In a time-dependent transport code, the flux of particles
across a surface would be determined by solving the particle
continuity equation, , where *S* is the local source rate. By *SNAP*'s
assumption of steady-state, .
Applying the divergence theorem, ,
i.e., the rate at which particles are born inside a given region
exactly balances the rate at which they cross the regions's surface.

From its beam and neutral calculations, *SNAP* has a complete
calculation of the total source rate for electrons (from *
ionization* of beam neutrals, wall neutrals, and halo neutrals) and
ions (from ionization of wall neutrals and beam-ion thermalization).
So it can compute the flux-surface-averaged flux of particles across
any flux surface directly by volume integrating the particle source
inside the region, and dividing by the flux-surface area.

*SNAP* has no models for impurity sources. It effectively assumes
for all impurities.

Fri Jul 11 15:18:44 EDT 1997