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.