This article deals with the experimental verification of a theoretical radial model, developed by the authors, for the sheath that surrounds a cylindrical Langmuir probe immersed in a plasma in which the positive ion temperature, Ti, is not negligibly small compared to the electron temperature, Te. The theoretical model is a generalization of the classical one developed for cold ions by Allen, Boyd, and Reynolds for the case of spherical probes, and extended by Chen for cylindrical ones. According to our theory, due to the positive ion thermal motion the ion current collected by the probe is increased with respect to the case of cold ions, so its influence must be considered in plasma diagnosis. An experimental device to accurately measure the I-V characteristic of a cylindrical probe in plasma, for which Ti/Te ≠ 0, has been developed. Very good agreement has been found between the theoretical positive ion I-V probe characteristic and the experimental values by using a Sonin plot.

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.

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.

Articles you may be interested inDeduction of edge electron density with multiply charged ions in ORNL volume-type electron cyclotron resonance ion sourcea) Rev. Sci. Instrum. 79, 02A319 (2008); 10.1063/1.2816911Effect of dc bias control on the power absorption in low-pressure, radio-frequency capacitive sheaths Transition of the sheath structure in an electrostatic probe from electropositive to electronegative plasma

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.