Beamstop-free small-angle X-ray scattering for multiscale structure measurement of high-energy-density plasma

Abstract

The advent of free-electron lasers has enabled in-situ small-angle X-ray scattering (SAXS) measurements of laser-produced high-energy-density plasma at solid density, with nanometer- and femtosecond-scale resolution. Such plasmas generally exhibit gentle density gradients, which suppress large-angle scattering signals while small-angle scattering components dominate near the direct X-ray beam; the latter signals were hard to access in the conventional SAXS method due to the use of a beamstop. Here, we demonstrate a beamstop-free SAXS system comprising layered metallic filters for optimal attenuation of diffraction signals. It enables continuous acquisition of wide q -range/angular scattering signals with a large dynamic range, including the critical small-angle region containing the direct X-ray beam. This method was applied to a two-dimensional array of submicron columnar plasmas generated by irradiating a high-intensity laser pulse onto an assembly of high-aspect-ratio silicon rods with well-defined sharp edges. We experimentally resolve the microscale and macroscale rarefaction dynamics of bulk electrons driven by the pump-laser pre-pulse and main pulse, revealing the initial plasma scale length as a key parameter governing laser absorption in structured targets. Beamstop-free SAXS enables single-shot multiscale structure measurement and establishes a route toward exploration of stochastic plasma dynamics under high-energy-density conditions, including filamentation instability and turbulence.