# Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL
## 6 Conclusions  
We reported on the status of the European XFEL two-color project at the soft X-ray SASE3 undulator. The simplest two-color setup, based on Figure 1, will be installed and made available to users in 2021. This scheme will provide two femtosecond soft X-ray pulses of different colors, separated in time by up to 3 ps, depending on electron beam energy.  
The more advanced setup depicted in Figure 2a, based on the addition of an optical delay line, can be implemented in order to enable zero-crossing of both pulses in the time domain. Pulse durations of about 1 fs can be achievable in the 20 pC low-charge operation mode, hence the \"low-charge\" scheme name. The maximum negative delay would be \(-85\) fs, while the positive delay would depend on electron beam energy: from 1550 fs at 8.5 GeV to 250 fs at 17.5 GeV. As a possible upgrade alternative, the fresh slice generation scheme, see Figure 2b, could be chosen, allowing 3100 fs and 730 fs of positive delay, respectively. The range for negative delays would depend on the precise electron beam properties and chosen photon energies, resulting in about \(-10\) fs.  
Regardless of the technique, both pulses are emitted in different locations, separated by about 50 m along the undulator. Therefore, upon refocusing, the waists of both pulses at the sample location will also be separated. The desired ratio of photon density on the sample may be optimized by adjusting the settings of the KB focusing system. Experiments can be modeled by simulating the generation as well as the transport of the two-color FEL pulses from the source up to the sample position. In order to illustrate this possibility we presented simulations for radiation generation and transport at SASE3 for a particular pump-probe experiment, whose scientific relevance has been discussed in Section 3. Two-color generation and delivery can be tailored for each experiment. In fact, pulse duration and photon density on the sample strongly depend on the generation technique, on the number of active undulators, on the distance between the sources of undulators as well as on settings of the KB mirror system and location of the sample with respect to the waists of each focused pulse.  
Finally, we reported on the first realization and observation of generating two temporally superimposed two-color pulses of photon energies about 700-1000 eV and pulse energies up to 1 mJ at the SASE3 undulator line. Radiation spectra were diagnosed with both grating spectrometer and a gas-based time-of-flight photo-electron spectrometer (PES). The latter can be used as a non-invasive diagnostic device to determine pulse energies of both colors on a shot-to-shot basis.  
Conceptualization, V.K., E.S. ([PERSON]) and G.G.; Data curation, S.S., S.K., E.S. ([PERSON]), M.Y., I.Z.; G.G.; Funding acquisition, E.K.; Investigation, S.S., N.G., J.G., A.K., J.L.; T.M. ([PERSON]); Methodology, M.H., E.K., T.M. ([PERSON]) and M.M.; Resources, W.D., L.F., M.G., M.H., Y.K., M.S., T.W., M.V.; Software, S.S., K., E.S. ([PERSON]), S.T., M.Y., I.Z., G.G., P.L.; Supervision, G.G.; Writing--original draft, S.S., N.G., E.K., J.L., T.M. ([PERSON]), M.M.; G.G. All authors have read and agreed to the published version of the manuscript.  
The Authors acknowledge the help of European XFEL and DESY support groups in the technical developments of the setup.  
The instrumentation of the two-color project at the European XFEL is funded by the Academy of Finland with co-funding by the European XFEL.  
The authors declare no conflict of interest.