I What makes a plasma collisionless?

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The distinction between collisionless and collisional plasmas is primarily determined by the mean free path and particle density. Collisionless plasmas occur when the mean free path is large enough that particle collisions are negligible, allowing long-range electromagnetic interactions to dominate. In these plasmas, charged particles are influenced more by the overall charge distribution and magnetic fields rather than direct collisions. Understanding this concept is crucial for studying phenomena in astrophysics and space physics. The discussion highlights the importance of electromagnetic effects in defining plasma behavior.
Jd1431
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Hi, I'm trying to understand what determines whether a plasma will be collisionless or collisional. I understand that a diffuse plasma with large mean free path will be collisionless but I don't really understand it from an electromagnetic point of view
 
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Jd1431 said:
Hi, I'm trying to understand what determines whether a plasma will be collisionless or collisional. I understand that a diffuse plasma with large mean free path will be collisionless but I don't really understand it from an electromagnetic point of view
Succinctly put, "Collisionless plasmas are weakly coupled, i.e., the coupling between the particles due to collisions is generally negligible, and instead, the long-range collective interactions bind particles together." In other words, the charged particles are mainly affected by the net charge distribution and the magnetic field.

So a relatively large mean free path, or low particle density, would be a key factor.

Some good discussions here - https://www.sciencedirect.com/topics/earth-and-planetary-sciences/collisionless-plasmas

See the discussion under

Magnetohydrodynamics and Solar Dynamo Action

E.R. Priest, in The Sun as a Guide to Stellar Physics, 2019
 
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