Characterization of free-electron laser third harmonic radiation at FLASH

The Free-electron LASer in Hamburg - FLASH - generates intense ultrashort pulses with femtosecond duration in the range of 3.3 - 90 nm in the fundamental. In addition, higher harmonic contributions are always present in the spectral distribution, extending this range to much shorter wavelengths. This study presents a detailed exploration of the evolution of the radiation properties of the third harmonic contribution in the self-amplified spontaneous emission (SASE) amplification process. Utilizing a terahertz streaking setup, we measured and analyzed the pulse durations and energies of the fundamental and third harmonic during the amplification process under various operational configurations and compared these with results from the 3D time-dependent free electron laser (FEL) simulation code FAST.