If L is of the order of the Debye length, then Eq.
(6) tells us that the quasineutrality condition can be violated.
This is what happens near the walls around a
plasma and near objects, such as probes, inserted into the
plasma. Adjacent to the surface, a sheath of thickness
about 5λD, forms, in which the ions outnumber the electrons,
and a strong electric field is created.