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To surpass the limitations of this jitter, experiments involving the fastest physical phenomena (e.g. transitions of core shell electrons) measure the relative timing between the laser and X-rays on a shot-by-shot basis by means of X-ray/optical cross-correlation techniques, referred to as the time tool, pictured in Figure 5. Ultrafast ionization by the x-ray beam in YAG windows allows its synchronization relative to the MEC laser to be precisely set. The ionization causes the window to become opaque showing definite before/after timing. By angling a window relative to the x-ray beam, the time tool gives synchronization to within a few fs. The time tool is cross-referenced to the target plane using another YAG window at the laser focus position, with a small f-stop temporarily inserted to spread the laser focus at the target plane. The time tool measures an RMS jitter of ~ 100 fs between the MEC laser and the x-ray beam.
Figure 5. Image of the MEC timing tool.
Figure 6. Synchronization of laser to the X-ray the time tool. Left: geometry of x-ray and probe laser on YAG crystal. Right/Top: laser shadography of a target window on two consecutive laser shots, arriving just before (left) and just after (right) an X-ray pulse (square shaped beam); Right/bottom: The time tool shows arrival time of the ionizing X-ray pulse inline, demonstrating that the synchronization jitter between the two consecutive pulses shown here is less than 100 fs.
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