## 3 Photon Beamlines
### Photon Diagnostics
#### 3.3.1 Position and Intensity Diagnostics  
The first photon diagnostics devices downstream of the undulators and the beam dump is the gas-based photon beam intensity and photon beam position monitor (PBIG and PBPG). This dual-purpose device, developed by the photon diagnostics group at DESY for hard X-ray FEL pulse characterization [95; 96], uses a gas-filled ionization chamber and sets of split electrodes and multipliers to measure the position of the beam and its absolute and relative intensity on a shot-to-shot basis. The gas-based nature of the device leads to non-destructive diagnostics for photon energies between 25 and 12,000 eV, with the transverse position of every X-ray photon pulse position being measured to an accuracy of 10 \(\upmu\)m, and the beam flux measured to a relative accuracy of about 1%, and an absolute value accuracy of about 10%. The gas-based detector is always on, but is located before any optics or mirrors in the beamline. The position and flux values measured before the optics section are not the same as the values at the experimental stations, since the intensity and position of the photon beam is affected by the beamline optics. However the position and flux values of the photon pulses at the experimental stations can be easily calculated when the data from the detector is combined with the X-ray transmission values of the beamline. The next device downstream of the gas detector is the photon backscattering monitor (PBPS). The device, similar to those developed at LCLS [97] and SACLA [98], use thin CVD diamond disks with thicknesses of 30, 50 and 100 \(\upmu\)m to scatter a portion of the incoming light onto four photodiodes that are placed out of the path of the beam in a backscattering geometry (see Figure 8). In addition to the device downstream of the gas detector, SwissFEL also has a PBPS placed at the entrance to the KB mirrors before each of the experimental stations, and has the option of being placed behind the experiment. This device measures the relative flux to about 1% accuracy and the absolute position of the beam to about 10 \(\upmu\)m accuracy. The thickness of the disks means that the transmission of the incoming X-ray beam is somewhat compromised, with about 15% of the intensity lost at 5 keV. However, the transmission is higher than 90% at photon energies higher than 6 keV, and reaches 95% at energies above 8 keV. Operating the device at energies lower than 4 keV raises the risk of damaging the diamond crystals used for the backscattering due to the amount of energy that would be deposited in them from the incoming X-ray pulses. To allow the PBPS to operate a lower photon energies the disks can be switched out for thinner targets (down to 10 \(\upmu\)m) if required. The next device for profile and position measurement of the FEL beam is the photon profile monitor (PPRM). This is a Ce:YAG screen that scintillates when a photon pulse impacts it, with the image relayed to a 100 Hz camera via a mirror and a lens. This setup is destructive since no beam is transmitted through the device, though it can be used as an online diagnostic after the experimental stations where the experiment allows for this. Since this technique is common in both synchrotron [99; 100] and FEL [101] facilities worldwide it does not need further discussion. The only unique features of the device are that it sports three different thicknesses of Ce:YAG crystals (30, 50, and 100 \(\upmu\)m), and has an optical geometry developed at PSI [74] that has the mirror always being in the same spot relative to the camera, ensuring that any movement we see on the screen comes only from the motion of the beam itself and limiting the damage the mirror may suffer from the FEL beam. Additional diagnostics are available for pre-SASE beam, mainly used for the setup and commissioning of SwissFEL.