# I Plasma physics questions

1. Feb 4, 2017

### physicist 12345

hello every body

in fact currently i began to study plasma physics and i confused about some points

1) why should my plasma must be greater than debye length to have a collective behavior ??? and is it the quasi neutrality is the main condition for plasma state ??

2) i want a discussion about the condition that plasma frequency must be greater than ordinary collision frequency such the electromagnetic collisions dominate .. and what is the hydrodynamic collisions ..

thank you

2. Feb 5, 2017

### the_wolfman

One of difficulties encountered in plasma physics is trying to identify when a gas becomes a plasma. Unlike other phase transitions there is no clear phase transition where a gas becomes a plasma. There is no analogue of a "boiling point" for a plasma. So we need to find some other criteria to distinguish between a gas and a plasma. Chen argues for a set of criteria that is based on Debye shielding and plasma oscillations. These two effects give rise to phenomena that you would not observe is a conducting gas. The two effect arise from the long range electro-static interaction between the charged particles.

1) A substance does not have to be greater than a Debye length to exhibit collective behavior. However, if a substance is smaller than the Debye length then it's not clear that there is an advantage to calling that substance a plasma versus a collection of charges.

I would not claim that quasi-neutrality is the main condition for a plasma. Gases are quasi-neutral! There are also non-neutral plasmas.

2) Chen's third criteria for a plasma is that the plasma frequency is larger that the electron-neutral collision frequency. If the electron-neutral collision frequency is larger than the plasma frequency then the electron motion will be dominated by "hydrodynamic like" collisions with the neutral gas and not long range electro-static interactions. Plasma oscillations (and many waves) will be strongly damped.

3. Feb 5, 2017

### physicist 12345

deep thanks .. it seems that i start to solve some of confusion

but i saw at many sources that the chen's 1st condition must be satisfied .. i still confused here .. could we say that to say that this is plasma then we want it to be quasi-neutral and that only achieved if our plasma is larger than debye length ? (but you mensioned that there in a non neutral plasma)

i cant understand the first condition till now

4. Feb 6, 2017

### the_wolfman

I don't have the book with me, but if I remember correctly Chen's first condition for a plasma is that the number of particles in a Debye sphere is large. This is a little different than saying that the plasma has to be larger than the Debye length. Recall in the derivation of Debye shielding we assume that the electron distribution is obeys Boltzmann's law. In order for this assumption to be valid there has to be enough electrons present for a statical description to make since. If there are not enough electrons present than the assumptions that went into the Debye length calculation are not valid. As I said, Chen's criteria for a plasma are based in part on Debye shielding.

Now to add to your confusion, there are things called strongly coupled plasmas where the number of particles in a Debye sphere is small. For example strongly coupled plasmas can be found in inertial confinement fusion experiments or in the cores of some stars. It's debatable whether or not to call these object plasma. One way to think of it is this: If a plasma is an ionized gas, then a strongly coupled plasma is analogous to an ionized liquid.

I'm not trying to confuse you. There is a textbook definition of a plasma, and it's important to understand this definition. But plasma physics is a very diverse field and not everything that we study in plasma physics fits nicely into this definition.

5. Feb 7, 2017

### physicist 12345

mmm thank you very much ... but i as told you i just beginner at this topic :)