Abstract
We have developed a timing control system to stabilize the long-term timing drift between X-ray free-electron laser (XFEL) and optical laser pulses using an out-of-loop balanced optical-microwave phase detector and an arrival-timing monitor for pump-probe experiments at the SPring-8 Ångstrom Compact free-electron LAser (SACLA). The timing jitter and drift between the XFEL and the optical laser pulses have been reduced to less than 50 fs (RMS) over ∼49 h. The performance of the timing stabilization system was investigated by measuring the correlation of the long-term simultaneous timing monitoring on two branches of BL3 over 8 h. A linear correlation was observed with an RMS error of 8.6 fs.