## 4 Discussion
### Advective Ambipolar Self-Coagulation
#### 4.2.2 Semi-Circle Structure for Ambipolar Type  
The ambipolar type of self-coagulation is constructed based on the semi-circle-shaped coagulated structure of Ar/SF\({}_{6}\) plasma to explain the internal neutral plasma. It applies to the neutral plasma core of comet-shaped coagulated structure in the Ar/O\({}_{2}\) plasma as well, as notified. Sequentially, in Section (a), the chemical self-coagulation in the Ar/SF\({}_{6}\) plasma is compared to the physical coagulation driven by the pre-push of ambipolar potential. It is seen that the external and physical coagulation that occurs early creates the cliffy border of anion density, while the internal and chemical coagulation that occurs afterwards creates the soft border of anion density. In Section (b), the formulae of ambipolar diffusion are presented at first and then introduce the ambipolar self-coagulation concept, which is actually the collective interaction of ion-pair plasma (i.e., electrically attracting force between the anions that are being self-coagulated and the cations that are still, spatially). Different from the significant gradient that the ambipolar diffusion potential has, the ambipolar self-coagulation creates neither the potential barrel nor the potential well due to the similarity of their masses. In Section (c), due to the high electronegativity, the decouple of electron dynamics from the ambipolar self-coagualtion of anion-cation pair plasma is  
Figure 15: Simulated two-dimensional profiles of O\({}^{-}\) density at the _late_ stage for four simulation times, (**a**) \(10^{-5}\) s, (**b**) \(1.8\times 10^{-5}\) s, (**c**) \(2.4\times 10^{-5}\) s, and (**d**) \(3.2\times 10^{-5}\) s, by means of fluid model. The discharge conditions are the same as Figures 1 and 2.  
presented, which is characterized by the potential collapse and more expansive electron density profile than the ionic profiles. 1. Chemical coagulation versus physical coagulation  
In Figure 16, the temporal evolution of anion density that is sampled under the coil (the coordinates of sampled position can be referred to the figure) is plotted. In the time range of \(10^{-5}\) s \(\sim 10^{-4}\) s, the anion density increases swiftly, which indicates the self-coagulation occurrence. In Figure 17, the anions densities and their chemical source of the two times are shown, and it is validated again that the self-coagulation is accompanied by a negative source of anions in Figure 17d. Before the self-coagulation, the chemical source is positive in Figure 17c. In Figure 18, the curves of anion density close to the border under the coil are compared before and after the self-coagulation. It is seen that the curve is cliffy at \(10^{-5}\) s, which is physically formed by the pushing role of ambipolar diffusion potential and double layer [40]. The curve is then softened by the self-coagulation at \(10^{-4}\) s, implying the occurrence of chemistry process that is different to the physics process. This temporal behavior is then characterized by the mode transition from the physical coagulation to chemical coagulation. In addition, the coagulated structure found in the Ar/SF\({}_{6}\) ICP simulation is a semi-circle shape, as seen in Figure 17b. 2. Ambipolar diffusion versus ambipolar self-coagulation  
In Equations (43) and (44), the balances of flux and density are shown. Herein, \(\overset{\rightarrow}{\Gamma}_{e}\) and \(\overset{\rightarrow}{\Gamma}_{i}\) are the fluxes of electrons and ions, respectively. They are both equal to \(\overset{\rightarrow}{\Gamma}\). \(n_{e}\) and \(n_{i}\) are the densities of electrons and ions, respectively. They are approximately equal to \(n\). \[\overset{\rightarrow}{\Gamma}_{e}=\overset{\rightarrow}{\Gamma}_{i}= \overset{\rightarrow}{\Gamma}, \tag{43}\]  
\[n_{e}\approx n_{i}=n. \tag{44}\]  
Figure 16: Temporal variation of simulated total anion density sampled under the coil (refer to the previous two-dimensional density profile of Figure 4a) in Ar/SF\({}_{6}\) plasma, by means of fluid model.