# Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL
## 5 Results of First Measurements  
First performance tests of the two-color setup at the SASE3 line of the European XFEL (Figure 1) have been carried out without the chicane, which is expected to be installed in 2020. We used a 250 pC electron beam at the energy of 14 GeV. The SASE3 undulator was divided into two roughly equal parts, and tuned to different photon energies, ranging between 700 eV and 1 keV, selected such that the two emission spectra did not overlap. Typical measured pulse energies were above the 350 uJ-range for each of the two colors. Figure 12a shows the radiation spectrum for the case of 835 eV emitted in U1 and 805 eV emitted in U2 with an average pulse energy of about 1.2 mJ in each color. Subfigures (b), (c) and (d) indicate stability of pulse intensities, photon energies and absence of significant correlation due to the presence of a large number of statistically independent SASE modes.  
The spectra were measured using a soft X-ray grating spectrometer which makes use of the SASE3 monochromator beamline [32]. This is achieved by introducing a YAG:Ce crystal at the exit slit plane; the crystal converts X-rays into optical luminescence, which is detected by a CCD or MCP-coupled CCD. If not limited by the detection system (imaging crystal and CCD), the performance of the spectrometer is similar to the performance of the SASE3 beamline monochromator [34]. The grating spectrometer, although providing high resolution up to \(5\times 10^{3}\), is an invasive device with a limited  
Figure 11: Simulated radiation and electron beam properties at the end of U2 for the fresh-slice technique. Top subplot: electron beam current (dashed black line) and radiation power (green line). Middle subplot: mean electron beam energy (blue line), energy spread \(\pm\sigma\) (area within the dashed red lines) and density modulation factor—bunching—(gray line). Bottom subplot: on-axis radiation spectrum. The head of the beam is located on the right, and emits after the tail with a photon energy of 1980 eV.  
spectral window which does not allow to detect two pulses simultaneously in case they are largely separated in photon energy.  
In a second campaign a set of 16 Time-Of-Flight (TOF) Photo-Electron Spectrometers (PES) [35], which are arranged around the intersection volume of the FEL radiation and a gas-phase target was used for the experiments to overcome this issue. A subset of the PES TOFs was optimized for the detection of the first color (950 eV) and another subset for the second color (910 eV). This arrangement allows for simultaneous non-invasive shot-to-shot detection of both colors, as shown in Figure 13. A more detailed description of the diagnostics system used can be found in Appendix B.  
Figure 12: Experimental demonstration of two-color lasing: (**a**) single shot spectral distribution measured with grating spectrometer (grey line) and average over 3100 consecutive events (black line). The light grey area shows the span of spectral densities in the ensemble; the 835 eV pulses were emitted in U1, and the 805 eV pulses in U2, as indicated with numbers “1” and “2”; (**b**) integrals under the spectra for both colors over time; (**c**) positions of the spectra over time; (**d**) correlation between the two integrals depicted on subfigure (**c**).  
Figure 13: Single shot spectra measured with grating spectrometer (black line), with PES optimized for 910 eV emitted in U2 (red line) and with PES optimized for 950 eV emitted in U1 (blue line). An average pulse energy was about 600 \(\upmu\)J in each color.