Abstract
A model for the coherence properties of free-electron lasers (FELs) in time and frequency domains is introduced within the framework of classical second-order coherence theory of nonstationary light. An iterative phase-retrieval algorithm is applied to construct an ensemble of field realizations in both domains, based on single-pulse spectra measured at the Linac Coherent Light Source (LCLS) in self-amplified spontaneous emission mode. Such an ensemble describes the specific FEL pulse train in a statistically averaged sense. Two-time and two-frequency correlation functions are constructed, demonstrating that the hard X-ray free-electron laser at LCLS in this case behaves as a quasistationary source with low spectral and temporal coherence. We also show that the Gaussian Schell model provides a good description of this FEL.
Highlights
Owing to their high brightness, ultrashort pulse duration, and high degree of transverse spatial coherence, X-ray free-electron lasers (XFELs) [1,2,3] enable novel research in material science, structural biology, and condensed matter physics [4]
In this paper we present a numerical method for generating ensembles of pulses representing beams such as those produced by XFELs
A single execution of the iterative Fourier-transform algorithm (IFTA) algorithm was found to lead to a temporal pulse of varying complicated shape, which depends on the chosen random initial spectral phase
Summary
Owing to their high brightness, ultrashort pulse duration, and high degree of transverse spatial coherence, X-ray free-electron lasers (XFELs) [1,2,3] enable novel research in material science, structural biology, and condensed matter physics [4]. Rather than assuming average spectral and temporal data as in [14], we employ actual measured spectra of individual pulses together with partial knowledge of their average time-domain intensity to construct the spectral phases and the temporal pulse realizations. Due to lack of detailed information, the mean temporal intensity is constructed to be Gaussian with the known average pulse length This is accomplished by employing in the time domain a Gaussian weighting function whose width is appropriately adjusted as the iteration proceeds. From the overall ensemble of spectral and temporal field realizations we construct two-frequency and two-time correlation functions [17], which describe pulse trains of partial spectral and temporal coherence fully within the framework of the classical second-order coherence theory of nonstationary light. The coherence properties of the XFEL source are found to match extremely closely the predictions of the Gaussian Schell model [18, 21]
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