Quasineutrality

Quasineutrality is the condition in plasmas and conducting fluids in which the net electric charge density is approximately zero on macroscopic length scales. In a quasi-neutral region, the densities of positive and negative charges balance so that n_e ≈ Z_i n_i, where n_e is the electron density, n_i is the ion density, and Z_i is the average ion charge. As a result, electric fields are small on those scales, and Gauss’s law implies only tiny charge separation.

The concept underpins many plasma models. When quasineutrality holds, Poisson’s equation can be effectively replaced by a constraint that the electron and ion densities match closely, allowing fluid or kinetic descriptions to ignore rapid electrostatic rearrangements. This simplification is central to magnetohydrodynamics, two-fluid theories, and gyrokinetic or gyrofluid approaches used to study space, laboratory, and astrophysical plasmas.

Validity and limits. Quasineutrality is valid when the characteristic length L is much larger than the Debye length λ_D and the timescales of interest are longer than the plasma period. It is typically applicable in the bulk of a hot plasma but breaks down near boundaries and in regions where space charge accumulates, such as sheaths, double layers, or during rapid transients. In non-neutral plasmas or under strong electric fields that drive significant charge separation, the approximation can fail.

Relation to other concepts. It is an approximation to Gauss’s law and electroneutrality, distinct from exact neutrality but often a very accurate description for large-scale plasma behavior.