## 3 Photon Beamlines
### X-ray Optics  
Free Electron Lasers are by design single user machines in the sense that every electron bunch serves only once as a source of light. This is in contrast to storage rings, where a finite number of bunches circulate for many hours, or even days, and repetitively deliver photons to many beamlines in parallel. From this it is evident that FELs can supply only one beamline at a time, meaning that beamtime is quite expensive and in high demand. To make full use of the valuable beamtime it is mandatory to have several experimental setups in parallel with the ability to switch between them in a fast and reproducible way. The beamline design [88] has to provide the ability to alternate between several experimental stations, while giving full access to the unused experimental hutches. This is achieved with the help of pairs of offset mirrors (OM) in front of the beamline as shown in Figure 6. They are located as far upstream as possible to achieve a reasonable lateral separation at the end. The beam is subsequently directed towards two independent double crystal monochromators (DCM) and finally focused by sets of bendable Kirkpatrick-Baez mirrors [89] (KB) to the experiments. Two instruments are installed in line at experimental station Alvra on Aramis-1 (see Section 4.1) and will be operated by changing the focal distance of the KB system. A similar pair of KB mirrors are installed at experimental station Bernina on Aramis-2, but as instrument exchange is accomplished using a rail system perpendicular to the photon beam, all three planned instruments can take advantage of the minimum focus of the mirrors (see Section 4.2). A pair of harmonic rejection mirrors (HRM) after the DCM is foreseen in  
Figure 5: Measured SwissFEL pulse energy gain curve as a function of inserted undulator modules at an electron energy of 900 MeV (left). Saturation is reached after insertion of 9 modules. Pulse energy measured with the gas-based photon beam intensity and position monitor (see Section 3.3.1). An image of the photon beam at saturation, as recorded with a Ce:YAG scintillating screen, is shown on the right. the Aramis-1 beamline from the beginning and as an optional extension for the Aramis-2 beamline. The Aramis-3 beamline is not yet specified and is under design concept phase in combination with the third experimental station, Cristallina. As one option, a simple pink beam beamline without monochromator and nanometer focusing at the end is considered and is shown in Figure 6.  
A set of two horizontally deflecting offset mirrors direct the beam into the Aramis-1 beamline with a total deflection angle of 12 mrad. The central Aramis-2 beamline stays in the direction of the FEL beam and uses two vertically deflecting offset mirrors in a zigzag geometry. For Aramis-3 a second pair of horizontal offset mirrors with a total deflection angle of 8 mrad is considered as worst case scenario with respect to spatial restrictions along the beamlines and the end stations. All offset mirrors are coated with low- and mid-Z materials to prevent single shot damage by the intense FEL beam [90; 91; 92]. However, this limits the deflection angle and two mirrors with small deflection angles instead of a single one with large deflection angle became necessary. The mirrors are coated with two bilayers leaving a blank area of uncoated silicon between them. The low-Z bilayer, 10 nm B4C on top of 36 nm SiC, should withstand the FEL beam at all photon energies and reflect over the full energy range of the FEL. Its B4C-layer is effective at lower photon energies and prevents the drop in reflectance at 1.8 keV due to the silicon K-edge absorption of the subjacent SiC-layer. At higher photon energies the B4C layer becomes transparent and reflectance is supported by the SiC layer with a critical energy of 12.4 keV. The mid-Z bilayer, 15 nm B4C on top of 20 nm Mo, extends the operation range up to the Mo K-edge at 20 keV but has a higher damage risk and may be used only at reduced fluence.