Determination of the XUV Frequency Chirp at the Free-Electron Laser FLASH via THz Streaking and Electron Beam Diagnostics

Free-electron lasers (FELs) operating in the extreme ultraviolet (XUV) and X-ray regions deliver ultrashort pulses with unprecedented intensity, enabling groundbreaking research across various scientific disciplines. A potential chirp (frequency change within the pulse) of these pulses influences their spectral properties, directly impacting the experimental outcomes and FEL performance. The accurate characterization of the chirp is, therefore, important for optimizing FEL operation and interpreting experimental results. This study presents a comprehensive comparison of two techniques determining the chirp of the XUV pulses at FLASH by directly measuring the XUV pulses with THz streaking and by detecting the chirp of the electron bunches by a Transverse Deflection Structure (PolariX TDS) to infer the XUV chirp. We conducted simultaneous measurements using both techniques at FLASH2 while tuning the FEL to produce various energy chirps on the electron bunch.