A simple method for timing an XFEL source to high-power lasers

We propose a technique, to be used for time-resolved pump-probe experiments, for timing an x-ray free electron laser (XFEL) to a high-power conventional laser with femtosecond accuracy. Our method takes advantage of the same electron bunch to produce both an XFEL pulse and an ultrashort optical pulse with the help of an optical radiator downstream of the x-ray undulator. Since both pulses are produced by the same electron bunch, they are perfectly synchronized. Application of cross-correlation techniques will allow to determine relative jitter between the optical pulse (and, thus, the XFEL pulse) and a pulse from an external pump-laser with femtosecond resolution. Technical realization of the proposed timing scheme uses an optical replica synthesizer (ORS) setup to be installed after the final bunch-compression stage of the XFEL. The electron bunch is modulated in the ORS setup by an external optical laser. Subsequently, it travels through the main undulator, and produces the XFEL pulse. Finally, a powerful optical pulse of coherent edge radiation is generated as the bunch passes through a long straight section and a separation magnet downstream of the main undulator. Our study shows that at a moderate (about 10%) density modulation of the electron bunch at the location of the optical radiator allows production of high power x-ray and optical pulses. Relative synchronization of these pulses is preserved by using the same mechanical support for both x-ray and optical elements transporting radiation down to the experimental area, where single-shot cross-correlation between optical pulse and pump-laser pulse is performed. We illustrate the potential of the proposed timing technique with numerical examples referring to the European XFEL facility.