THE NEUTRAL DIFFUSION-APPROXIMATION IN A CONSISTENT TOKAMAK EDGE PLASMA-NEUTRAL COMPUTATION

A diffusion approximation for neutral transport in a plasma is developed for incorporation in a finite difference 2-D coupled plasma-neutral fluid computation model (EPIC). One energy group neutrals are assumed in thermal equilibrium locally with the plasma ions. Validity criteria for the model: lambda-0 < L(Ti) is discussed where lambda-0 is the neutral mean free path and L(Ti) is the scale length for the ion temperature. Boundary conditions are derived to include particle recycling at the edge-core plasma interface, and neutral particle reflection from the pumping duct. The neutral diffusion model is incorporated in the 2-D computational code coupling the plasma-neutrals and compared extensively to Monte Carlo (DEGAS) calculations, showing that the neutral diffusion results are realistic over a wide range of limiter and divertor edge plasma conditions. Examples of the time dependent flux and the steady state density and temperature contours are shown for the ASDEX diverted tokamak, and are consistent with experimental data. The flux across the separatrix is seen to be dominated by recycled particles from the edge plasma. The neutral interaction source terms in the plasma density, parallel momentum and temperature equations are each seen to influence the plasma solution.