![]() In this work, we report the first direct measurement result of the coherence time of ultra-fast hard x-ray FEL pulses through a conceptually different approach. Inspired by this idea, we propose the method utilizing the cross-correlation between the x-rays and the microbunched electrons to characterize the coherence time of hard x-ray FEL pulses. However, since the electrons are almost ‘fresh’, the coherence information is smeared out. Meanwhile, x-ray pulse duration characterization method based on FEL dynamics has been well established 16, 17, in which the cross-correlation between ‘fresh’ electrons and x-rays has been used to measure the x-ray pulse length. ![]() Hence, currently, there is no effective method to characterize the coherence time of SASE FEL in the time domain. Therefore the measured coherence time will be much longer than the intrinsic SASE FEL coherence time. However, this method cannot be implemented to SASE FEL coherence time characterization, since these crystals lead to external strong purification on the incident pulse spectrum, due to Bragg diffraction. Experiments employing crystals as mirrors to generate an effective delay has been carried out to measure the coherence time of a monochromatized hard x-ray pulse 15. Although it has been proven that a combination of laser beam splitter and mirror based optical delay can be used to implement autocorrelation to characterize the FEL coherence time in the extreme ultraviolet and soft x-ray regime 13, 14 for the hard x-ray pulses, due to the lack of mirrors, it is very difficult to realize autocorrelation. Obviously, autocorrelation depends on the mirrors to generate optical delays, which limits its applicability for different photon energy ranges. The output interference versus delay gives the information of coherence time. Typically, the radiation pulse is split into two identical pulses, one pulse is delayed by some optical mirrors and eventually the two pulses are recombined. To directly measure the FEL coherence time, one straight forward way is to implement conventional optical method: autocorrelation, which is a widely used method in the optical wavelength. Hence, a direct characterization method of the coherence time of the ultra-fast hard x-ray FEL pulse in time domain is a primary requirement in this field. ![]() ![]() Foreknown information about x-ray coherence time would benefit experiments focusing on ionization dynamics 10, spectro-holography 11, nonlinear mixing-wave experiment 12, etc. Due to starting from electron shot-noise, SASE FELs usually provide partially temporal coherence corresponding to temporal isolated spikes 9. Most of the hard x-ray FELs rely on the self-amplified spontaneous emission (SASE) scheme. These pulses are able to encode valuable structural and dynamical information on atomic scales. Femtosecond, high brightness, hard x-ray pulses generated by free-electron lasers (FELs) 1, 2, 3, 4, 5 opens the door to a new frontier of high-intensity x-ray experiments in various research fields, e.g., physics, chemistry 6, life 7 and material sciences 8.
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