Techniques for Pump-Probe Synchronisation of FSEC Radiation Pulses

The production of ultra-short photon pulses for UV, VUV or X-ray Free-Electron Lasers demands new techniques to measure and control the arrival time of the FEL-pulses. For pump-probe experiments using optical-lasers, the desired synchronization between the pump and the probe pulses is on the order of tens of femtoseconds. This corresponds to the typical duration of the FEL pulses. Since accelerators are large scale facilities with a length of several hundred meters or even kilometers, the problem of synchronization has to be attacked twofold. First, the RF acceleration sections upstream of the magnetic bunch compressors need to be stabilized in amplitude and phase to high precision. Second, the remaining electron beam timing-jitter needs to be determined with femtosecond accuracy for off-line analysis. In this paper, several techniques of measuring the FEL beam arrival-time are presented.