Abstract
Presheath and sheath structures of collisional two ion species (helium and argon) plasma in the presence of bi-Maxwellian electrons have been investigated by using a fluid model. As the thermal energy of hot electrons is higher than cold electrons, the electron impact ionization process is governed by the concentration of hot electrons. The velocity of positive ions at the sheath boundary, i.e., the Bohm criterion, gets modified in the presence of ion–neutral drag force, source term, and bi-Maxwellian electrons. It is found that the ion–neutral drag force, ionization rates, and volumetric composition of electrons affect the characteristics of the presheath and sheath. The scale length of the sheath region widens from about 1.09 mm to 5.80 mm with the increase in the concentration of hot electrons. The acoustic speed of helium ions at the sheath boundary is higher than its common speed, whereas the acoustic speed of argon ions is lower than its common speed. The common speed of positive ions at the sheath boundary is slower by 13% in magnitude than in the collisionless case. Furthermore, the effect of ion–neutral drag force on streaming instability for two ion species plasma has been systematically presented.
Highlights
The study of magnetized and unmagnetized plasma–wall transition (PWT) has received paramount importance to understand the characteristics of bounded plasmas, which has useful applications in various fields such as divertor and limiter in fusion devices, etching, semiconductor plasma processing, and many more.1–5 Among these examples, in most of the cases, plasmas often consist of two species of positive ions, and the understanding of such plasmas is fundamental before going into the plasma applications.6–10 The collisional unmagnetized PWT has two distinct regions: the presheath and the sheath.11 The presheath is a quasineutral region where the positive ions accelerate to attain the Bohm velocity before entering the sheath region
In the account of collision force, the common speed with which the positive ions reach the sheath boundary is found to be reduced by 13% in terms of magnitude than in the collisionless case
Since the electric field is relatively weak in the presheath region than in the sheath region, one can assume that both ion species are drifting with nearly equal speed at the sheath boundary, i.e., V1 ≈ V2
Summary
The study of magnetized and unmagnetized plasma–wall transition (PWT) has received paramount importance to understand the characteristics of bounded plasmas, which has useful applications in various fields such as divertor and limiter in fusion devices, etching, semiconductor plasma processing, and many more. Among these examples, in most of the cases, plasmas often consist of two species of positive ions, and the understanding of such plasmas is fundamental before going into the plasma applications. The collisional unmagnetized PWT has two distinct regions: the presheath and the sheath. The presheath is a quasineutral region where the positive ions accelerate to attain the Bohm velocity before entering the sheath region. Baalrud and Hegna and Baalrud et al. studied the Bohm criterion for multiple ion species plasma in which ion–ion two-stream instability has a key role in the determination of flow of each ion species at the sheath boundary They showed that each ion species attains a common speed at the sheath boundary when the temperature of ion species tends to zero. We have considered the ion–neutral collision force as velocity dependent, which is lacking in the previous works while determining the Bohm sheath criterion for multiple ion species plasmas. It is found that the presence of hot electrons, ionization rate, and ion–neutral collision force affect the presheath and sheath structures and the velocity of positive ion species at the sheath boundary. In the account of collision force, the common speed with which the positive ions reach the sheath boundary is found to be reduced by 13% in terms of magnitude than in the collisionless case
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