## 3 Setup and Results  
In the following, we report on several experimental results recently obtained at the SASE3 line of the European XFEL. Table 1 summarizes the main parameters and output radiation pulse energy for the various experiments. In all cases, the bunch charge was 250 pC. Since SASE3 has a period of 68 mm and consists of 21 variable-gap undulator segments, the total magnetic length amounts to 105 m [3]. After the 11 th segment, a magnetic chicane for 2-color pump-probe (2 CPP) experiments has been recently installed [8]. The undulator takes several hours to be configured and optimized for frequency mixing lasing, an operation that was carried out independently for the various experiments. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Quantity** & **Exp. 1** & **Exp. 2** & **Exp. 3** & **Exp. 4** \\ \hline \(E_{FM}\) (eV) & 500 & 1100 & 700 & 500–600 \\ \(E_{2}\) (eV) & 1200 & 2400 & 1600 & 1300–1400 \\ \(E_{1}\) (eV) & 700 & 1300 & 900 & 800 \\ \(E_{e}\) (GeV) & 11.5 & 16.5 & 14 & 11.5 \\ \(E_{r,1}\) (uJ) & - & 10 & - & - \\ \(E_{r,2}\) (uJ) & 30 & 40 & 40 & 20 \\ \(E_{r,3}\) (uJ) & 150 & 180 (no tap\({}^{*}\)) & 300 & 200 \\ \(E_{r,4}\) (uJ) & 400 & - & 1100 & 1100 \\ \(E_{r,5}\) (uJ) & 700 (no tap\({}^{*}\)) & - & 2000 (no tap\({}^{*}\)) & 2800 \\ \(E_{r,6}\) (uJ) & - & - & - & 4500 (tap\({}^{*}\)) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the main parameters for four different freqeuncy mixing experiments (Exp. 1–4) at SASE3: photon energy of the target signal \(E_{FM}\) and of the two frequencies \(E_{2,1}\), electron energy \(E_{e}\), radiation pulse energy after 1 up to n radiator segments \(E_{r,1}\ldots E_{r,n}\). The values correspond to the actual signal \(E_{FM}\), where the background due to the other colors has been substracted from the XGM reading. The comment (no) tap\({}^{*}\) means (no) tapering applied. Note that readings below 20–30 uJ are within the measurement uncertainty, and only serve as a rough estimation of the actual pulse energy. However, once an optimal configuration file is saved, it can be loaded later on, within a few minutes. For the sake of illustration, Figure 1 shows one of the configurations used during the fourth experiment, Exp. 4 (see Table 1). The first part of the undulator comprises segments 2-15 (SASE3 begins from segment number 2 and segment number 13 corresponds to the 2 CPP chicane) in an alternating sequence slightly favouring the second color at \(\omega_{2}\) (equivalent to the photon energy \(E_{2}=1400\) eV), as it corresponds to the shortest wavelength. Up to the present, it has not been not possible to keep the generation of \(\omega_{1}\) and \(\omega_{2}\) before the 2 CPP chicane, which would have been optimal in terms of longitudinal dispersion control. However, we could still optimize the chicane to obtain some gain in the bunching. In the case shown in Figure 1, the optimum delay amounted to 0.7 fs. Four segments (16-19) were used as dispersive elements with scrambled values of the \(K\) parameter optimized for optimum output, while the last four segments were tuned around 600 eV (the exact value was 604 for this case) with no taper applied. The actual values in the last line of Exp. 4 in Table 1 refer to a slightly different case of an optimum delay of 0.6 fs, two segments (16-17) used as dispersive elements, and the last six segments, tapered, used as a radiator at the difference frequency. A single X-ray Gas Monitor (XGM) device [9; 10] could be used to investigate each color separately. This was achieved while suppressing the others by detuning or opening the relevant segments. We initially optimized the second color at 1400 eV in the first part of the undulator (up to segment 16) in normal SASE mode, removing every taper, and having care of avoiding saturation.