Experimental method for warm dense matter ionization distribution based on X-ray fluorescence pectroscopy

Abstract

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