What is quasineutral in plasma physics?

Question

What is quasineutral gas? In plasma physics, what exactly quasineutral means? In definition of plasma, it is written that plasma is a quasineutral gas of charged and neutral particles which exhibit collective behaviour.

Answer 1

To put it simply:

Plasma is described as a quasi-neutral gas consisting of ions and electrons. You know that electrons are negatively charged and ions (typically ionised by losing an electron) are positive. To sustain a stable plasma, it must be subject to an electric field (lets say between an alternating bias of electrodes) and at a gas pressure. The alternating field is accelerating the electrons rapidly in the direction of the field, but the ions are more massive and more-or-less move at random due to their higher inertia.

Now the definition of "quasi-" basically means something is 'apparently what it says it is, but not really'. So when we think of a plasma, it is fluctuating between net-negative and net-positive charge very quickly with all these charges flying about but if you squint and take an average it looks neutral, but not really.

That is what quasi-neutral means, macroscopically it is easier to say it is "pretty much neutral", rather than try to put a name to the high energy chaos it takes to sustain a plasma.

Answer 2

Quasi-neutrality in a plasma frequency is just saying that the following holds on distances larger than the the Debye length: $$ n_{e} = \sum_{s} \ Z_{s} \ n_{s} \tag{0} $$ where $n_{s}$ is the number density of electrons, $Z_{s}$ is the charge state of ion species $s$, and $n_{s}$ is the number density of ion species $s$.