Study of the electropositive to electronegative sheath transition in weakly ionized plasmas

Abstract

In this paper, we studied the electropositive to electronegative sheath transition in a weakly ionized plasma and the mathematical condition that describes it. This condition is derived from the Bohm criterion in a two-scale formalism. We show that this transition is characterized by the formation of a triple layer structure in the sheath. Finally, we analyze the sheath thickness as a function of the temperature and density of the negative ions, i.e. for different values of plasma electronegativity. We prove that this sheath thickness has a universal dependence on the ionization rate, due to the universality of the Bohm criterion. When the plasma goes from electropositive to electronegative behavior, it continuously varies even though the sheath edge and its corresponding electric potential displays a discontinuity.

Boundary layer structure of the sheath surrounding a cylindrical or spherical probe in electronegative plasmas

“…Furthermore, using the same reasoning as expressed by Crespo et al [12], it can be shown that the point on the quasi-neutral solution (ξ s , η 0s ) that satisfies (19a) and (19b) is unique and provides a boundary layer structure for the sheath in the limit → 0; the electric potential grows from η 0s to y p in a shrinking region given by (22). Figure 4 (26)…”

“…where δ 0 = δ 0 (y p , α 0 , γ ) and 0 < m = m(y p , α 0 , γ ) < 1 [11,12]. As will be discussed in section 6, this relationship is only based on the two-scale formalism and does not depend on the electric potential at the sheath edge.…”

“…r s − r p = δ • λ D where δ = δ( , y p , α 0 , γ ). In the limit → 0 and from the point of view of the pre-sheath [12,11] (l < ∞), the sheath thickness is finite, ξ s − ξ p < ∞, whereas based on the sheath scale, the size of the sheath is infinite,…”

Modelling of electronegative collisional warm plasma for plasma-surface interaction process

“…Such a thin layer is termed as sheath that plays a momentous role in analyzing the plasma-surface interactions [8]. The investigation of sheath characteristics in different plasma systems has been carried out by several researchers [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. The sheath characteristics have been found to show a distinct nature, when negative ions are introduced in the system [23,[29][30][31][32].…”

“…Hatami [40] and Moulick et al [17] have investigated the presence of two varieties of positive ions in the electropositive and electronegative plasma, respectively, under the collisional environment. The impact of ionization rate on the sheath transition from electropositive to electronegative plasma has been examined by Crespo et al [18]. Yasserian et al [26] have investigated the influence of the magnetic field in electronegative discharge.…”