SELF-CONSISTENT NUMERICAL BOUNCE-AVERAGED TRANSPORT COMPUTATIONS IN STELLARATORS EMPLOYING A MULTI-MESH APPROACH

A numerical code FLOCS (flow code for stellarators), allowing a self-consistent computation of neoclassical stellarator transport, has been developed. Starting from a bounce-averaged kinetic equation, the distribution function < f > is found as a function of one spatial variable (poloidal angle) and two velocity-space variables (energy and a pitch-angle (pa) variable). Special care is needed at the phase-space boundary where particles can entrap (detrap) collisionlessly into (out of) a helical ripple. The natural choice for the pa variable is therefore the ripple-depth parameter y being 1 at the trapping boundary, approaching 0 for very deeply trapped particles, and going to ∞ for very passing particles. Since < f > is to contain information from which, in addition to the radial fluxes (and thus the confinement times and the ambipolar electric field), the parallel flows and currents should be computable, the entire velocity space must be covered, including very large y values. Numerical stability arguments in combination with CPU-time considerations, on the one hand, versus the physical behavior of < f > for very large y values, on the other hand, suggested the use of a multimesh approach (in the variable y) up to a reasonable value of y ∼ 100. FLOCS extends the range of validity of a previously developed bounce-averaged code FPSTEL. After presenting some issues concerning FPSTEL, which are of relevance to our approach, the code FLOCS is described in detail, and some initial results on the distribution function are given. © 1991.