Temporal characterization on FLASH FEL Pulses

Since the free-electron laser FLASH at DESY in Hamburg, Germany lases in Self-Amplified Spontaneous Emission (SASE) mode each pulse is “unique” and has different pulse energy, XUV spectrum and pulse duration. Due to very small fluctuations in the acceleration, the arrival time of the XUV pulses jitters within several tens of femtoseconds. A Terahertz (THz) field driven streak camera has the potential to deliver single-shot pulse duration information basically wavelength independent and with a high dynamic range (in pulse duration and FEL energy) and it is able to be operated with repetition rates up to several hundred kHz (potentially even MHz). In addition, it can provide arrival time information between the XUV pulse and the laser driving the THz generation for each single pulse with accuracy well below 10 fs resolution. In this paper, several different FEL operation settings have been used to commission the technique in a wide range of pulse durations from 200 fs to less than 20 fs (FWHM). For each setting the FLASH single-shot pulse duration as well as the arrival time of the XUV pulses with respect to the THz generating optical laser was measured. A time sequence of 5 minutes shows the inherent fluctuations of the pulse duration. As a second important result of the commissioning experiments, the assumption that the electron beam arrival time monitor (BAM) measuring the arrival time of the electron bunches with high precision in the FLASH accelerator is verified as an acceptable measure for the arrival time of the XUV photon pulses in all measured cases. The arrival time of the electrons determined almost 200 meters upstream of the experimental hall in the accelerator tunnel (with respect to the FEL master timing) is in good agreement with the arrival time of the XUV pulses with respect to the pump-probe laser (generating the THz and also synchronized to the master timing) at the experiment. Shot-to-shot arrival time measurement shows a correlation width of 20 fs rms. Thus, the time resolution of user pump-probe experiments (using the pump-probe laser) can be significantly improved by sorting their data with the arrival times measured by the BAM.