Assuming that all ions have the same temperature *T*_{i} and the same
thermal diffusivity , *SNAP*'s power balance for the ion
population is:

where *A*_{s} is the area of the flux surface, is the
RF power density to ions including both direct heating and
heating from a minority tail, *q*_{cx} is the net power density lost
to charge-exchange, *q*_{ion-i} is the power density lost by thermal
ions in ionizing neutrals, represents heating due to
viscous dissipation, is the flux of the *jth* ion species,
*n*_{j} is the density of the *jth* ion species, *C*_{v} is the convective
multiplier, specified by the user, and the other terms have their
usual meanings.

When the routine for solving the ion power balance is called, *SNAP*\
has either calculations or measurements for all variables appearing in
Eq.10 except . It can therefore solve the equation
for . If, instead, the user has specified a *model* for
, then Eq. 10 can be solved to yield a new
estimate of the local , given a previous estimate of the
*T*_{i}(*r*) profile (which is necessary to determine the *q*_{ei} and
other terms appearing on the left hand side). By iterating, a
*T*_{i}(*r*) profile consistent with the chosen model for is
obtained.

The corresponding equation for electrons is

As for the ions, all terms in this equation are known to *SNAP*\
except the electron thermal diffusivity , which it proceeds to
solve for.

Fri Jul 11 15:18:44 EDT 1997