First Observation of Synchrotron Radiation Spikes for Transverse Electron Beam Size Measurements at a Free-Electron Laser

We report the observation of transverse intensity fluctuations—spikes—in synchrotron radiation from a single undulator cell at the European XFEL after monochromatization. Autocorrelation analysis of the recorded events confirms that these fluctuations originate from the partial transverse coherence of the radiation. By calculating the second-order autocorrelation function, we determined the averaged transverse slice size of the electron beam along the SASE1 undulator. The measurements were performed with 5.49 keV x-ray photons using a commissioning silicon monochromator with (111) Bragg reflection and an x-ray imager. This technique enables undulator cell-by-cell diagnostics of the transverse electron beam size at free-electron laser facilities.