Normally, electron populations in the low temperature plasma follows the Maxwellian distribution and corresponding that only one temperature. if we look at the I - V curve, then it saturates at the plasma potential. now, in the case of two temperatures , I - V curves are also saturated at plasma potential and we observed only some current values at higher negative bias. This current also follows the exponential variations toward the floating point.
Now, if I - V curves having the two saturation regions ( one at negative bias and another at higher positive bias). Before the saturation point, current variation follow the Maxwellian trend for both regions and we can say that there exist two electron populations in the system. I am not understanding the main difference between two temp. and two electron populations in the plasma system. so please put your ideas on this problem.
The idea of two electron population is based on the (classical) thermodynamic picture that temperature is the average of undirected velocities of particles with given mass (so basically the statistic mean value of the kinetic energ of your electrons).
In some plasmas (as you stated correctly) it can be that you have some non-negligible "amount" of electrons that are much faster moving than the others. Such an example would be beam electrons with very high velocity, which have already undergone some direction changing collisions (but please note that an electron beam itself usually has no temperature in the classical sense, because the beam electrons move in more or less the same direction with more or less the same energy and that makes thermodynamical statistics obsolete).
If you have some electrons like the scattered moving beam electrons you can assign a second distribution function to them.
You can apply the definition of temperature only to equilibrium systems. Most of the laboratory plasmas are non-equilibrium. Therefore, it is not accurate to say that non-equilibrium plasma has some temperature. However, electron energy distribution function (EEDF) of many non-equilibrium plasmas consists of groups which are fitted by Maxwellian EEDFs. As a consequence, you can find in the literature how researchers write that they obtain two- or even three-Maxwellian EEDFs. Such groups of electrons may appear due to emissions from the electrodes (beam electrons as Johannes wrote) or any non-elastic collisions like excitation of different vibrational states or attachments.
Yes, Dmitry made a good point, I should have mentioned that also: the classical temperature is only valid for equilibrium systems. However, you might find a phyiscal "workaround" if you are not too strict on your definition of "equilibrium".
For example in many plasmas the electrons are not in equilibrium with the neutrals or ions. Hence you get one "temperature" for the electrons and a second one for the ions (in atomic gases). This can be done by adding to the general definition of equilibrium that the electrons are in equilibrium with the other electrons but not with the neutrals and ions (and the same holds vice versa for the heavier particles).
In molecular gase you may also talk about rotational or vibrational temperature of the gas molecules (which again comes with individual distribution functions).
thanks, johannes and Dmitry for giving a good description of the stated question.
Agree with the above answers. I think ionization, excitation, second emission processes contribute significantly in the non-equilibrium as the inelastic collision in these processes give rise to the change of initial equilibrium distribution of electrons. This happens more frequently in non-local regimes.
None of the reactions to date has yet referred to what is needed to achieve the various equilibria, between the particles of one species, between the particles of different species, the different excited states, etc.: the collision time (in a notionally uniform plasme: the corresponding quantity in a non-uniform but stationary plasma might be the gradient scale length).
In a uniform plasma, after many collision times you will achieve thermal equilibrium. In a collision the two particles share their energy and momentum (mostly these in the plasma's I know of; in other plasmas they may share angular momentum too). When the particles look like hard spheres, that is, the collision cross section does not depend on the relative velocity between the two particles and the collision process doesn't transfer energy to an excitation in one of them, the cross section is well-defined and there's only one collision time to characterize the process. Room-temperature gases are good examples, and those examples are what informs much of our intuition.
Unfortunately, plsamas are more complicated. You can have vastly different time scales, hence there are vastly different equilibria, one additional one for each longer time scale. When a plasma's electrons lose energy when they excite some energy level in the plasma's atoms, the electron energy distribution takes on a particular shape on the time scale relevant to this particular process if this time is long enough to establish equilibrium. This is all in the textbooks: it leads to electron energy distributions such as the Druyvestein distribution, which differs from the much better-known Maxwellian exp(-3mv^2/2kT).
For highly ionized plasmas the Compton collision cross section is the largest. It is not a hard-sphere-like collision because the force between two charged particles varies with distance r like 1/r^2; as a result, the collision cross section varies strongly with velocity v, like 1/v^3. Now there is a continuum of collision times, easily 10x different for electrons with much lower energy than the average energy per particle (which would be the thermal energy if there were equilibrium) than for higher-energy electrons.On a particular time scale you can then have equilibrium for the low-energy electrons, but not (yet) for the energetic electrons. If the time scale is defined by some kind of a source of high-energy electrons, say a directed beam, and an energy sink, say of atomic excitations, you can end up with an electron energy distribution that is not maxwellian but still in equilibrium.
As already remarked by Johannes, the electrons exchange energy amongst themselves on a much faster time scale that they do with ions. Hence, you can have a well-defined electron temperature that differers from an equally well-defined ion temperature.
In short, in a plasma even equilibria can be complicated.
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