In a low temperature filament discharge plasma, plasma potential increses if we place a positively biased electrode(spherical or disk shape) in the plasma volume. It is obvious that electrons loss leaves plasma more positive. If at the same time, we expect the low plasma density due to the loss of electrons at the additional surface. But this situation is possible if the plasma formation rate does not change with the loss of electrons or with changing the plasma potential. In experiments, we definitely expect the modification in the ionization rate and therefore change in plasma density and average energy..Does it possible to increase the plasma density with increasing the positive bias on an electrode?. Could we expect the negligible change in an average energy of electrons eventhough the density is increased?..Plesse share your views on the mentioned points..
The plasma is getting from impact higher temperatures degree and higher pressure is occurring in up layers of spacee
Of course, it is possible to increase the plasma density with increasing anode potential. Just picture the extreme case where the positive bias is so high that the auxiliary anode becomes the primary anode (in contrast to the chamber wall). What you will get in low pressure is a Townsend-like discharge between the primary filament and the externally biased anode.
Furthermore, if the anode surface is small and the voltage is high - the collision induced ionisation processes can exceed the possible electron losses over the anode by far.
Thank you Gruenwald..In our case, size of anode is around 8 cm diameter..I don't see the secondary discharge near the filaments. The positive biased electrode is a sink for electrons and makes plasma more positive wrt chamber wall. Is this potential well responsible for extra ionization? I think the energetic electrons get trapped into the potential well, hence increses the ionization cross section due to the higher mean free path. .Is it expected?
Hi Choudhary,
What do you mean by 'secondary discharge' near the filament?
In fact, it is the other way around: the electrons, which are most likely to be trapped are the low energy electrons, which are not really responsible for ionisation. However, secondary electrons from the surrounding plasma are also accelerated in the sheath of the anode and can cause further ionisation processes. The ionisation cross section is a value that is dependent on the particle energy and not on the mean free path - it is the other way around: the mean free path is dependent on the cross section. As the ionisation cross section is a function of Ekin a stronger bias on the electrode might lead to higher ionisation cross section and increased ionisation rates.
cheers
Johannes
ok, I think where is the impact of resistance of filament and pressure and temperature degree.
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