Comparisons and analyses of the aluminum K-shell spectroscopic models

Abstract

Comparing different collisional-radiative models is of great importance for validating the models for plasma spectroscopy and improving the diagnostic accuracy of plasma parameters. In this paper, the widely applied K-shell spectroscopic models, FAC and FLYCHK, are compared based on their calculation results of the aluminum K-shell emissivity and absorption coefficient. The state abundances, K-shell line ratios, K-shell emissivities and absorption coefficients in a wide range of plasma temperatures and densities are calculated and compared, and the reasons for the differences between these two models are discussed. In an electron temperature range from 200 to 800 eV, and an electron density range from 1017 to 1024 cm-3, the Al ions in the plasma are mainly composed of H-like and He-like ions. The ground-state populations of the H-like and He-like ions, calculated from FAC model, are in good agreement with the results from FLYCHK. Number densities of the excited states are two orders or more less than those of the ground states from both the models, and significant differences are observed in the number densities of n=2 and n=3 states of both the H-like and He-like ions. These differences will further result in the differences in spectral line emissivity and their line emissivity ratio, such as He-IC/He-αup and H-βup/He-βup, which are key parameters used to diagnose the electron temperature and density. The line emissivity ratio Ly-αup/(He-αup+He-IC) is less dependent on the electron density, and the difference in line emissivity ratio between the two models mainly lies in the parameter region where both the electron temperature and density are high. The ratio He-IC/He-αup is less dependent on the electron temperature when the electron density is more than 1019 cm-3 while significant differences are observed at a lower electron density.#br#The reason for the difference between the number densities of the low-energy excited states from FAC and FLYCHK models is analyzed by comparing the rate coefficients of various collisional and radiative processes in the rate equation of each state. The differences in the n=2 excited states of H-like ions come from the fact that FAC and FLYCHK models use the detailed-level model and the super-configuration model respectively to construct the rate equations of these states. The FAC model ignores the collisional excitation and de-excitation processes between the n=3 state and higher excitation states (e.g. n = 4) in H-like and He-like ions, which are responsible for the density difference in the n=3 excited state. Higher Rydberg states considered in FLYCHK model do not have any significant influence on the density of the ground-states. The difference in the absorption coefficient between the two models is smaller than that in the emissivity as discussed above, for the absorption coefficient mainly depends on the number density of the ions in ground state.