What makes a plasma collisionless?

[Jd1431]

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

 

[Astronuc]

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