A Highly Stable-Output Kilohertz Femtosecond Hard X-ray Pulse Source for Ultrafast X-ray Diffraction

Abstract

Femtosecond hard X-ray pulses generated by laser-driven plasma sources are eminently suitable to probe structural dynamics due to the angstrom spatial resolution and sub-picosecond time resolution. However, the insufficient flux of X-ray photons and high pulse-to-pulse instability compared with the large-scale ultrashort X-ray source, such as X-ray free-electron laser and synchrotrons, largely restricts its applications. In this work, we have optimized automation control and mechanical designs to significantly enhance the reliability and photon flux in our femtosecond laser plasma-induced X-ray source. Specifically, the optimized source provides a reliable pulse-to-pulse stability with a fluctuation of less than 1% (root-mean-square) and a total flux of Cu-Kα X-ray photons above 1011 photons/s. To confirm its functionality, ultrafast X-ray diffraction experiments are conducted on two different samples and the high consistency with previous results verifies the system’s superior performance.

K[alpha] x-ray source size based on high-intensity femtosecond laser-solid interaction: generation and applications

“…Those oscillations are launched with an inertia corresponding to the delay required to convert the laser energy deposited in the electron sub-system into internal mechanical constraints which accounts for  100'ps delay time. Figure 5b then illustrates the dynamics of those transient oscillations in gold with a characteristic period of  235 ps (in fair agreement with measurements on crystalline 200-nm Au (111) [18]), when it is excited with a 400-nm pump of fluence below the damage threshold of the sample in regime of accumulation (F  5 mJ/cm²). Completing the full delay curve shown on figure 5b requires capturing rocking curves for an extended set of delays and for the reader information, one rocking curve for a given delay was obtained within 8 seconds of operation at 100-Hz repetition rate.…”

“…delay from which change in the rocking curve of gold is detected (Bragg peak angular shift as shown in figure 5b) following 400-nm laser excitation. The change corresponds to lattice oscillations resulting from the propagation of acoustic strain following pump laser excitation [18]. Those oscillations are launched with an inertia corresponding to the delay required to convert the laser energy deposited in the electron sub-system into internal mechanical constraints which accounts for  100'ps delay time.…”