Study of experimental method for warm dense matter ionization distribution based on x-ray fluorescence spectroscopy

Abstract

Warm dense matter (WDM), a state of matter which lies at the frontiers between condensed matter and plasma, is one of the main research objects of high energy density physics (HEDP). Compared to the isolated atom, the electron structure of WDM will change because of the influence of density and temperature effect. Both the accurate theoretical represent and the accurate experimental study of WDM electron structure are challenging, as it is strongly coupled and partially degenerated. In this paper, an experimental method for studying the ionization distribution of warm dense matter based on x-ray fluorescence spectroscopy is developed. In the experiment, warm dense titanium with several tens eV and near solid density is created by a simultaneous drive from high energy xray heating and shock compression in a special designed hohlraum. Then, using the characteristic line spectrum emitted by the laser irradiation on pump material (Vanadium) as pump source, the titanium emits fluorescence. The x-ray fluorescence spectroscopy of titanium with different states (cold sample, 1.8-4.5 g/<i>cm</i><sup>3</sup> and 1-25 eV) is diagnosed by changing the experimental strategy. The experimental results indicate that the line profiles of <i>K<sub>α</sub></i> and <i>K<sub>β</sub></i> fluorescence spectrum of the heated sample change obviously relative to that of the cold sample. Associating a theoretical calculation from two-step Hartree-Fock-Slater (TSHFS) method, the reason for the variation of the line profile is the change of ionization distribution mainly caused by temperature rise. The future work will focus on optimizing the experimental method of x-ray fluorescence spectroscopy, such as improving the spectrum resolution, characterizing the temperature and density experimentally, obtaining a set of ionization distribution data, and then study the influence of dense environment on electronic structure.