## 2 Overview European XFEL
### The FEL Undulator Sources  
The initially three FEL undulator sources denoted SASE1, SASE2 and SASE3 will provide FEL radiation ranging from the carbon K-edge to very hard X-rays for user experiments. SASE1 and SASE2 serve the hard X-ray regime from approximately 3 to 25 keV in the first harmonic. SASE3 produces soft X-rays from approximately 250 eV to 3 keV. These ranges are achieved by a combination of electron energy set points and gap tuning (see Table 2). The lengths of the FEL undulators have been determined after simulation of the saturation length for the highest photon energy at the largest considered electron beam emittance. All FELs are therefore much longer than the saturation length in the middle of the tuning range, around 10 keV (SASE1 and SASE2) or up to 1 keV (SASE3), which allows for special modes of operation, e.g., the implementation of self-seeding. In addition, each FEL  
Figure 2: Time pattern of the electron bunch train in the linac. The RF field is pulsed with 10 Hz and has a flat top region of \(\sim\)1.2 ms clearly exceeding the duration of the electron bunch train of 600 \(\upmu\)s. The bunch train can be separated into portions with different function. The header H is typically used for fast intra-bunch feedback. The next portion S is dedicated for the South branch with SASE2. Following a short gap to switch the flat top kicker magnet the last portion N will be send to the North branch with SASE1 and SASE3. The smallest separation of electron bunches is 222 ns in standard operation, corresponding to 4.514 MHz and up to 2700 bunches per train. Operation at bunch separations of 886 ns (1.128 MHz) and 10 \(\upmu\)s (0.1 MHz) is possible, too. has additional space before and after the device for optional extensions, e.g., for laser-driven beam manipulation or so-called afterburners. All FELs are segmented into 5 m long planar undulators and 1.1 m long intersections. Undulators are equipped with permanent hybrid NdFeB magnet technology for a minimum magnetic gap of 10 mm allowing the use of aluminum vacuum chambers with an inner opening of 8.8 mm for the electron beam. Out of vacuum magnetic structures were chosen to minimize radiation damage of the magnets, but also to reduce resistive wall wake fields. The intersections carry a quadrupole for electron beam focusing, a phase shifter for matching the radiation field and the micro-bunched electron beam, an electron beam position monitor, and vacuum devices. All parts for the total 91 undulator segments have been produced and assembled by industry. Magnetic tuning was performed by the European XFEL undulator group using the pole-height tuning technique [25]. The FEL source properties depend on a large number of parameters, not only of the FEL undulators, but also the electron beam properties, e.g., peak current, emittance, or energy spread. Table 3 shows simulation results for the saturation point for a selection of photon energies and electron beam parameters. The full set of properties can be found in refs. [26; 27]. In practice, FELs are often operated well beyond saturation thereby boosting the emitted pulse energies but also sacrificing some of the other properties. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **SASE1/SASE2** & **SASE3** \\ \hline Period length (mm) & 40 & 68 \\ Maximum B-field (T; @10 mm) & 1.11 & 1.68 \\ Number of poles per segment & 248 & 146 \\ Number of segments & 35 & 21 \\ Total system length (m) & 205 & 121 \\ Gap range (mm) & 10–20 & 10–25 \\ K-parameter range & 1.65–3.9 & 4–9 \\ Photon energy range (keV; @8.5 GeV) & 1.99 1–7.2 & 0.243–1.08 \\ Photon energy range (keV; @12 GeV) & 3.97–14.5 & 0.485–2.16 \\ Photon energy range (keV; @17.5 GeV) & 8.44–30.8 1 & 1.031–4.6 1 \\ \hline \end{tabular} \({}^{1}\) Beam transport does not allow reaching these photon energies.  
\end{table} 
Table 2: FEL undulator source parameters [24].