## 3 Linear Streaking Measurements at Free-Electron Lasers
### Solutions and experience
The schematic setup of the experiment from [110] is shown in Figure 22.  
The experiment conducted in [110] compared the arrival time measurements of the spatial-encoding SACLA timing tool and the PALM, finding that the two agreed to within about 17 fs of each other at 9 keV photon energy. Earlier measurements presented in [109] showed a better arrival time agreement, which can be explained by the altered geometry for the PALM in [109]. The entrance opening to the TOF was twice as far away from the interaction region than ideal due to a manufacturing issue with the PALM chamber, which affected the number of electrons that weredetected. This in turn caused the peaks to be less well-defined and reduced the accuracy of the measurement accordingly. The measurements are presented in Figure 23.  
Figure 23: Comparison of the spatial-encoding timing tool with the Photon Arrival and Length Monitor (PALM) at SPring-8 angstrom compact free-electron laser (SACLA) (reproduced with permission from [110], Copyright Optical Society, 2017). (**a**) compares the SACLA timing tool (blue solid line) and the PALM (red dashed line) arrival time measurements for a set of consecutive pulses; (**b**) shows the correlation \(\rho\) between the two methods for several thousand shots; (**c**) shows the histogram of the deviations between the two methods from (**b**). Figure 22: THz streak camera setup for experiments in [110] (reproduced with permission from [110], Copyright Optical Society, 2017). To demonstrate the flexibility of the PALM, two-color mode measurements at SACLA showed that the PALM could also measure the temporal separation of multiple FEL pulses. In this mode, the FEL was set to create two FEL pulses at photon energies of 9 keV and 8.8 keV, with the ability to shift their relative arrival times with respect to each other by manipulating the X-ray generating electron beam with the help of a magnetic chicane. The average rms jitter of the two pulses with respect to each other at a given delay setting was about 31 fs, of which about 17 fs are attributed to the uncertainty of the PALM measurements. These results are presented in Figure 24.  
The experiment presented in [109] also checked the streaking effect at several photon energies ranging from 5 keV to 9 keV. The goal was to observe the streaking shift, and the accompanying streaking slope, get stronger or weaker depending on the X-ray photon energy. The experiment showed the expected relationship between the accuracy of the arrival time measurement and the kinetic energy of the electrons ionized by the FEL pulse, shown in Figure 25.  
In conclusion, the experiments on THz streaking of hard X-ray pulses worked as expected and the signal strengths on a shot-to-shot basis, as reported in [109], were more than sufficient for single-shot temporal measurements. Based on these experiments, there seems to be no reason why the technique could not be extended to even higher photon energies, and ideas for adapting the device to handle larger jitters between the FEL laser and the experimental laser are also being considered [113].