The effect of bound states on X-ray Thomson scattering for partially ionized plasmas

Abstract

Abstract. X-ray Thomson scattering is being developed as a method to measure the temperature, electron density, and ionization state of high energy density plasmas such as those used in inertial confinement fusion. X-ray laser sources have always been of interest because of the need to have a bright monochromatic x-ray source to overcome plasma emission and eliminate other lines in the background that complicate the analysis. With the advent of the xray free electron laser (X-FEL) at the SLAC Linac Coherent Light Source (LCLS) and other facilities coming online worldwide, we now have such a source available in the keV regime. An important challenge with x-ray Thomson scattering experiments is understanding how to model the scattering for partially ionized plasmas. Most Thomson scattering codes used to model experimental data greatly simplify or neglect the contributions of the bound electrons to the scattered intensity. In this work we take the existing models of Thomson scattering that include elastic ion-ion scattering and inelastic electron-electron scattering and add the contribution of bound electrons in the partially ionized plasmas. Except for hydrogen plasmas, most plasmas studied today have bound electrons and it is important to understand their contribution to the Thomson scattering, especially as new x-ray sources such as the X-FEL will allow us to study much higher Z plasmas. To date, most experiments have studied hydrogen or beryllium plasmas. We first analyze existing experimental data for beryllium to validate the code. We then consider several higher Z materials such as Cr and predict the existence of additional peaks in the scattering spectrum that requires new computational tools to understand. For a Sn plasma, we show that bound contributions change the shape of the scattered spectrum in a way that would change the plasma temperature and density inferred from experiment.

Characterizing plasma conditions in radiatively heated solid-density samples with x-ray Thomson scattering

“…The literature contains many examples in which the inelastic scattering feature is compared with theoretically generated fits to infer plasma properties such as electron density and temperature [4,23,24,26]. While the modeling behind the free-free scattering feature is thought to be well understood, the shape and scaling of the bound-free term can differ with different theoretical treatments [39,[43][44][45][46][47]. The bound-free term is of-ten modeled using the impulse approximation, which is known to be a reasonable approximation for the case of k → ∞ [28].…”

“…The bound-free term is of-ten modeled using the impulse approximation, which is known to be a reasonable approximation for the case of k → ∞ [28]. However, there remains uncertainty as to the proper scaling of the bound-free feature with respect to other features in the spectrum [43][44][45]. Because of such uncertainties, some recent work seeks to interpret scattering spectra outside of the Chihara decomposition, with methods like density functional theory [9,35,48].…”

Average-atom treatment of relaxation time in x-ray Thomson scattering from warm dense matter

“…[17][18][19][20][21] to investigate anomalous dispersion in C, Al, Ag, and other plasmas in the soft X-ray region of the spectrum and has been used to investigate Thomson scattering in Refs. [22][23][24][25][26][27][28][29][30].…”