Temperature and Electrical Potential

[gareth]

Hi,

I posted this in General but to no avail, any ideas?

Hi,

Hope I'm in the right place for this question,

I am currently studying the basic properties of a plasma and I've come to the understanding that the plasma potential is the potential energy that exists between any two particles in the plasma (electrons).

The temperature of a plasma is the thermal energy of these particles.

Also I have found that hotter plasmas have a higher plasma potential.

(please correct me here)

Is this because that in a hotter plasma the electrons will be closer together and hence have higher potential?

If not, what is the relation between plasma electron temperatures and plasma potentials?

Thanks
Gareth

 

[miknsk]

Is this because that in a hotter plasma the electrons will be closer together and hence have higher potential?

Plasma temperature does not depend on the distance between charged particles.
Very hot plasma with low density of charged particles can be obtained, and high-density cold plasma can be obtained too.

 

[gareth]

Thanks Miknsk,

Then why is the plasma potential strongly linked to the plasma temperature?

 

[Astronuc]

Electron mobility is related to temperature (temperature is a measure of average kinetic energy) and the electrons are more easily separated from positive charges as temperature increases. One site mentioned electrons moving out of a plasma to the metal containment, such that the plasma develops a net positive charge while the container develops a net negative charge. This would then cause the plasma (+ ions) to drift (expand) toward the negative charged container.

Plasmas try to maintain charge neutrality.

See - http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html

 

[gareth]

Thanks Astronuc

So hotter electrons,

1) are more mobile
2) reach the containment wall faster
3) cause a positive charge to build up in the plasma (plasma potential)

and hence a higher electron temperature causes a higher plasma potential

However, should the flux of ions to the surface not also increase as the plasma temperature increases? This would cause all plasma potentials to be the same would it not?

 

[Astronuc]

It's the separation of charges that causes local potential field. The plasma and metal container are good conductors, so any electrons leaving a confined plasma and making it to the containment would spread out very quickly. The plasma then has a net positive charge (because they are not balanced by the absent electrons) and that's distributed primarily at the surface.

Is the question primarily about confined plasmas? Astrophyiscal (e.g. stellar) plasmas on a much larger scale are much more complicated.

 

[miknsk]

Thanks Astronuc

So hotter electrons,

1) are more mobile
2) reach the containment wall faster
3) cause a positive charge to build up in the plasma (plasma potential)

and hence a higher electron temperature causes a higher plasma potential

However, should the flux of ions to the surface not also increase as the plasma temperature increases? This would cause all plasma potentials to be the same would it not?

Plasma potential acts as potential barrier for electrons confined in plasma. So what will happen if the electron energy is suddenly arisen, for example, by an order of magnitude?

Electrons will overcome the potential barrier and negative charge located on the walls will increase rapidly, so the plasma potential should increase too till it stops electrons.
I think, it will happen when the plasma potential is arisen by an order of magnitude too.

 

[gareth]

Astronuc,

Yes it a question regarding confined plasmas, in particular, flowing plasmas diagnosed with a Langmuir probe.

So I supose in this case the probe (metal tip) acts as the containment, which, when confronted by the plasma absorbs electrons, when the potential in the plasma (caused by the depletion of the electrons) is strong enough to hold the electrons in the plasma we have arrived at the plasma potential.

So, the idea of plasma potential only really exists in contained plasmas, in space for example, would their generally speaking, be an overall charge neutrality in the plasma?

Or do electrons manage to escape because of their kinetic energy?

 

[gareth]

Any thoughts on this folks?

Just to re-iterate, if a plasma expanding in free space, does it reach a plasma potential? I.e do some of the electrons just escape because of their kinetic energies and keep on going with their thermal energy converted into translational kinetic energy?

Thanks