Experimental Investigation of Optical Properties of Nonideal Plasma within the Electron Concentration Range $10^{17}$ cm$^{-3}< N_e < 10^{22}$ cm$^{-3}$

The optical properties of nonideal plasmas within the electron concentration range $10^{17}$ cm$^{–3}$ $< N_e < 10^{22}$ cm$^{–3}$ are studied experimentally. According to these results, as well as to the data available in the literature, we obtain the experimental dependence of the $\Delta E$ (the difference between the atom ionization potential and the excitation potential of the upper level of the last observed emission or absorption line) value on the electron concentration within the range taken. The dependence is compared to the earlier obtained theoretical formulas. Recommendations on application of the experimental formula to determine the electron concentration in the external layers of the non-ideal plasma at $N_e < 10^{22}$ cm$^{–3}$ are presented.