## 3 Single-Particle Imaging with X-ray Lasers
### Light Source and Instruments
Two pairs of pnCCD detectors are located in different positions downstream of the IR; (**b**) For the CXI instrument, each chamber is colored to match its designed KB mirrors. A timing tool (TT) is designed to obtain the fine time between X-ray laser and pump laser. (Figures were reproduced from Reference [145; 146] with permission from International Union of Crystallography/IUCr). Another principle for the photon energy selection is the optimized X-ray energy range for specified experiment instruments. For different X-ray instruments at LCLS or different beamlines at SACLA, the optimized photon energy varies. The commonly used photon energy for SPI is 1700 eV that just below the Si K-edge at AMO, 7 keV at CXI that below the Fe edge of 7.1 keV, LCLS. Also, the photon count ability of detectors has also been considered during the photon energy selection. Other photon energies, such as 1250 eV, 1800 eV, 5 keV and 10 keV have also been used and tested, with varying degrees of success. A balance has to be struck between the resolution and scattering efficiency, and the ability for the various detectors to count photons with a reasonable signal to noise ratio. High harmonics are an inevitable aspect of XFEL sources. Radiation at the fundamental wavelength dominates but appreciable levels of high harmonics also present at the 1% level. These harmonics may extend the capability of instruments but also may contribute background to experiments. The Soft X-ray Offset Mirror System (SOMS) and Hard X-ray Offset Mirror System (HOMS) located in the Front End Enclosure of LCLS have harmonic cutoffs at ~2.5 and ~25 keV respectively. The SOMS operate in the FEL energy range from 0.20 keV to 2.0 keV and the reflectivity is above 90% [151]. The system reflectivity above 2.48 keV is below 20%, which contributes to suppressing the third harmonic FEL peak [152]. The fundamental operation energy range for HOMS is 2.0-12.0 keV, and be able to deliver 2-25 keV photons up to the third harmonic. From former measurement, it is known that the second harmonic content in normal operation at soft X-ray region (near 1 keV) is 0.04-0.1%, and the third harmonic is as high as 2.0-2.5% [153]. The third harmonic for hard X-ray wavelength (6\(\sim\)8 keV) is 0.2-2%. The reflectivity for the second and third harmonics are different for the downstream KB mirrors located at the experiment station, which provide some level of harmonic rejection. The third harmonic can produce undesired noise, for example via fluorescence by crossing above an absorption edge such as iron, a material making up most of the vacuum chamber. It is also less efficiently block by apertures and slits and can again increase the background noise. Knowing and optimizing the focused beam size and the wavefront is another key aspect of SPI experiments [154]. The position and angles of KB mirrors should ideally be aligned for every SPI experiments. Imprint methods have been employed for the AMO and CXI instruments to find the best focusing position [155; 156]. This is a time-consuming procedure for the current experiments. Two to three hours are usually required. According to the nature of SASE mode, the beam position and intensity fluctuates shot-to-shot. The SPI initiative has also taken the phenomenon into consideration. A backscattering position and intensity monitor was used for diagnostic for the CXI instruments [157]. The parasitic scattering of particle injectors is the primary source of scatter, the team is still working on this problem. Apart from particle injector scattering, the left stray scattering comes from the upstream components of the beamline. For the AMO instrument, the parasitic scattering comes from the roughness of KB mirrors. For the CXI instrument, the stray scattering comes from the diamond window that is located downstream of the KB mirrors to protect the mirrors from particles. The detailed information will come out soon by the SPI team. For SPI experiments, the alignment of apertures is also of great importance.