4. Non-interacting plasmas

When the density of a plasma is fairly low (upper limit 10 ¹² to 10 ¹⁴ m ^–3 ), particle interactions can often be neglected; the motions of electrons and ions are then determined solely by the electromagnetic fields prevailing in the plasma. This results in what can be called a gas of free electrons and ions; of course, this model is a limiting case; all real gases also contain neutrals, so electrons will always undergo collisions with a certain frequency ν _e0 .