where n i is the ion density and n n the neutral density (in number of particles per unit volume). In the case of fully ionized matter, α α = 1 . Because of the quasineutrality of plasma, the electron and ion densities are related by n e = ⟨ ⟨ Z i ⟩ ⟩ n i , where ⟨ ⟨ Z i ⟩ ⟩ is the average ion charge (in units of the elementary charge).