quasineutral

Quasineutrality is the property of many plasmas and plasma-like systems in which the net electric charge density is nearly zero at macroscopic length scales. In a plasma consisting of electrons and ions, the electron density ne and the ion density ni satisfy ne ≈ ∑ Zi ni, where Zi is the charge number of ion species. In a singly ionized plasma, ne ≈ ni. The approximation relies on the rapid collective response of the mobile electrons that neutralizes local charge imbalances. On scales much larger than the Debye length, charge separation is suppressed, so the electrostatic potential varies slowly and the net charge density is small.

The quasineutral approximation enables fluid and magnetohydrodynamic descriptions of plasmas, in which Poisson’s equation is replaced by a constraint enforcing ne ≈ ∑ Zi ni. It is widely used in space plasmas, laboratory fusion devices, and laser-produced plasmas, as well as in semiconductor plasma modeling. It underpins concepts such as ambipolar diffusion, where electron and ion fluxes are coupled by the requirement of nearly equal charge.

Limitations and breakdowns: the approximation breaks down near boundaries and in sheath regions, double layers, or in situations with strong charge separation or very low densities where the Debye length is not small compared with the system size. In such cases, full kinetic treatments or Poisson’s equation must be used to resolve the charge distribution and electric fields.