Investigation of intensity distribution by image analysis for DC discharge.
There is a high interesting in the study of the characteristics of DC and AC discharges due to their potential application in various industrial application for surface modification, etching, and plasma enhanced chemical vapor deposition. Analysis for DC glow discharge images using argon gas at constant pressure and different applied voltage were done to study the plasma glow regions. Electron density and temperature were measured by optical emission spectroscopy measurements from neutral atom and ion lines emissions to make comparison between the results of the analysis of the spectroscope at different position between the two electrodes. The results of the image analysis for photo taken of the plasma generated between the electrodes with different applied voltage, show distinct change in the intensities distribution between the electrodes, with some peaks and valleys in of the glow intensities indicates bright and dark plasma regions. These zones vary depending on the applied voltage as close to each other and to cathode with increasing applied voltages. Finally, the glow discharge zones cannot be distinguish and disappear at high voltage. The results of optical emission spectroscopy analysis showed that the electron temperature values increase near the cathode with increasing voltages, while the opposite behavior in the center and near the anode. The biggest values of the electrons density in the plasma center, and increase the applied voltages leads to increase it near the anode and in the plasma center, while decrease near the cathode.
KEYWORDS: DC discharge, Glow discharge regions, Optical emission spectroscopy, Image
Sputtering deposition using DC discharge is an important method in many thin film applications . DC glow discharge includes of the separate regions, Aston dark space, cathode glow, cathode dark space, negative glow, faraday dark space, positive column, anode dark space. The dark spaces are regions relatively poor of ions comparing with plasma region dc discharge .
The spectroscopic methods used to obtain the electron temperature using some information about the excited states population, plasma density and the chemical compositions within plasma . Wavelength of emitted light depends on energy difference between levels. While the intensity, depending on Boltzmann distribution if the plasma at local thermal equilibrium, can be described as 
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
Where [I.sub.ij] and [[lambda].sub.ij] is the intensity and wavelength corresponds to transition from i to j, h is the Planck's constant, n number density of emitting species, c is the speed of light, U(T) partition function, [A.sub.ij] is the transition probability between level i and j. The ratio equation to determine electron temperature .
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)
While the electron density calculated utilizing the Stark broadening effect, while another line broadening such as Doppler broadening and ionic broadening is so small comparing with Stark broadening that can be neglected . So the electron density can be given by the formula :
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)
where [omega] is the electron impact parameter (nm) that can be found in the standard tables [N.sub.r] is the reference electron density which equal to [10.sup.16] (c[m.sup.-3]) for neural atoms and [10.sup.17] (c[m.sup.-3]) for singly charged ions, [DELTA][[lambda].sub.1/2] is the FWHM of Stark broadening lines.
D.C discharge system consist of two electrodes (Anode made from stainless steel and cathode from copper) with 7.5 cm diameter and 4 cm inter electrode distances. Double stage rotary pump were used to vacuumed a glass chamber. The working pressure was adjusted at constant value (0.2 mbar) by control the flow of argon gas using needle valve and pirani gauge. High resolution camera at fixed position, and the camera seating so it keeps the aperture to be constant, to study the changes happening in the plasma intensity zones. The spectroscopy data was taken by optical spectroscope connected with computer and its lens was fitted at three different position from anode (1, 2 and 3) cm as shown in Fig. (1). The applied voltage across the chamber is varied from -290 to -1000 V.
[FIGURE 1 OMITTED]
RESULTS AND DISCUSSIONS
The result Images for glow at different voltage were analyzed using (image j software), for the region between the electrodes, to find the distribution of intensities as shown in Fig.(2), which shows a high variation in glow distribution between the electrodes.
[FIGURE 2 OMITTED]
The extracted curves from above figure of intensity distribution between the electrodes at central line (r = 0) were shown in Fig. (3). At 290 volts, which represents the beginning of breakdown voltage, two region of weak glow appear one near the anode at z [approximately equal to] 0.1 cm and the second near the cathode at z [approximately equal to] 3.9 cm. Plasma regions were distinguish, especially at 300 and 400 volts, where the peaks at 3.3 cm and 3.9 cm represent the negative glow and the cathode glow in addition to areas of darkness between them. Then these regions be narrow and get closer to the cathode, this indicate that the charge clouds in the space move toward the cathode, with increasing applied voltage. This result is in agreement with Xing et al. (1999) . Because of the increased intensity significantly and because the fixed camera aperture the high-intensity cannot be discrimination and the curves appeared as broken.
[FIGURE 3 OMITTED]
Spectrometer measurements were taken at three points and at different voltages as shown in Fig (4). This figure shows many peaks corresponding to atomic and ionic lines for argon and some peaks identical with atomic and ionic copper lines Which has been matched with the standard lines in the NIST sites . This figure shows how increasing intensity in general when increasing voltages, and be the largest at the center while getting more near the cathode at high voltages. Peaks, which has been used [lambda] [approximately equal to] 750 nm belonging to Ar I and [lambda] [approximately equal to] 487 nm, which is for Ar II to calculate the electron temperature.
[FIGURE 4 OMITTED]
Fig. (5) shows the peak profile for 750.38 nm Ar I at different position (near anode, in the center and near cathode) with different applied voltage (600, 700, 800, 900 and 1000) volt. The full width at half maximum for all peaks were found by using Gaussian fitting to calculate electron density ([n.sub.e]) at different condition. The electron impact parameter standard values of broadening for this line (0.085 nm) .
[FIGURE 5 OMITTED]
The variation of electron temperature ([T.sub.e]), calculated by ratio method. Fig. (5) shows that the electron temperature values increase with voltages near the cathode due to increasing the electric field near cathode, while decreasing at the center and near the anode.
[FIGURE 6 OMITTED]
Fig (7) shows the variation of electron density at three regions with different applied voltage. In general, the maximum values of electron densities at the center, due to electrons diffusion to the chamber walls , and increase with increasing voltage, as a result of increasing electrons energy which lead to increase ionization reaction rate which is related with the mean electron energy  which is the mean reaction feed the plasma with more electrons, while decrease near cathode with increasing applied voltage.
[FIGURE 7 OMITTED]
Table 1 shows the calculated values of Debye length ([[lambda].sub.D]), plasma frequency ([f.sub.p]) and Debye number ([N.sub.d]) for DC discharge in argon gas at the three regions with different applied voltage. All the calculated electron temperature values about 1 eV. All the calculated plasma parameters, Debye length, plasma frequency and plasma number, show they were satisfied the plasma conditions.
* The data extracted from the picture analysis showed that it was an effective way to find out the plasma regions
* The electron temperature values increase with voltages near the cathode, while decreasing at the center and near the anode.
* Electron density in general have maximum values at the center and increase with increasing voltage
* Calculated plasma parameter show they were satisfied the plasma conditions
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Kadhim A. Aadim and Mohammed O. Salman
University of Baghdad, College of Science, Department of Physics, Iraq, Baghdad.
Address For Correspondence:
Mohammed Oudah Salman, University of Baghdad, College of Science, Department of Physics, Iraq, Baghdad.
This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/
Received 12 January 2016; Accepted 10 March 2017; Available online 26 March 2017
Table 1: plasma parameters from spectroscopy lines at three region with different applied voltage V (V) KT (eV) [n.sub.e] x [10.sub.18] (c[m.sup.-3]) Near anode 600 1.044 1.118 700 1.018 1.147 800 1.007 1.176 900 1.002 1.247 1000 1.000 1.259 In the center 600 1.047 1.235 700 1.002 1.294 800 0.999 1.324 900 0.993 1.412 1000 0.989 1.441 Near cathode 600 1.057 1.206 700 1.067 1.176 800 1.066 1.118 900 1.076 1.118 1000 1.082 1.059 [[lambda].sub.D] x [10.sup.-6] (cm) Near anode 7.152 6.970 6.845 6.634 6.594 In the center 6.812 6.511 6.428 6.205 6.130 Near cathode 6.929 7.048 7.228 7.262 7.480 [f.sub.p] (Hz) x [10.sup.9] [N.sub.d] Near anode 3.009 1713 3.048 1627 3.087 1580 3.178 1525 3.193 1512 In the center 3.163 1635 3.238 1496 3.274 1472 3.382 1413 3.417 1390 Near cathode 3.125 1681 3.087 1725 3.009 1768 3.009 1793 2.929 1856
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|Author:||Aadim, Kadhim A.; Salman, Mohammed O.|
|Publication:||Advances in Environmental Biology|
|Date:||Mar 1, 2017|
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