No abstract

A new smoothing method has been used to obtain the electron energy distribution function (EEDF) in plasmas by evaluating the second derivative of the I-V characteristic of a probe inmersed in the plasma. The smoothing method is based on the use of the instrument function. A comparison with other smoothing techniques has permitted us to show the advantages in using the new smoothing method. The experimental setup used to measure the I-V probe characteristic fast and accurately is also presented. The smoothing method was tested by measuring the EEDF in an argon dc discharge at different conditions of the gas pressure and discharge current. The plasma parameters (electron density and temperature) evaluated from the EEDF were compared with those evaluated by using other classic diagnostic methods obtaining a quite good consistency among them.

A new theoretical model for the potential distribution in the surroundings of a cylindrical conductor placed inside a neutral plasma permitted us to analyse the effect of the positive ion thermal motion on the ion current collected by a cylindrical Langmuir probe. The new theoretical model includes the ABR (Allen - Boyd - Reynolds) theory as a limiting case, which is that of a negligible ion temperature compared to the electron temperature.

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.

The influence of the positive ion temperature in cold plasma diagnosis by using Langmuir probes is analyzed. The positive ion zone of the I-V characteristic is used. This zone is distinguished because the charge drained from the plasma is small, diminishing the perturbation due to the measurement process. Nevertheless, it is much affected by the positive ion temperature, thus the traditional methods give inaccurate values for the electron density. Moreover, for an accurate measurement of that current, a good calibration of the instrument used must be ensured. The authors propose the floating potential as the proper parameter to control that calibration.

Some general aspects of the sheath structure in electronegative plasmas are analysed, through an extensive review of the authors' contributions to this subject. An analysis of the positive ion flux as a function of the electric potential, in the region in which neutrality still holds, is shown to be useful to establish the structure of both the sheath and the presheath. Depending on the plasma parameters, the sheath can show the structure of a triple layer in which a region with a net positive charge is followed by a region with net negative charge and finally by the positive ion sheath. However, the electric potential distribution is always a monotonic function in the sheath. In the parameter space region in which the presheath becomes stratified, a separate analysis of the sheath fails to give the correct positive ion flux in the sheath. In any case, the analysis of the sheath gives either the exact value for the positive ion flux or a good approximation and, therefore, a separate analysis of the sheath is also useful in the case of electronegative plasmas. Finally, a criterion to establish when the presheath-sheath structure is governed by the negative ions instead of by the electrons is introduced. This criterion determines when the main properties of the sheath, such as the sheath thickness or the floating potential, begin to be governed by the negative ions.

We solve a radial theoretical model that describes the ion sheath around a cylindrical Langmuir probe with finite non-zero ion temperature in which singularity in an a priori unknown point prevents direct integration. The singularity appears naturally in fluid models when the velocity of the ions reaches the local ion speed of sound. The solutions are smooth and continuous and are valid from the plasma to the probe with no need for asymptotic matching. The solutions that we present are valid for any value of the positive ion to electron temperature ratio and for any constant polytropic coefficient. The model is numerically solved to obtain the electric potential and the ion population density profiles for any given positive ion current collected by the probe. The ion-current to probe-voltage characteristic curves and the Sonin plot are calculated in order to use the results of the model in plasma diagnosis. The proposed methodology is adaptable to other geometries and in the presence of other presheath mechanisms.

This article shows a transition in the behavior of the positive ions movement around a cylindrical Langmuir probe which we have experimentally observed in a low pressure plasma. In the case of helium plasma, depending on the plasma conditions, the ion current collected by the probe behaves as predicted by a radial motion theory, by an orbital motion theory, or by none of them when the transition between the two behaviors takes place. In the case of argon and neon plasmas, the ion current is well described by radial motion theories. The knowledge of the positive ions behavior is essential to diagnose the plasma parameters by using the ion saturation zone of the current-voltage characteristic curve. The use of this zone is one of the less intrusive in probe plasma diagnostics methods providing local information about the plasma parameters, since the charge drained from the plasma is very low.