Microscopy X-ray Imaging enriched with Small Angle X-ray Scattering for few nanometer resolution reveals shock waves and compression in intense short pulse laser irradiation of solids

Abstract

Understanding how laser pulses compress solids into high-energy-density states requires diagnostics that simultaneously resolve macroscopic geometry and nanometer-scale structure. Here we present a combined X-ray imaging (XRM) and small-angle X-ray scattering (SAXS) approach that bridges this diagnostic gap. Using the Matter in Extreme Conditions end station at LCLS, we irradiated 25-micrometer copper wires with 45-fs, 0.9-J, 800-nm pulses at 3.5e19 W/cm2 while probing with 8.2-keV XFEL pulses. XRM visualizes the evolution of ablation, compression, and inward-propagating fronts with about 200-nm resolution, while SAXS quantifies their nanometer-scale sharpness through the time-resolved evolution of scattering streaks. The joint analysis reveals that an initially smooth compression steepens into a nanometer-sharp shock front after roughly 18 ps, consistent with an analytical steepening model and hydrodynamic simulations. The front reaches a velocity of about 25 km/s and a lateral width of several tens of micrometers, demonstrating for the first time the direct observation of shock formation and decay at solid density with few-nanometer precision. This integrated XRM-SAXS method establishes a quantitative, multiscale diagnostic of laser-driven shocks in dense plasmas relevant to inertial confinement fusion, warm dense matter, and planetary physics.

Figures (7)

• Figure 1: Experimental setup. The MEC HI laser (red) is focused onto the Copper wire target under $45^\circ$ in p-polarisation, the XFEL (blue) is probing the plasma density. The detector records the Small-angle X-ray scattering (SAXS) image normalised at small $q$ values by the retractable absorber system. The diagnostics are on motorised stages so the absorbers, beam block and SAXS detector can be easily exchanged for the CRL stack and imaging detector. The figure is not to scale. The insets show raw images for XRM and SAXS of HI-laser driven wires after $100\text{\,}\mathrm{ps}$.
• Figure 2: X-ray microscopy results. Sequence of representative XRM images showing the evolution of the Cu wire after laser irradiation (shown is the ratio of the pumped wire image over the cold image). Distinct features (A–D) mark ablation, surface compression, hole-boring-like fronts, and inward-propagating compression fronts, respectively. See main text for details.
• Figure 3: Evolution of the SAXS pattern for representative probe delays grouped by the two experiment days (a) day 1 and (b) day 2. The tilt, number, and sharpness of streaks increase with time, reflecting the evolving curvature, multiplicity of compression fronts, and shock formation. Note that for each image the X-ray absorber positions are slightly different for an individual optimisation.
• Figure 4: (a) Tilt angle of SAXS scattering streak angles from SAXS. (b) Profiles along the streaks of cold (blue) and HI laser pumped (orange) wires for those images from Fig.\ref{['fig:SAXS']} where the streaks can be clearly separated and are sufficiently intense. (c) Fitted change of the roughness parameter as function of probe delay.
• Figure 5: Correlation between SAXS and XRM. (a) Overlay of SAXS streak orientations with the corresponding XRM-derived fronts. (b) Density model projection used to reproduce the observed transmission and scattering patterns by using a wide Gaussian (corresponding to streaks ai) and a narrower one (which produces two apparent fronts upon rotation, corresponding to streaks bi, ci). The combined analysis links specific SAXS features to the distinct compression fronts identified in XRM. (c) Fourier Transform of the density projection shown in (b) with the XFEL beam spot on target assumed to be transversely offset w.r.t. the laser axis (dashed lines in (b)). Note, that the high frequency oscillations are a numerical effect of the superposition of scattering from the perfectly aligned layers in the model.