A PHENOMENOLOGICAL MODEL FOR CROSS-FIELD PLASMA TRANSPORT IN NON-AMBIPOLAR SCRAPE-OFF LAYERS

A simplified two-fluid transport model which includes phenomenological coefficients of particle diffusion, mobility, and thermal diffusivity is used to investigate the effects of nonambipolar particle transport on scrape-off layer (SOL) plasma profiles. A computer code (BSOLRAD3) has been written to iteratively solve for 2-D cross-field plasma density, potential, and electron temperature profiles for arbitrary boundary conditions, including segments of "limiters" that are electrically conducting or non-conducting. Numerical results are presented for two test cases: (1) a 1-D slab geometry showing the interdependency of the density, potential, and temperature gradient scale lengths on particle diffusion, mobility, and thermal diffusivity coefficients and limiter bias conditions, and (2) a 2-D geometry illustrating E X B plasma flow effects. It is shown that the SOL profiles can be quite sensitive to non-ambipolarity conditions imposed by the limiter and, in particular, whether the limiter surfaces are biased. Such effects, if overlooked in SOL transport analysis, can lead to erroneous conclusions about the magnitude of the local ambipolar diffusion coefficient.