We characterize the spatial and temporal coherence properties of hard X-ray pulses from the Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL, Pohang, Korea). The measurement of the single-shot speckle contrast, together with the introduction of corrections considering experimental conditions, allows obtaining an intrinsic degree of transverse coherence of 0.85 ± 0.06. In the Self-Amplified Spontaneous Emission regime, the analysis of the intensity distribution of X-ray pulses also provides an estimate for the number of longitudinal modes. For monochromatic and pink (i.e. natural bandwidth provided by the first harmonic of the undulator) beams, we observe that the number of temporal modes is 6.0 ± 0.4 and 90.0 ± 7.2, respectively. Assuming a coherence time of 2.06 fs and 0.14 fs for the monochromatic and pink beam respectively, we estimate an average X-ray pulse duration of 12.6 ± 1.0 fs.
Bibliographical noteFunding Information:
We thank A. Robert (Linac Coherent Light Source) and H.-S. Kang (PAL-XFEL) for useful discussion. This research was supported by the National Research Foundation of Korea (NRF-2014R1A2A1A10052454, 2015R1A5A1009962, 2016R1A6B2A02005468). The experiments were performed at NCI end-station of PAL-XFEL (proposal no. 2017-2nd-NCI-008) funded by the Ministry of Science and ICT of Korea.
© 2019, The Author(s).