The plasma density was seen to increase from 2.0×1020 m-3 to 3.0×1020 m-3 for the full variation of the plasma current.
Gas temperature was seen to get reduced with increasing plasma current, which varied between 2.7 eV to 2.1 eV.
Why this is so?
To answer this question more information are needed.
1) how do you measure gas temperature and plasma density?
2) how do you generate the current (what kine of power supply are you using)?
3) do you have a uniform plasma?
4) do you increase the current in the same experiment or you are running different test cases?
1). Gas temperature was measured using doppler broadening of the spectral line and plasma density was measured by stark broadening of the same spectral line.
2). We used a 80 kW DC power supply having 200 V of open circuit voltage and current capacity of 400 Amps.
3). Its a steady state helium plasma where plasma was confined by magnetic field of 0.4 Tesla. Plasma was uniform near the source exit but expand out towards the end of the chamber.
4). Yes we increase the current in the same experiment form 200 Amps to 300 Amps with increment of 50 Amps.
Where are you measuring the plasma parameters? At the exit or in the discharge?
I think there are different possibility of your results. A probable answer is that you are increasing the current, but the electric field given to the discharge is reduced. Increasing the current you are increasing the ionization degree, therefore less energy is transferred to the gas temperature. If you are supplying the same power to your discharge, increasing the current can correspond to a decrease of the power lost by Joule effect.
You can give a look the the following papers
Article Boltzmann and Master Equations for Magnetohydrodynamics in W...
Article A parametric study of electron energy distribution functions...
I think, the reason is simply current continuity: j=q*ne*ve
if this holds (on average - as it might not be entirely satisfied due to the high number of collisions in a high pressure plasma), ne will go up linearly with the discharge current, which it does according to your data. On the other hand, ve or Te as a result will go down and, thus, the gas temperature will be decreased as well - not as strong as the electron temperature because an electron has low mass - hence it can only transfer a small amount of it's thermal energy to a neutral or an ion in each collision.
First of all thanks for your answer. I again have a confusion i.e, how does gas temperature different from plasma electron temperature. Is it right to call gas temperature as temperature of the plasma?
You are very welcome.
Usually the electron temperature is not equal to the gas temperature because mostly they have different energy distribution functions. However, you wrote about a thermal plasma - thermal plasma indicates that the gas and ion temperature are roughly the same as the electron temperature. This is the case, for example, in fusion devices where the temperature of the gas particles is very high.
That been said, most atmospheric pressure plasmas are non-thermal, which mean Te >> Tgas - if this would not be the case the gas would have to have ~2.5 eV as well, which would correspond to nearly 30,000 Kelvin. But you can have very hot flames, which might be nearly thermal - as you did not mention, which plasma you have, I was not sure but I assumed that it was an arc plasma or something similar.
our plasma is thermal plasma generated with a non transferred torch
what is the density You write 'Gas temperature was seen to get reduced'
So density is : n=P/(k*T). If T (the gas temperature) to get reduced then n increase for P fixed .
what is 'Gas temperature was seen to get reduced with increasing plasma current '
T~W=U*I If 'Gas temperature was seen to get reduced with increasing plasma current ' but to reduced U ( the floating potential ). Make a measurement the floating potential.
nothing unique is there )
In my opinion, at first, in a transient state electron temperature increases with current. These electrons will cause ionization and excitation. The net result is that plasma density increases and gas temperature decreases.
Any correction in this regard would be appreciated.
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