Table-top laser-driven hard x-ray sources with kilohertz repetition rates are an attractive\nalternative to large-scale accelerator-based systems and have found widespread\napplications in x-ray studies of ultrafast structural dynamics. Hard x-ray pulses of 100\nfs duration have been generated at the Cu Ka wavelength with a photon flux of up to\n109 photons per pulse into the full solid angle, perfectly synchronized to the sub-\n100-fs optical pulses from the driving laser system. Based on spontaneous x-ray emission,\nsuch sources display a particular noise behavior which impacts the sensitivity of\nx-ray diffraction experiments. We present a detailed analysis of the photon statistics\nand temporal fluctuations of the x-ray flux, together with experimental strategies to optimize\nthe sensitivity of optical pump/x-ray probe experiments. We demonstrate measurements\nclose to the shot-noise limit of the x-ray source.
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