I have generated plasma at 720 *10^-3 mbar pressure and AC voltage of about 6kV frequency 50Hz. The discharge gas used is air. I am having difficult time understanding the formation of bluish disc at the electrodes.I want to know why and how this disc is formed.Also i will greatly appreciate the information regarding the color of bulk and some extra information i might need.
You got first positive system of nitrogen molecules glowing predominantly in the centre of your discharge and second positive system of N2 in near electrode regions. In near electrode regions you have common cathode layers of the glow discharge - it takes place only in half periods of your voltage when certain electrode is cathode. If you will insert rectifying diode bridge in your discharge circuit you will see bluish glow at one electrode that will be cathode. See special literature on the glow discharges.
Best regards.
Can you please refer me some literature? I am new in this field so i am having hard time understanding things.
Dr. Mikhail Yu. Yakimov explains right, I think it is the cathode glow region.
If you use the DC discharge instead of AC, only one blue region, near the cathode may be observed.
You can review the following websites for more information:
https://en.wikipedia.org/wiki/Glow_discharge
Article Foundations of DC plasma sources
See, for instance, Fridman A., Kennedy L.A. Plasma Physics and Engineering, CRC Press, 2004.
Or/and Raizer Yu.P. Gas Discharge Physics, Springer, 1991.
Simplified consideration you can find in web by kew words: gas discharge physics, glow discharge physics, cathode layer, cathode spot, negative glow, positive column, Faraday dark space, Townsend discharge.
I also suggest two books:
1- Industrial Plasma Engineering, Vol. 1, by J Reece Roth
2- Principles of Plasma Discharges and Materials Processing, by M. A. Lieberman and A. J. Lichtenberg.
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