Extended X-ray Absorption Fine Structure (EXAFS) measurements in ramp compressed tantalum at the National Ignition Facility

Dynamic compression is now a widespread technique for investigating material properties at extraordinary pressure, density, and temperature. However, there is a nearly complete lack of termperature measurements across the full scope of this field. As a result, thermal effects remain a large source of uncertainty in equation of state constructrion. Extended X-ray Absorption Fine Structure (EXAFS), which refers to modulations in the x-ray absorption spectra in the region just above an edge that arise from the photoelectron scattering off of neighboring atoms, is a powerful diagnostic for characterizing material properties. EXAFS is particularly sensitive to density, temperature, and crystal structure in the range 100s-10000 K, where most materials form a solid at high pressure. Here we present results of experiments at the National Ignition Facility (NIF) that measured EXAFS from tantalum ramp compressed to ~2 Mbar following different initial shock states. These measurements are made possible by the high flux x-ray source [1] and high fidelity laser pulse shaping available at the NIF, as well as the high-resolution x-ray spectrometer design [2]. L-edge EXAFS, which is required for high atomic number materials, is particularly challenging due to intrinscially small amplitude EXAFS oscillations compared to K-edge. We discuss these results and prospects for this capability at NIF.