Abstract
X-ray emission spectroscopy is a well-established technique used to study continuum lowering in dense plasmas. It relies on accurate atomic physics models to robustly reproduce high-resolution emission spectra, and depends on our ability to identify spectroscopic signatures such as emission lines or ionization edges of individual charge states within the plasma. Here we describe a method that forgoes these requirements, enabling the validation of different continuum lowering models based solely on the total intensity of plasma emission in systems driven by narrow-bandwidth x-ray pulses across a range of wavelengths. The method is tested on published Al spectroscopy data and applied to the new case of solid-density partially-ionized Fe plasmas, where extracting ionization edges directly is precluded by the significant overlap of emission from a wide range of charge states.
Highlights
Spectroscopic methods are based on the strong dependence that the atomic physics of ions in a plasma exhibits to CL effects
Any plasma modelling needs to include some CL model for comparison with experiment, and it is not immediately obvious that there is a single combination of plasma conditions, atomic physics model and CL model that will provide a unique agreement with the experimental result
Reported isochoric heating experiments performed at the LCLS free-electron laser (FEL)[20,21,22] provide a rich spectroscopy dataset for x-ray-driven aluminium plasmas that we shall employ for the validation of our proposed approach
Summary
X-ray emission spectroscopy is a well-established technique used to study continuum lowering in dense plasmas It relies on accurate atomic physics models to robustly reproduce high-resolution emission spectra, and depends on our ability to identify spectroscopic signatures such as emission lines or ionization edges of individual charge states within the plasma. Spectroscopic methods are based on the strong dependence that the atomic physics of ions in a plasma exhibits to CL effects This dependence makes it possible to identify specific features in x-ray emission and absorption spectroscopy, such as bound-bound transition lines[18] or the presence of absorption or emission edges, that can be linked, directly or indirectly, to the ionization threshold energy, and to the level of CL. We propose that by measuring the integrated emission intensity as a function of excitation photon energy it is possible to validate specific CL models in a way that is both experimentally practical and computationally efficient
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