## 3 Results
### Comet Structure  
The comet-shaped density profile of anions (O\({}^{-}\)) is found in the fluid simulation of Ar/O\({}_{2}\) plasma, as shown in Figure 1c. This novel density profile of plasma species provokes our profound interest since it has never been reported before. As seen, the comet-shaped O\({}^{-}\) density looks like a Dirac delta function, especially in the axial direction. To investigate the mechanism behind this, the relevant quantities are observed, such as the electron density, argon ion Ar\({}^{+}\) density, plasma potential in Figure 1, and its chemical processes, including the reaction rates of O\({}^{-}\) generation and loss and the summed rate shown in Figure 2. An important feature that the O\({}^{-}\) species have a negative chemical source in the located density peak is noticed. In the mature analytic works (see Ref. [1], pp. 330-333), the species density morphologies of electropositive plasma, such as the parabola, cosine, and Bessel at high pressure limit, the variable mobility model of intermediate pressure range, and the Langmuir solution at low pressure limit, are all based on the continuity equation of \begin{table}
\begin{tabular}{c c} \hline \hline
**No.** & **Reaction** \\ \hline
13 & \(\mathrm{e}+\mathrm{O}^{-}\to 2\mathrm{e}+\mathrm{O}\) \\
26 & \(\mathrm{O}^{-}+\mathrm{O}^{+}_{2}\to\mathrm{O}+\mathrm{O}_{2}\) \\
27 & \(\mathrm{O}^{-}+\mathrm{O}\to\mathrm{e}+\mathrm{O}_{2}\) \\
28 & \(\mathrm{O}^{-}+\mathrm{O}^{+}\to 2\mathrm{O}\) \\
29 & \(\mathrm{O}^{-}+\mathrm{O}^{+}_{2}\to 3\mathrm{O}\) \\
37 & \(\mathrm{O}^{-}+\mathrm{O}^{+}_{2}\to\mathrm{O}+\mathrm{O}_{2}+\mathrm{e}\) \\
42 & \(\mathrm{O}^{-}+\mathrm{Ar}^{+}\to\mathrm{O}+\mathrm{Ar}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Reactions collection for O\({}^{-}\) loss. \begin{table}
\begin{tabular}{l c c} \hline \hline
**No.** & **Surface Reactions** & **Reference** \\ \hline
1 & \(\mathrm{O}^{+}_{2}+\mathrm{wall}\to\mathrm{O}_{2}\) & [38] \\
2 & \(\mathrm{O}^{+}+\mathrm{wall}\to\mathrm{O}\) & [38] \\
3 & \(\mathrm{O}^{+}_{2}+\mathrm{wall}\to\mathrm{O}_{2}\) & [38] \\
4 & \(\mathrm{O}^{+}+\mathrm{wall}\to\mathrm{O}\) & [38] \\
5 & \(\mathrm{O}^{-}+\mathrm{wall}\to\mathrm{O}\) & [38] \\
6 & \(\mathrm{O}+\mathrm{wall}\to 1/2\mathrm{O}_{2}\) & [38] \\
7 & \(\mathrm{O}+\mathrm{wall}\to 1/2\mathrm{O}_{2}^{*}\) & [38] \\
8 & \(\mathrm{Ar}^{+}+\mathrm{wall}\to\mathrm{Ar}\) & [34] \\
9 & \(\mathrm{Ar}^{*}+\mathrm{wall}\to\mathrm{Ar}\) & [34] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Surface reactions for Ar/O\({}_{2}\) discharges included in the fluid model. \begin{table}
\begin{tabular}{c c} \hline \hline
**No.** & **Reaction** \\ \hline
3 & \(\mathrm{e}+\mathrm{O}_{2}\to\mathrm{O}^{-}+\mathrm{O}\) \\
4 & \(\mathrm{e}+\mathrm{O}_{2}\to\mathrm{e}+\mathrm{O}^{-}+\mathrm{O}^{+}\) \\
16 & \(\mathrm{e}+\mathrm{O}_{2}^{*}\to\mathrm{O}^{-}+\mathrm{O}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Reactions collection for O\({}^{-}\) generation. species with positive chemical source. This is logical since in the electropositive plasmas the species are depleted on the chamber wall through surface kinetics. In the electronegative plasmas, this is not always true. In Figure 2, at certain circumstances (low electronegativity), the negative source of anions given by the recombination exceeds the positive source given by the electron-impact dissociative attachment. The chemical kinetics of O\({}^{-}\) species can be referred to from Tables 5 and 6. The analysis above implies that when the chemical source is negative, it tends to create a coagulated structure in the electronegative plasma. It is seen in Figure 1c,d that the delta type of O\({}^{-}\) is located on the top of the ambipolar potential, and its main transport component is then free diffusion since the drift is negligible. To qualitatively interpret the forming mechanism of delta, the unsteady-state continuity equation of O\({}^{-}\) that consists of the free diffusion flux and negative chemical source of recombination is presented in Equation (16).