A simple model of the positive ion sheath in front of a plane probe in weakly ionized electropositive plasmas

Plasma Phys. Control. Fusion

et al. 2011

This work proposes a method to measure the I -V characteristic of a plane probe for each one of the ion species present in a low-pressure plasma, by using as the ammeter a mass spectrometer whose probe entrance can be biased at an arbitrary electric potential. From the I -V characteristic, the corresponding ion energy probability distribution function of each ion species in the plasma is obtained. Moreover, the corresponding ion temperature and the plasma potential are obtained. These results are in good agreement with those obtained from classical Langmuir probes and mass spectrometry methods.

Section: Measurement Methods and Resultsmentioning

confidence: 94%

Mass spectrometry diagnosis of ion species in low-pressure plasmas

Lucena-Polonio

Díaz‐Cabrera

Plasma Phys. Control. Fusion

et al. 2011

“…The sheath in front of a plane probe immersed in a weakly ionized electronegative plasma, in a low-pressure discharge, can be described by the continuity and the momentum-transfer equations for the positive ion motion, and Poisson's equation for the electric potential, according to a fluid description for positive ion motion [7]:…”

Section: Equations Of the Modelmentioning

confidence: 99%

“…For these purposes, it is important to know what is the relationship between some easily measurable parameters such as the probe size or the electric potential of the probe and other parameters related to the plasma characteristics through the sheath (i.e. positive ion current collected by the probe or the ionization rate) [4][5][6][7]. Several authors have studied positive sheaths in both electropositive and electronegative plasmas.…”

Section: Introductionmentioning

confidence: 99%

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

Crespo

Ballesteros

Plasma Sources Sci. Technol.

et al. 2011

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.

“…For the case of electronegative plasmas, it also shows that even for those values corresponding to an oscillatory electric potential, the Bohm criterion is uniquely determined and allows us to obtain the electric potential at the sheath edge. For this purpose, in section 2 we describe the model equations that will allow us to obtain the quasi-neutral solution [35,36]. This quasi-neutral solution provides the initial conditions that should be used to solve the equations of the model, to obtain the electric potential profile from the plasma to the probe surface.…”

Section: Introductionmentioning

confidence: 99%

Study of sheath thickness in weakly ionized plasmas and its dependence on the electric potential and position of the probe

Crespo

Plasma Sources Sci. Technol.

Ballesteros

et al. 2010

This paper studies the sheath in front of a plane probe immersed in an electropositive/electronegative plasma in the low ionization regime. We analyze the dependence of the model on the electric potential and the position of the probe. As will be shown, this relationship is a necessary condition to form the sheath and allows us to determine the thickness of the sheath. The above-mentioned relationship provides the mathematical structure of the boundary layer for the sheath in a two-scale formalism as it is well known. It also shows that the Bohm criterion unambiguously determines the potential at the sheath edge in electronegative plasmas even when the electronegativity of the plasma corresponds to values for which the electric potential has an oscillatory behavior, against the commonly accepted view.