# Two-Color Operation of a Soft X-ray FEL with Alternation of Undulator Tunes
## 4 Simultaneous Measurements of Two Colors
### Measurements of Temporal Properties  
In order to gain information about the pulse duration of the two XUV pulses, we used the THz streaking technique [29; 30]. THz streaking uses a noble gas target that is photoionized by the FEL pulses. The generated photoelectrons propagate in the time-varying electric field of the co-propagating THz radiation. After interaction with the terahertz field, the photoelectrons have changed their momentum component in the direction of the field. If the electron wave packet is short compared to the period length of the THz field, the temporal structure of the wave packet will be mapped onto the kinetic energy distribution of the emitted electrons and can be used to determine the pulse duration. Figure 8: Two-color OPIS measurement while scanning color 1 from \(\lambda_{1}\)= 13.6 nm to 20.6 nm. Color 2 was kept at \(\lambda_{2}\) = 22.6 nm. The time of flight directly translates into photon beam wavelength. The OPIS was operated with argon as a target gas. The main features to which the wavelength labels are assigned to are the Ar 3p photoelectron lines. A retardation voltage of 25 V had been applied to the flight tube. Figure 7: Correlation plot of pulse energies at 15 nm and 17 nm measured with the grating spectrometer. The measurements were performed at the dedicated photon diagnostic beamline FL21, which is equipped with a permanently installed THz streaking setup (for details, see e.g., [31; 32]). For the shown data, the FEL was operated at wavelengths of \(\lambda_{1}=15\) nm (83 eV) and \(\lambda_{2}=19\) nm (66 eV). Using Ne as target gas leads to kinetic energies of 61 and 45 eV for the 2p photoelectron lines, resulting from the two different wavelengths. Figure 9 shows the kinetic energy of the two photoelectron lines as a function of the delay between the THz field and the XUV pulse. This delay scan depicts the vector potential of the THz field and shows the linear slope which is used to map the temporal information into kinetic energy. Acquiring data at a fixed delay in the middle of the linear slope, we can determine the pulse-to-pulse variation of the pulse duration by monitoring the broadening of the streaked photoelectron line with respect to an unstreaked reference (THz off). The result for the two wavelength is shown in Figure 10. The averaged pulse duration of \(\lambda_{1}\) is 70 \(\pm\) 12 fs and 90 \(\pm\) 12 fs for the second wavelength, respectively. The given error denotes the RMS width of the distribution of the pulse durations. Analyzing the shot-to-shot fluctuations, it turns out that the pulse duration as well as the pulse energy (determined as integral over the photoelectron line) fluctuate independently for the two colors. No correlation was found; this is the consequence of the fact that FEL operated in a linear regime. The method of THz streaking also allows for determining the arrival time of the XUV pulses with respect to the THz [31]. Here, the technique can be used to investigate the relative arrival times between the two-color pulses. First of all, a temporal shift between the two streaking curves in Figure 9 indicates an average offset of the arrival time of the  
Figure 10: Measured pulse duration for 3000 FEL pulses. The shorter wavelength \(\lambda_{1}=15\) nm (red) shows a shorter pulse duration on average of 70 fs FWHM, while the pulse duration of \(\lambda_{2}=19\) nm (blue) is 90 fs FWHM. Figure 9: The plot shows the vector potential of the THz streaking field recorded in a delay scan. The THz pulses were temporally scanned with respect to the XUV pulses. For each delay, 30 photoelectron spectra were recorded. For the pulse duration analysis, the spectra in the middle of the linear slope (delay \(-0.2\) ps) were used. One can also see the broadening of the spectral lines with respect to the larger delays (with very low THz field), containing the information about the pulse duration. two pulses.