic FEL radiation properties throughout the amplification process was conducted using experimental measurements and FEL simulations. Variations in pulse duration, energy, and shot-to-shot fluctuations across different wavelengths were examined. The experimental results exhibit good agreement with the chirped three-dimensional time-dependent FEL simulation, confirming the existence of frequency chirp in FEL pulses. These insights enhance our understanding of the properties of FEL fundamental and harmonic radiation, paving the way for future FEL experiments.