Single-shot arrival timing diagnostics for a soft X-ray free-electron laser beamline at SACLA

Arrival timing diagnostics performed at a soft X-ray free-electron laser (FEL) beamline of SACLA are described. Intense soft X-ray FEL pulses with one-dimensional focusing efficiently induce transient changes of optical reflectivity on the surface of GaAs. The arrival timing between soft X-ray FEL and optical laser pulses was successfully measured as a spatial position of the reflectivity change. The temporal resolution evaluated from the imaging system reaches similar to 10 fs. This method requires only a small portion of the incident pulse energy, which enables the simultaneous operation of the arrival timing diagnostics and experiments by introducing a wavefront-splitting scheme.