# Opportunities for Two-Color Experiments in the Soft X-ray Regime at the European XFEL
## Appendix A Design Concept  
The two-color setup at SASE3 should allow for scanning from negative to positive temporal delays between the two X-ray pulses. As discussed, we proposed to achieve this goal with the introduction of an Optical Delay Line (ODL) or with the exploitation of a fresh-slice technique. Because of this, we consider two symmetrical designs for the magnetic chicane: with and without the ODL, located between second and third dipole magnets, as illustrated on Figure 11. Both designs assume 0.6 m-long magnet yokes with coils extending by 6 cm from each side of the magnets.  
In case the negative delay is enabled by a fresh-slice technique, the magnetic design with 15 cm separation between the central magnets (Figure 11, top picture) would allow to significantly delay the electron beam, and hence to separate the radiation pulses by up to 3 ps, assuming an electron beam energy of 8.5 GeV and dipole magnetic field of 1.2 T, see Figure 12 (left plot).  
If, instead, the negative delay is obtained by installing a compact ODL, the large distance of 135 cm between the middle dipoles (Figure 11, bottom picture) necessary to accommodate the optical chamber would limit the maximum electron beam delay to about 1.7 ps. The ODL consists of four 10 cm-long mirrors, see Figure 13. The electron beam is deflected horizontally, the radiation vertically. Such design allows to introduce a fixed delay of 152 fs to the U1 radiation. The resulting temporal separation between the two radiation pulses, presented on Figure 19 (right plot) is the sum of the tunable positive electron beam delay and a fixed negative radiation delay.  
When the ODL is introduced, minimal spatial separation between the electron beam and the edges of the mirrors should be respected. An offset from the optical axis of around 5 mm corresponds to 67 fs delay of the electron beam in U2. If we limit ourselves to a minimum distance between the electron beam and the mirror surface of 4 mm (the radiation footprint is located around 1 mm from the mirror edge), then the _maximum negative_ separation of radiation pulses would be \(-152+67=-85\) fs for all electron beam energies. The _maximum positive_ delay is the electron beam retardation with deduced 152 fs of optical delay: 1550 fs at 8.5 GeV and 250 fs at 17.5 GeV, see Figure 19 (left plot).  
The cut-off incidence angle of the B4C mirror coating material is 0.6 degrees at 3 keV (maximum nominal photon energy at SASE3). We choose 0.5 degrees incidence angle to maximize the total reflectivity. The latter reaches 80% at 300 eV and 90% at 3 keV for 4 mirrors.  
The mirror length is chosen to avoid geometrical clipping and related diffraction effects on the radiation at the most divergent photon energy of 250 eV. Given the distance from the radiation source (around the middle of the upstream undulator segment) to the first mirror of around 5.6 m, we can calculate the transverse radiation spot size at the mirror position and the consequent geometrical transmission. The latter exceeds 99% at 250 eV.  
Assuming 100 uJ of incidence pulse energy, the estimated radiation dose on the mirror coating does not exceed 0.1 eV/atom for 1 opened cell upstream the chicane at photon energies up to 750 eV and for 2 opened upstream cells--up to 2000 eV.  
Further investigation on the actual implementation of the ODL is being conducted.