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

Guanqun Zhou,Timothy J Maxwell,Cheng-Ying Tsai,Yi Jiao,Yuantao Ding,Jiuqing Wang,Tor O Raubenheimer,Aaron J Holman,Jerome Y Wu,Chuan Yang,Alberto A Lutman,Juhao Wu,Franz-Josef Decker,Weiwei Wu,Moohyun Yoon

doi:10.1038/s41598-020-60328-4

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.

Highlights

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]
We carry out experiments to measure the coherence time of hard x-ray pulses at Linac Coherent Light Source (LCLS) with the method we propose above
The experimental setup is based on the normal LCLS spontaneous emission (SASE) FEL configuration[25]

Summary

Introduction

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]. 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] 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. Since the electrons are almost ‘fresh’, the coherence information is smeared out 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. This process produces ultrashort bursts of x-rays at the wavelengths λr

Methods

Results

Conclusion