Influence of atomic kinetics in the simulation of plasma microscopic properties and thermal instabilities for radiative bow shock experiments

Abstract

Numerical simulations of laboratory astrophysics experiments on plasma flows require plasma microscopic properties that are obtained by means of an atomic kinetic model. This fact implies a careful choice of the most suitable model for the experiment under analysis. Otherwise, the calculations could lead to inaccurate results and inappropriate conclusions. First, a study of the validity of the local thermodynamic equilibrium in the calculation of the average ionization, mean radiative properties, and cooling times of argon plasmas in a range of plasma conditions of interest in laboratory astrophysics experiments on radiative shocks is performed in this work. In the second part, we have made an analysis of the influence of the atomic kinetic model used to calculate plasma microscopic properties of experiments carried out on MAGPIE on radiative bow shocks propagating in argon. The models considered were developed assuming both local and nonlocal thermodynamic equilibrium and, for the latter situation, we have considered in the kinetic model different effects such as external radiation field and plasma mixture. The microscopic properties studied were the average ionization, the charge state distributions, the monochromatic opacities and emissivities, the Planck mean opacity, and the radiative power loss. The microscopic study was made as a postprocess of a radiative-hydrodynamic simulation of the experiment. We have also performed a theoretical analysis of the influence of these atomic kinetic models in the criteria for the onset possibility of thermal instabilities due to radiative cooling in those experiments in which small structures were experimentally observed in the bow shock that could be due to this kind of instability.

Analysis of microscopic properties of radiative shock experiments performed at the Orion laser facility

“…For simplicity, the radiative properties dependent on time, position and propagation direction have been omitted, although are required for a complete description. In a previous work [39] , the contribution of opacity on the calculation of the divergence of radiative flux for xenon was analyzed, at plasma conditions quite similar to the ones of this work, and it was obtained that the relevance of that contribution was considerably lower than the radiative power loss (RPL), that is, the first term in the right-hand side of Equation (10). Therefore, the plasma can be considered as optically thin and the divergence of the radiative flux can be approximated as the RPL.…”

“…Equations (2) and (3) are coupled through the emissivity and the absorption coefficients ( j (r, t, ν) and κ(r, t, ν), respectively). The CR model as well as the Saha-Boltzmann (SB) equations used in this work is implemented in the MIXKIP code [39] , a code developed to calculate plasma atomiclevel populations of mono and multicomponent optically thin and thick plasmas in time-dependent and steadystate situations. The atomic processes included in the CR model implemented in MIXKIP are collisional ionization, three-body recombination, spontaneous decay, collisional excitation and deexcitation, radiative recombination, autoionization and electron capture.…”

“…A more detailed description of the expressions used for the rates of the atomic processes can be found in Ref. [39]. Plasma self-absorption (i.e., opacity effects) is modeled in MIXKIP in an approximate way using the escape factor formalism for the bound-bound opacity [40] .…”

Radiative properties for astrophysical plasma mixtures in nonlocal thermodynamic equilibrium

“…However, as the different elements of the multicomponent plasma are immersed in a common pool of free electrons, they will be coupled through the electron density since the plasma level population (and the average ionization) of each component has to be consistent with the same electron density, which ensures the electrical equilibrium keeping the overall plasma neutrality [54,55]. The CRSS model described is implemented in the numerical code MIXKIP [56].…”

“…For aluminum plasmas, we determined the plasma thermodynamic regimes as a function of plasma conditions and checked our simulations for the average ionization, CSDs, mean opacities, and monochromatic opacities and emissivities with other LTE and NLTE simulations [57,66]. Finally, we performed comparisons of the average ionization, CSDs, and RPLs of argon plasmas [56] with NLTE codes from [67], for electron temperatures from 3 to 35 eV and at the electron densities 10 16 and 10 20 cm −3 . The results obtained in all these studies were quite acceptable.…”