## 3 Linear Streaking Measurements at Free-Electron Lasers
### Solutions and experience  
The experiments performed with hard X-ray streaking at FELs have used photon energies as high as 10 keV [109; 110], and had the challenge of performing the streaking measurements for single FEL pulses. The experiment was performed at SACLA, which works at a repetition rate of 30 Hz and pulse energies between 150 \(\upmu\)J and 300 \(\upmu\)J. The choice of a gaseous target with suitable electron binding energies at 10 keV photon energy is limited, and the closest easily reachable noble gas absorption edge is that of the xenon L shell. In this particular case, the Xe 2p\({}_{3/2}\) photoionization line was the best suited for the measurements, as it had the largest cross section at this photon energy, and xenon was easily available for the experiment. To ensure that the number of photoelectrons generated by the ionization process was as large as possible, while keeping the mean background pressure in the vacuum chamber low enough for safe MCP operation at the same time, the experimental setup used pulsed piezo valves [111] that were synchronized to the FEL trigger signal and allowed short 15 \(\upmu\)s (FWHM) bursts of xenon gas into the chamber very close to the interaction region. A large 1200 L/s pump was placed directly below the gas jet to suck away the particles as quickly as possible and additional differential pumping was installed for the TOFs. The valve was backed by xenon gas at a pressure between 2 bar and 3 bar above atmosphere and was equipped with a conical nozzle with a 150 \(\upmu\)m exit diameter, which forced the gas atoms to cluster upon entering the chamber. This arrangement both increased the local density of the atoms at the point of photoionization due to the formation of large, thousand-strong Xe clusters, and prevented a large broadening of the gas jet as it streamed from the nozzle into the vacuum [112]. Overall, the choice of xenon gas as the interaction medium together with the specific nozzle type ensured that there was sufficient signal for single-shot measurements even at 10 keV, and worked well even when the intensity of the X-rays was reduced by introducing a monochromator into the beam path. Moreover, a non-streaked TOF photoionization measurement was implemented upstream of the THz streaking region, since both the bandwidth and the photon energy of FEL pulses change from shot to shot. The availability of a calibration measurement for every shot makes the evaluation of the single-shot arrival time jitter and the temporal characteristics of FEL pulses much easier. Because the kinetic energy of electrons set free at 10 keV X-ray photon energy amounts to about 5200 eV for the Xe 2p\({}_{3/2}\) shell, the streaking shift in electron momentum according to Equation (1) is much larger than that experienced with soft X-rays, even when fairly weak THz fields are applied. In the experiments performed in [109; 110], the electric field strength of the focused THz beam in the interaction region was only between 5 MV/m and 6 MV/m, but the maximum streaking energy shift observed was about 100 eV. To accommodate this large shift, the TOFs had to be designed to be able to detect electrons over a large energy window with constant resolution over the whole range and special care had to be taken to not accidentally cut off any signal by choosing the retardation potentials on the TOFs too high. For example, though the kinetic energies of streaked photoelectrons shifted by \(\pm\)100 eV lie between 5100 eV and 5300 eV in this case, the retardation potentials were set to an upper limit of about 4850 V to ensure that no signal was lost and the spectral resolution was not compromised due to contributions from the full FEL bandwidth or dispersive focusing effects of the TOF. In addition to these special components, the setup also included the standard components for a THz streak camera. A parabolic mirror with a centered hole inside the vacuum chamber focused the collimated THz beam collinearly with the FEL propagation direction onto the streaking interaction region.