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A beam-monitor system for XFEL/SPring 8, ''SACLA,'' has been constructed. In order to maintain a stable self-amplified spontaneous emission (SASE) interaction, the straightness and overlap of the axes to within 3 m between the electron beams and the radiated x rays for an undulator section of about 100 m length is necessary. This straightness means relative alignment to an experimental target sample. Furthermore, a temporal stability of 30 fs in order to maintain a constant peak beam current is also necessary to conduct stable SASE lasing. The monitor system was developed to satisfy these spatial and temporal stability and resolution criteria. The system comprises spatial monitors, such as cavity-type beam-position monitors and screen monitors, as well as temporal measurement instruments, such as current monitors, waveguide spectrometers, coherent synchrotron-radiation detectors, a streak camera, and an rf deflector. Commissioning of SACLA started from March 2011, and the monitors performed sufficient roles to tune the beams for lasing. The achieved overall performances of the system, including data acquisition, are: the beam position monitor has a spatial resolution of 600 nm in rms; the bunchlength monitors show ability to observe bunch lengths from 1 ns in an injector with velocity bunching to less than 30 fs after three-stage bunch compressors. The less than 3 m spatial resolution of the screen monitor was also confirmed during practical beam operation. Owing to these fulfilled performances, such as the high spatial and temporal resolutions, stable lasing of SACLA has been achieved.

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