The plasma is assumed to consist of three (charged) species; namely, electrons (), majority ions (), and impurity ions
(). The charges of the three species are , , and
, respectively, where
is the magnitude of the electron charge.
Quasi-neutrality  demands that
where is the species- number density. Here, is the flux-surface label introduced in Section 14.2.
is the effective ion charge number. (See Section 1.6.) It follows that