Retrieving Transport Properties from Multiscale HED Flows using X-ray Photon Correlation Spectroscopy (XPCS)*

Using coherent x-rays sources, X-ray Photon Correlation Spectroscopy (XPCS) can measure microscopic transport properties by probing the time correlation of density fluctuations. This is particularly challenging under high-energy-density (HED) conditions because of the extremely fast timescales and the multiscale shear commonly induced to achieve HED conditions using shock compression. We develop a theoretical framework for utilizing XPCS to study material diffusivity in multiscale flow environments. We derive a timescale that captures the combination of shear and diffusion dynamics across a wide range of lengthscales. Our theory is validated through synthetic XPCS. We demonstrate the versatility of our approach across several orders of magnitude in timescales using sequential-pulse XPCS, single-pulse X-ray Speckle Visibility Spectroscopy (XSVS), and double-pulse XSVS. Our framework enables the concurrent measurements of diffusion coefficients due to microscopic physics and characteristic macroscopic velocities and establishes the theoretical foundation for XPCS as a promising diagnostic for HED systems.