ZPEnergy.com - Plasma physics

This post from BlazeLabs yahoo group might be of interest to some of our readers:
Hello all,

Thanks Saviour for this very nice article introducing Debye Length and Plasma frequency. As a former student in plasma physics It is normal that I know Debye and Langmuir work which where for me a good starting point to understand some aspects of corona glows.

But although corona glow physic is closely related to plasma physics, Debye length and plasma frequency cannot be directly applied to the case of Lifters.

For those among you that didn't read the paper, let me resume the situation :

1 - A plasma, like conductors and insulators, has a tendency to be electrically neutral (E = 0)
2 - When a charge (microscopic or macroscopic) is introduce in a plasma the electric field created by the charge is neutralised by a screen of charges on a distance that is called Debye length.
3 - The Debye length depends on the plasma density (density of charges). The greater the density the smaller the length.
4 - (not in the article) When an area outside from electrical equilibrium exist (then necessarily smaller than the Debye length) this area is expelled by electrostatic forces to the external boundaries of the plasma.
5 - (not in the article) This external area far from electric equilibrium (E not null) is called the shield and was studied in details by Langmuir in low pressure luminescent glows.

This general statements applies to a great number of situations. In a conductor for instance the charges (when existing) are concentrated on a very thin layer (you can now calculated the length of the layer) on the surface of the conductor in such a way that the field is null inside.

In the corona glow the monopolar area outside from the luminescent glow where the mechanical effect (electric wind) appears is then also a kind of shield. In the corona glow however, the shield nearly fills the whole gap whereas in usual plasma the shield is a very small space compared to the size of the plasma (very well stated in the examples given in the articles).

It is then clear that : the Debye length formula do not apply sraitforward to the corona case.

The reason is simple the Debye formula was obtained in a case where the plasma is fully ionised in thermal equilibrium. This is not directly said in the article but can be guessed : charge dynamics is obtained by Newton second principal F = ma ( then no collision between ion and neutrals) and only one temperature T is taken into account (instead of three Te, Ti, Tn in the general case). Corona glows are low ionisation plasma (ions density a lot smaller than neutral density) far from thermal equilibrium (electron at high temperature, ion temperature very close from the neutral one). The physics of the luminescent glow in corona discharge is a lot more complex than the physics of tokomaks (which is the main background of all the plasma studies of the last thirty years) and then many aspect of the glow are still not clearly understood (such as streamers formation).On the other side the monopolar region where no ionisation process occurs and only one polarity is present is a lot simple. The main process there is only migration of ions through neutrals (v = kE).

It is possible however to derive simple formulas in the monopolaire drifting region following the Debye main idea (it is the reason why I have introduce his concept in the group). Because in our case charge is associated with drift and then with current the maximum density is related to a maximum current, the derivation is fully explained in my pages:

http://membres.lycos.fr/plasmapropulsion/the_lifters_dedicated_page.htm

Although the Debye formula cannot be applied to corona glows, the general conclusions as explained above are still valid, and among them : "the bigger the gap the smaller the charge density".

But there is also a very important specific result:

The bigger the gap the smaller the current, in order that the maximum rise of pressure is constant (do not depends on size).

Henri.


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