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Abstract
In this study a low pressure DC glow discharge plasma source has been designed. The aim of our study is to investigate the glow discharge plasma produced by such system. We used electrostatic probes (single and double) as diagnostic tools for the ignited plasma. Argon and helium were used as the working gases in our study.
Investigating the produced glow discharge plasma in both Ar and He discharges based on determining the electrical properties of such plasma like; plasma space potential, electric field distributions, plasma densities (electrons and ions), electron temperature, and the electron energy distribution function EEDF, for various discharge conditions, i.e., over a range of pressure and discharge voltage. The discharge pressure was varied from 0.08 to 0.5 torr for Ar gas, and from 0.2 to 1 torr for He gas. The discharge voltage ranged from 260 to 600 volt for Ar, and from 550 to 750 volt for He. The discharge condition designated according to the pressure–voltage combination.
The experimental work started with studying current–voltage characteristics of the DC glow discharge for various values of pressure, in the range (0.08 to 1 torr) for both argon and helium. This was followed by investigating the Paschen curve for both Ar and He DC glow discharge plasma.
The I-V characteristic curves for both single and double probes, upon which most of the plasma parameters could be derived, were obtained for Ar and He discharges over various discharge conditions. Measurements were carried out in different regions of discharges, i.e., the cathode fall and the negative glow. Measurements of positive column (if exist) was also performed.
Plasma space potential Vs for Ar and He at various discharge conditions were determined by using the single probe, and applying the zero-cross method, according to which the probe characteristic curve was differentiated twice. The axial and radial distributions of the plasma space potential were given, and by differentiating them with respect to distance gave the electric field distributions either axially or radially.
The electron temperatures for both Ar and He discharge were determined in this study by four ways: (1) the semi-log plot method applied to the I-V characteristic curve of the single probe, (2) by multiplying the average electron energy, , derived from the corresponding electron energy distribution function, by the factor 2/3, (3) calculated directly from the double probe characteristic by applying the equivalent resistance theory, and (4) Applying the logarithmic plot method to the double probe characteristic. Data obtained from these methods were compared to each other. An axial and radial distributions of electron temperature for Ar and He at different discharge conditions were investigated.
The axial and radial distributions of plasma density under various discharge conditions for both Ar and He were also investigated. The electron density has been obtained by three methods: (1) by using the single probe, determining the value of electron current at plasma potential can lead to know the electron density, (2) by integrating the area under the electron energy distribution function EEDF, (3) and it was also determined using the double probe method. Two theories were applied to determine the ion number densities, the orbital motion limited theory, OML, for ion collection and the parameterization of Laframboise’s numerical results. A differentiation between the results obtained from both methods was held as a trial to eliminate the over estimation of ion density compared with the electron density over the range of our discharge conditions.
The electron energy distribution functions, EEDFs, for both Ar and He were determined at different discharge conditions by applying the double differentiation method to the single probe characteristic curves. The EEDFs were measured and distributed along the discharge axis.
The effects of discharge conditions, especially, discharge voltage and pressure on the determined plasma parameters namely, electron temperature, electron density, ion density, and EEDFs, were also under the scope of our study.