Attosecond Coherence Time Characterization in Hard X-Ray Free-Electron Laser

Abstract

One of the key challenges in scientific researches based on free-electron lasers (FELs) is the characterization of the coherence time of the ultra-fast hard x-ray pulse, which fundamentally influences the interaction process between x-rays and materials. Conventional optical methods, based on autocorrelation, are very difficult to realize due to the lack of mirrors. Here, we experimentally demonstrate a novel method which yields a coherence time of 174.7 attoseconds for the 6.92 keV FEL pulses at the Linac Coherent Light Source. In our experiment, a phase shifter is adopted to control the cross-correlation between x-ray and microbunched electrons. This approach provides critical diagnostics for the temporal coherence of x-ray FELs and is universal for general machine parameters; applicable for wide range of photon energy, radiation brightness, repetition rate and FEL pulse duration.

Figure 1

Schematic description of hard x-ray SASE FEL coherence time measurement based on cross-correlation between microbunched electrons and x-ray pulse. Hard x-ray SASE FEL is generated in the before-delay undulators, then a phase shifter is employed to generate a relative delay between the electrons and the x-ray and the final undulator converts the correlation to x-ray pulse energy. (a) Visualizing the radiation wave and the microbunched electrons at the beginning of the phase shifter. (b) Illustrating the case that the phase shifter induced delay is small. (c) Showing that when the delay is quite large, the microbunched electrons would work with x-rays in other coherence spikes.

Figure 2

Experimental results of hard x-ray SASE FEL coherence time measurement at LCLS: (a) Pulse energy versus delay, in which the black line is the average pulse energy for each delay and the grey points represent the single-shot pulse energy. (b) Moving variance with respect to delay, where blue points are obtained by Eq. (6) in Method and the red line is the fitting curve.

Figure 3

Fluctuation source analysis. (a) Time-domain description of hard x-ray SASE FEL pulse. (b) Bunching factor profile. (c) Pulse energy versus delay (ideal). (d) Pulse energy versus delay (initial shot noise fluctuation considered). (e) Pulse energy versus delay (experimental fluctuation sources considered), in which the black line is the average pulse energy and the grey points represent the pulse energy for each single-shot. (f) The moving variance obtained by Eq. (6) together with its fitting curve.